The new Firebuild release contains plenty of small fixes and a few notable improvements.

Experimental macOS support The most frequently asked question from people getting to know Firebuild was if it worked on their Mac and the answer sadly used to be that well, it did, but only in a Linux VM. This was far from what they were looking for. Linux and macOS have common UNIX roots, but porting Firebuild to macOS included bigger challenges, like ensuring that dyld(1), macOS s dynamic loader initializes the preloaded interceptor library early enough to catch all interesting calls, and avoid using anything that uses malloc() or thread local variables which are not yet set up then. Preloading libraries on Linux is really easy, running LD_PRELOAD=my_lib.so ls just works if the library exports the symbols to be interposed, while macOS employs multiple lines of defense to prevent applications from using unknown libraries. Firebuild s guide for making DYLD_INSERT_LIBRARIES honored on Macs can be helpful with other projects as well that rely on injecting libraries. Since GitHub s Arm64 macOS runners don t allow intercepting binaries with arm64e ABI yet, Firebuild s Apple Silicon tests are run at Bitrise, who are proud to be first to provide the latest Xcode stacks and were also quick to make the needed changes to their infrastructure to support Firebuild (thanks! ). Firebuild on macOS can already accelerate simple projects and rebuild itself with Xcode. Since Xcode introduces a lot of nondeterminism to the build, Firebuild can t shine in acceleration with Xcode yet, but can provide nice reports to show which part of the build is the most time consuming and how each sub-command is called. If you would like to try Firebuild on macOS please compile it from the GitHub repository for now. Precompiled binaries will be distributed on the Mac App Store and via CI providers. Contact us to get notified when those channels become available.

Dealing with the Epochalypse Glibc s API provides many functions with time parameters and some of those functions are intercepted by Firebuild. Time parameters used to be passed as 32-bit values on 32-bit systems, preventing them to accurately represent timestamps after year 2038, which is known as the Y2038 problem or the Epochalypse. To deal with the problem glibc 2.34 started providing new function symbol variants with 64-bit time parameters, e.g clock_gettime64() in addition to clock_gettime(). The new 64-bit variants are used when compiling consumers of the API with _TIME_BITS=64 defined. Processes intercepted by Firebuild may have been compiled with or without _TIME_BITS=64, thus libfirebuild now provides both variants on affected systems running glibc >= 34 to work safely with binaries using 64-bit and 32-bit time representation. Many Linux distributions already stopped supporting 32-bit architectures, but Debian and Ubuntu still supports armhf, for example, where the Y2038 problem still applies. Both Debian and Ubuntu performed a transition rebuilding every library (and their reverse dependencies) with -D_FILE_OFFSET_BITS=64 set where the libraries exported symbols that changed when switching to 64-bit time representation (thanks to Steve Langasek for driving this!) . Thanks to the transition most programs are ready for 2038, but interposer libraries are trickier to fix and if you maintain one it might be a good idea to check if it works well both 32-bit and 64-bit libraries. Faketime, for example is not fixed yet, see #1064555.

Select passed through environment variables with regular expressions Firebuild filters out most of the environment variables set when starting a build to make the build more reproducible and achieve higher cache hit rate. Extra environment variables to pass through can be specified on the command line one by one, but with many similarly named variables this may become hard to maintain. With regular expressions this just became easier:

firebuild -o 'env_vars.pass_through += "MY_VARS_.*"' my_build_command

If you are not interested in acceleration just would like to explore what the build does by generating a report you can simply pass all variables:

firebuild -r -o 'env_vars.pass_through += ".*"' my_build_command

Other highlights from the 0.8.3 release

• Fixed and nicer report in Chrome and other WebKit based browsers
• Support GLibc 2.39 by intercepting pidfd_spawn() and pidfd_spawnp()
• Even faster Rust build acceleration

For all the changes please check out the release page on GitHub! (This post is also published on The Firebuild blog.)

• Joachim Bauch (fancycode)
• Alexander Kj ll (capitol)
• Jan Moj (janmojzis)
• Xiao Sheng Wen (atzlinux)

• Alberto Bertogli
• Alexis Murzeau
• David Heilderberg
• Xiyue Deng
• Kathara Sasikumar
• Philippe Swartvagher

The coffee shop Ibtehal and me used to visit during the midnight

Grating at a chemist shop in Mombasa, Kenya

A visit to Nairobi s Historic Cricket Ground On this trip, I got a unique souvenir that can t be purchased from the market a cricket jersey worn in an ODI match by a player. The story goes as follows: I was roaming around the market with my friend Benson from Nairobi to buy a Kenyan cricket jersey for myself, but we couldn t find any. So, Benson had the idea of visiting the Nairobi Gymkhana Club, which used to be Kenya s main cricket ground. It has hosted some historic matches, including the 2003 World Cup match in which Kenya beat the mighty Sri Lankans and the record for the fastest ODI century by Shahid Afridi in just 37 balls in 1996. Although entry to the club was exclusively for members, I was warmly welcomed by the staff. Upon reaching the cricket ground, I met some Indian players who played in Kenyan leagues, as well as Lucas Oluoch and Dominic Wesonga, who have represented Kenya in ODIs. When I expressed interest in getting a jersey, Dominic agreed to send me pictures of his jersey. I liked his jersey and collected it from him. I gave him 2000 shillings, an amount suggested by those Indian players.

Me with players at the Nairobi Gymkhana Club

Cricket pitch at the Nairobi Gymkhana Club

A view of the cricket ground inside the Nairobi Gymkhana Club

Scoreboard at the Nairobi Gymkhana cricket ground

Giraffe Center in Nairobi Kenya is known for its safaris and has no shortage of national parks. In fact, Nairobi is the only capital in the world with a national park. I decided not to visit one, as most of them were expensive and offered multi-day tours, and I didn t want to spend that much time in the wildlife. Instead, I went to the Giraffe Center in Nairobi with Pragya and Rabina. The ticket cost 1500 Kenyan shillings (1000 Indian rupees). In Kenya, matatus - shared vans, usually decorated with portraits of famous people and play rap songs - are the most popular means of public transport. Reaching the Giraffe Center from our hotel required taking five matatus, which cost a total of 150 shillings, and a 2 km walk. The journey back was 90 shillings, suggesting that we didn t find the most efficient route to get there. At the Giraffe Center, we fed giraffes and took photos.

A matatu with a Notorious BIG portrait.

Inside the Giraffe Center

Train ride from Nairobi to Mombasa I took a train from Nairobi to Mombasa. The train is known as the SGR Train, where SGR refers to Standard Gauge Railway. The journey was around 500 km. M-Pesa was the only way to make payment for pre-booking the train ticket, and I didn t have an M-Pesa account. Pragya s friend Mary helped facilitate the payment. I booked a second-class ticket, which cost 1500 shillings (1000 Indian rupees). The train was scheduled to depart from Nairobi at 08:00 hours in the morning and arrive in Mombasa at 14:00 hours. The security check at the station required scanning our bags and having them sniffed by sniffer dogs. I also fell victim to a scam by a security official who offered to help me get my ticket printed, only to later ask me to get him some coffee, which I politely declined. Before boarding the train, I was treated to some stunning views at the Nairobi Terminus station. It was a seating train, but I wished it were a sleeper train, as I was sleep-deprived. The train was neat and clean, with good toilets. The train reached Mombasa on time at around 14:00 hours.

SGR train at Nairobi Terminus.

Interior of the SGR train

Arrival in Mombasa

Mombasa Terminus station.

Mombasa was a bit hotter than Nairobi, with temperatures reaching around 30 degrees Celsius. However, that s not too hot for me, as I am used to higher temperatures in India. I had booked a hostel in the Old Town and was searching for a hitchhike from the Mombasa Terminus station. After trying for more than half an hour, I took a matatu that dropped me 3 km from my hostel for 200 shillings (140 Indian rupees). I tried to hitchhike again but couldn t find a ride. I think I know why I couldn t get a ride in both the cases. In the first case, the Mombasa Terminus was in an isolated place, so most of the vehicles were taxis or matatus while any noncommercial cars were there to pick up friends and family. If the station were in the middle of the city, there would be many more car/truck drivers passing by, thus increasing my possibilities of getting a ride. In the second case, my hostel was at the end of the city, and nobody was going towards that side. In fact, many drivers told me they would love to give me a ride, but they were going in some other direction. Finally, I took a tuktuk for 70 shillings to reach my hostel, Tulia Backpackers. It was 11 USD (1400 shillings) for one night. The balcony gave a nice view of the Indian Ocean. The rooms had fans, but there was no air conditioning. Each bed also had mosquito nets. The place was walking distance of the famous Fort Jesus. Mombasa has had more Islamic influence compared to Nairobi and also has many Hindu temples.

The balcony at Tulia Backpackers Hostel had a nice view of the ocean.

A room inside the hostel with fans and mosquito nets on the beds

Visiting White Sandy Beaches and Getting a Hitchhike Visiting Nyali beach marked my first time ever at a white sand beach. It was like 10 km from the hostel. The next day, I visited Diani Beach, which was 30 km from the hostel. Going to Diani Beach required crossing a river, for which there s a free ferry service every few minutes, followed by taking a matatu to Ukunda and then a tuk-tuk. The journey gave me a glimpse of the beautiful countryside of Kenya.

Nyali beach is a white sand beach

This is the ferry service for crossing the river.

During my return from Diani Beach to the hostel, I was successful in hitchhiking. However, it was only a 4 km ride and not sufficient to reach Ukunda, so I tried to get another ride. When a truck stopped for me, I asked for a ride to Ukunda. Later, I learned that they were going in the same direction as me, so I got off within walking distance from my hostel. The ride was around 30 km. I also learned the difference between a truck ride and a matatu or car ride. For instance, matatus and cars are much faster and cooler due to air conditioning, while trucks tend to be warmer because they lack it. Further, the truck was stopped at many checkpoints by the police for inspections as it carried goods, which is not the case with matatus. Anyways, it was a nice experience, and I am grateful for the ride. I had a nice conversation with the truck drivers about Indian movies and my experiences in Kenya.

Diani beach is a popular beach in Kenya. It is a white sand beach.

Selfie with truck drivers who gave me the free ride

Back to Nairobi I took the SGR train from Mombasa back to Nairobi. This time I took the night train, which departs at 22:00 hours, reaching Nairobi at around 04:00 in the morning. I could not sleep comfortably since the train only had seater seats. I had booked the Zarita Hotel in Nairobi and had already confirmed if they allowed early morning check-in. Usually, hotels have a fixed checkout time, say 11:00 in the morning, and you are not allowed to stay beyond that regardless of the time you checked in. But this hotel checked me in for 24 hours. Here, I paid in US dollars, and the cost was 12 USD.

Almost Got Stuck in Kenya Two days before my scheduled flight from Nairobi back to India, I heard the news that the airports in Kenya were closed due to the strikes. Rabina and Pragya had their flight back to Nepal canceled that day, which left them stuck in Nairobi for two additional days. I called Sahil in India and found out during the conversation that the strike was called off in the evening. It was a big relief for me, and I was fortunate to be able to fly back to India without any changes to my plans.

Newspapers at a stand in Kenya covering news on the airport closure

Experience with locals I had no problems communicating with Kenyans, as everyone I met knew English to an extent that could easily surpass that of big cities in India. Additionally, I learned a few words from Kenya s most popular local language, Swahili, such as Asante, meaning thank you, Jambo for hello, and Karibu for welcome. Knowing a few words in the local language went a long way. I am not sure what s up with haggling in Kenya. It wasn t easy to bring the price of souvenirs down. I bought a fridge magnet for 200 shillings, which was the quoted price. On the other hand, it was much easier to bargain with taxis/tuktuks/motorbikes. I stayed at three hotels/hostels in Kenya. None of them had air conditioners. Two of the places were in Nairobi, and they didn t even have fans in the rooms, while the one in Mombasa had only fans. All of them had good Wi-Fi, except Tulia where the internet overall was a bit shaky. My experience with the hotel staff was great. For instance, we requested that the Nairobi Transit Hotel cancel the included breakfast in order to reduce the room costs, but later realized that it was not a good idea. The hotel allowed us to revert and even offered one of our missing breakfasts during dinner. The staff at Tulia Backpackers in Mombasa facilitated the ticket payment for my train from Mombasa to Nairobi. One of the staff members also gave me a lift to the place where I could catch a matatu to Nyali Beach. They even added an extra tea bag to my tea when I requested it to be stronger.

Food At the Nairobi Transit Hotel, a Spanish omelet with tea was served for breakfast. I noticed that Spanish omelette appeared on the menus of many restaurants, suggesting that it is popular in Kenya. This was my first time having this dish. The milk tea in Kenya, referred to by locals as white tea, is lighter than Indian tea (they don t put a lot of tea leaves).

Spanish Omelette served in breakfast at Nairobi Transit Hotel

I also sampled ugali with eggs. In Mombasa, I visited an Indian restaurant called New Chetna and had a buffet thali there twice.

Ugali with eggs.

Tips for Exchanging Money In Kenya, I exchanged my money at forex shops a couple of times. I received good exchange rates for bills larger than 50 USD. For instance, 1 USD on xe.com was 129 shillings, and I got 128.3 shillings per USD (a total of 12,830 shillings) for two 50 USD notes at an exchange in Nairobi, while 127 shillings, which was the highest rate at the banks. On the other hand, for smaller bills such as a one US dollar note, I would have got 125 shillings. A passport was the only document required for the exchange, and they also provided a receipt. A good piece of advice for travelers is to keep 50 USD or larger bills for exchanging into the local currency while saving the smaller US dollar bills for accommodation, as many hotels and hostels accept payment in US dollars (in addition to Kenyan shillings).

Missed Malindi and Lamu There were more places on my to-visit list in Kenya. But I simply didn t have time to cover them, as I don t like rushing through places, especially in a foreign country where there is a chance of me underestimating the amount of time it takes during transit. I would have liked to visit at least one of Kilifi, Watamu or Malindi beaches. Further, Lamu seemed like a unique place to visit as it has no cars or motorized transport; the only options for transport are boats and donkeys. That s it for now. Meet you in the next one :)

• 18% used a custom postinst script
• 11% provided their ssh pub key for passwordless root login
• 50% of the jobs didn't included a desktop environment at all, the others used GNOME, XFCE or KDE or the Ubuntu desktop the most.
• The biggest ISO was a FAIme job which created a live ISO with a desktop and some additional packages This job took 30min to finish and the resulting ISO was 18G in size.

Designed by Freepik

What is an online system? Networking is a complex topic, and there is lots of confusion around the definition of an online system. Sometimes the boot process gets delayed up to two minutes, because the system still waits for one or more network interfaces to be ready. Systemd provides the network-online.target that other service units can rely on, if they are deemed to require network connectivity. But what does online actually mean in this context, is a link-local IP address enough, do we need a routable gateway and how about DNS name resolution? The requirements for an online network interface depend very much on the services using an interface. For some services it might be good enough to reach their local network segment (e.g. to announce Zeroconf services), while others need to reach domain names (e.g. to mount a NFS share) or reach the global internet to run a web server. On the other hand, the implementation of network-online.target varies, depending on which networking daemon is in use, e.g. systemd-networkd-wait-online.service or NetworkManager-wait-online.service. For Ubuntu, we created a specification that describes what we as a distro expect an online system to be. Having a definition in place, we are able to tackle the network-online-ordering issues that got reported over the years and can work out solutions to avoid delayed boot times on Ubuntu systems. In essence, we want systems to reach the following networking state to be considered online:

1. Do not wait for optional interfaces to receive network configuration
2. Have IPv6 and/or IPv4 link-local addresses on every network interface
3. Have at least one interface with a globally routable connection
4. Have functional domain name resolution on any routable interface

A common implementation NetworkManager and systemd-networkd are two very common networking daemons used on modern Linux systems. But they originate from different contexts and therefore show different behaviours in certain scenarios, such as wait-online. Luckily, on Ubuntu we already have Netplan as a unification layer on top of those networking daemons, that allows for common network configuration, and can also be used to tweak the wait-online logic. With the recent release of Netplan v1.1 we introduced initial functionality to tweak the behaviour of the systemd-networkd-wait-online.service, as used on Ubuntu Server systems. When Netplan is used to drive the systemd-networkd backend, it will emit an override configuration file in /run/systemd/system/systemd-networkd-wait-online.service.d/10-netplan.conf, listing the specific non-optional interfaces that should receive link-local IP configuration. In parallel to that, it defines a list of network interfaces that Netplan detected to be potential global connections, and waits for any of those interfaces to reach a globally routable state. Such override config file might look like this:

[Unit]
ConditionPathIsSymbolicLink=/run/systemd/generator/network-online.target.wants/systemd-networkd-wait-online.service

[Service]
ExecStart=
ExecStart=/lib/systemd/systemd-networkd-wait-online -i eth99.43:carrier -i lo:carrier -i eth99.42:carrier -i eth99.44:degraded -i bond0:degraded
ExecStart=/lib/systemd/systemd-networkd-wait-online --any -o routable -i eth99.43 -i eth99.45 -i bond0

In addition to the new features implemented in Netplan, we reached out to upstream systemd, proposing an enhancement to the systemd-networkd-wait-online service, integrating it with systemd-resolved to check for the availability of DNS name resolution. Once this is implemented upstream, we re able to fully control the systemd-networkd backend on Ubuntu Server systems, to behave consistently and according to the definition of an online system that was lined out above.

Future work The story doesn t end there, because Ubuntu Desktop systems are using NetworkManager as their networking backend. This daemon provides its very own nm-online utility, utilized by the NetworkManager-wait-online systemd service. It implements a much higher-level approach, looking at the networking daemon in general instead of the individual network interfaces. By default, it considers a system to be online once every autoconnect profile got activated (or failed to activate), meaning that either a IPv4 or IPv6 address got assigned. There are considerable enhancements to be implemented to this tool, for it to be controllable in a fine-granular way similar to systemd-networkd-wait-online, so that it can be instructed to wait for specific networking states on selected interfaces.

A note of caution Making a service depend on network-online.target is considered an antipattern in most cases. This is because networking on Linux systems is very dynamic and the systemd target can only ever reflect the networking state at a single point in time. It cannot guarantee this state to be remained over the uptime of your system and has the potentially to delay the boot process considerably. Cables can be unplugged, wireless connectivity can drop, or remote routers can go down at any time, affecting the connectivity state of your local system. Therefore, instead of wondering what to do about network.target, please just fix your program to be friendly to dynamically changing network configuration. [source].

• AS2697 Education and Research Network
• AS9885 NKN Internet Gateway
• AS23770 Tata Institute of Fundamental Research (used as National Centre for Biological Sciences network)
• AS38021 Network of Indian Institute of Foreign Trade
• AS38620 National Knowledge Network
• AS38872 Indian School of Business
• AS45340 B.M.S College of Engineering
• AS55296 National Institute of Public Finance and Policy
• AS55479 Indian Institute of Technology, Kanpur
• AS55566 Inter University Centre for Astronomy and Astrophysics
• AS55824 NKN Core Network
• AS56056 AMITY-IN
• AS55847 NKN Edge Network
• AS58703 Amrita Vishwa Vidyapeetham
• AS58758 Tata Institute of Fundamental Research (used as Homi Bhabha Centre for Science Education (HBCSE) network)
• AS59163 GLA University
• AS59193 Indian Institute of Technology, Hyderabad
• AS131226 Indian Institute of Technology, Roorkee
• AS131473 SRM University
• AS132423 Indian Institute of Technology, Bombay
• AS132524 Tata Institute of Fundamental Research (used as main campus network)
• AS132749 Indraprastha Institute of Information Technology, Delhi
• AS132780 Indian Institute of Technology, Delhi
• AS132984 Uka Tarsadia University
• AS132785 Shiv Nadar Institution of Eminence Deemed to be University
• AS132995 South Asian University
• AS133002 Indian Institute of Tropical Meteorology
• AS133233 S.N. Bose National Centre for Basic Sciences
• AS133273 Tata Institute of Social Sciences
• AS133308 Indira Gandhi Centre For Atomic Research
• AS133313 Saha Institute of Nuclear Physics
• AS133552 B.M.S. College of Engineering
• AS133723 Institute for Plasma Research
• AS134003 Centre For Cellular And Molecular Platforms
• AS134023 Aligarh Muslim University
• AS134322 Tata Institute of Fundamental Research (used as International Centre for Theoretical Sciences (ICTS) network)
• AS134901 Indian Institute of Science Education And Research
• AS134934 Institute For Stem Cell Biology And Regenerative Medicine
• AS135730 Datta Meghe Institute Of Medical Sciences
• AS135734 Birla Institute of Technology And Science
• AS135835 Sardar Vallabhbhai Patel National Police Academy
• AS136005 Raman Research Institute
• AS136304 Institute of Physics, Bhubaneswar
• AS136470 B.M.S. College of Engineering
• AS136702 Physical Research Laboratory
• AS137136 Indian Agricultural Statistics Research Institute
• AS137282 Kalinga Institute of Industrial Technology
• AS137617 Indian Institute of Management, Ahmedabad
• AS137956 Indian Institute of Technology, Ropar
• AS138155 Jawaharlal Nehru University
• AS138231 Indian Institute of Information Technology, Allahabad
• AS140033 Indian Institute of Technology, Bhilai
• AS140118 Indian Institute of Technology Banaras Hindu University
• AS140192 Indian Institute of Information Technology and Management, Kerala
• AS140200 Panjab University
• AS141270 Indian Institute Of Technology, Indore
• AS141340 Indian Institute Of Technology, Madras
• AS141477 Indira Gandhi National Open University
• AS141478 Director National Institute Of Technology, Calicut
• AS141288 National Institute of Science Education And Research Bhubaneswar
• AS141507 National Institute of Mental Health And Neurosciences
• AS142493 Sri Ramachandra Institute Of Higher Education And Research
• AS147239 Lal Bahadur Shastri National Academy of Administration (LBSNAA)
• AS147258 Dayalbagh Educational Institute
• AS149607 National Forensic Sciences University
• AS151086 Amrita Vishwa Vidyapeetham
• AS152533 National Institute of Technology, Karnataka

• AS132926 Allen Career Institute
• AS141841 Indian Institute of Hardware Technology Limited

• Only 14 (out of 67) networks have/announce IPv6 prefixes.
• Two networks have IX presence; AS132785 Shiv Nadar at Extreme IX Delhi and AS56056 Amity at DE-CIX Delhi.
• TIFR as an organization has 4 ASNs (AS23770, AS58758, AS132524 and AS134322), BMS College of Engineering has 3 (AS45340, AS133552 and AS136470) and Amrita Vishwa Vidyapeetham has 2 (AS58703 and AS151086).
• AS2697, AS9885, AS38620, AS55824 and AS55847 ie Education and Research Network (ERNET) and National Knowledge Network (NKN) form the Indian NREN.
• 43 network have NKN as their upstream, while BSNL comes second as upstream for 21.

• [1] https://etbe.coker.com.au/2024/08/24/wifi-6e-mesh/

A Starting Point for Investigation: /sys/power/state and /sys/power/disk These two system files exist to allow debugging of hibernation, and thus control the exact state used directly. Writing specific values to the state file controls the exact sleep mode used and disk controls the specific hibernation mode¹. This is extremely handy as an entry point to understand how these systems work, since we can just follow what happens when they are written to.

Show and Store Functions These two files are defined using the power_attr macro: kernel/power/power.h:80

#define power_attr(_name) \
static struct kobj_attribute _name##_attr =     \
    .attr   =               \
        .name = __stringify(_name), \
        .mode = 0644,           \
     ,                  \
    .show   = _name##_show,         \
    .store  = _name##_store,        \
 

show is called on reads and store on writes. state_show is a little boring for our purposes, as it just prints all the available sleep states. kernel/power/main.c:657

/*
 * state - control system sleep states.
 *
 * show() returns available sleep state labels, which may be "mem", "standby",
 * "freeze" and "disk" (hibernation).
 * See Documentation/admin-guide/pm/sleep-states.rst for a description of
 * what they mean.
 *
 * store() accepts one of those strings, translates it into the proper
 * enumerated value, and initiates a suspend transition.
 */
static ssize_t state_show(struct kobject *kobj, struct kobj_attribute *attr,
			  char *buf)
 
	char *s = buf;
#ifdef CONFIG_SUSPEND
	suspend_state_t i;
	for (i = PM_SUSPEND_MIN; i < PM_SUSPEND_MAX; i++)
		if (pm_states[i])
			s += sprintf(s,"%s ", pm_states[i]);
#endif
	if (hibernation_available())
		s += sprintf(s, "disk ");
	if (s != buf)
		/* convert the last space to a newline */
		*(s-1) = '\n';
	return (s - buf);
 

state_store, however, provides our entry point. If the string disk is written to the state file, it calls hibernate(). This is our entry point. kernel/power/main.c:715

static ssize_t state_store(struct kobject *kobj, struct kobj_attribute *attr,
			   const char *buf, size_t n)
 
	suspend_state_t state;
	int error;
	error = pm_autosleep_lock();
	if (error)
		return error;
	if (pm_autosleep_state() > PM_SUSPEND_ON)  
		error = -EBUSY;
		goto out;
	 
	state = decode_state(buf, n);
	if (state < PM_SUSPEND_MAX)  
		if (state == PM_SUSPEND_MEM)
			state = mem_sleep_current;
		error = pm_suspend(state);
	  else if (state == PM_SUSPEND_MAX)  
		error = hibernate();
	  else  
		error = -EINVAL;
	 
 out:
	pm_autosleep_unlock();
	return error ? error : n;
 

kernel/power/main.c:688

static suspend_state_t decode_state(const char *buf, size_t n)
 
#ifdef CONFIG_SUSPEND
	suspend_state_t state;
#endif
	char *p;
	int len;
	p = memchr(buf, '\n', n);
	len = p ? p - buf : n;
	/* Check hibernation first. */
	if (len == 4 && str_has_prefix(buf, "disk"))
		return PM_SUSPEND_MAX;
#ifdef CONFIG_SUSPEND
	for (state = PM_SUSPEND_MIN; state < PM_SUSPEND_MAX; state++)  
		const char *label = pm_states[state];
		if (label && len == strlen(label) && !strncmp(buf, label, len))
			return state;
	 
#endif
	return PM_SUSPEND_ON;
 

Could we have figured this out just via function names? Sure, but this way we know for sure that nothing else is happening before this function is called.

Autosleep Our first detour is into the autosleep system. When checking the state above, you may notice that the kernel grabs the pm_autosleep_lock before checking the current state. autosleep is a mechanism originally from Android that sends the entire system to either suspend or hibernate whenever it is not actively working on anything. This is not enabled for most desktop configurations, since it s primarily for mobile systems and inverts the standard suspend and hibernate interactions. This system is implemented as a workqueue² that checks the current number of wakeup events, processes and drivers that need to run³, and if there aren t any, then the system is put into the autosleep state, typically suspend. However, it could be hibernate if configured that way via /sys/power/autosleep in a similar manner to using /sys/power/state to manually enable hibernation. kernel/power/main.c:841

static ssize_t autosleep_store(struct kobject *kobj,
			       struct kobj_attribute *attr,
			       const char *buf, size_t n)
 
	suspend_state_t state = decode_state(buf, n);
	int error;
	if (state == PM_SUSPEND_ON
	    && strcmp(buf, "off") && strcmp(buf, "off\n"))
		return -EINVAL;
	if (state == PM_SUSPEND_MEM)
		state = mem_sleep_current;
	error = pm_autosleep_set_state(state);
	return error ? error : n;
 
power_attr(autosleep);
#endif /* CONFIG_PM_AUTOSLEEP */

kernel/power/autosleep.c:24

static DEFINE_MUTEX(autosleep_lock);
static struct wakeup_source *autosleep_ws;
static void try_to_suspend(struct work_struct *work)
 
	unsigned int initial_count, final_count;
	if (!pm_get_wakeup_count(&initial_count, true))
		goto out;
	mutex_lock(&autosleep_lock);
	if (!pm_save_wakeup_count(initial_count)  
		system_state != SYSTEM_RUNNING)  
		mutex_unlock(&autosleep_lock);
		goto out;
	 
	if (autosleep_state == PM_SUSPEND_ON)  
		mutex_unlock(&autosleep_lock);
		return;
	 
	if (autosleep_state >= PM_SUSPEND_MAX)
		hibernate();
	else
		pm_suspend(autosleep_state);
	mutex_unlock(&autosleep_lock);
	if (!pm_get_wakeup_count(&final_count, false))
		goto out;
	/*
	 * If the wakeup occurred for an unknown reason, wait to prevent the
	 * system from trying to suspend and waking up in a tight loop.
	 */
	if (final_count == initial_count)
		schedule_timeout_uninterruptible(HZ / 2);
 out:
	queue_up_suspend_work();
 
static DECLARE_WORK(suspend_work, try_to_suspend);
void queue_up_suspend_work(void)
 
	if (autosleep_state > PM_SUSPEND_ON)
		queue_work(autosleep_wq, &suspend_work);
 

The Steps of Hibernation

Hibernation Kernel Config It s important to note that most of the hibernate-specific functions below do nothing unless you ve defined CONFIG_HIBERNATION in your Kconfig⁴. As an example, hibernate itself is defined as the following if CONFIG_HIBERNATE is not set. include/linux/suspend.h:407

static inline int hibernate(void)   return -ENOSYS;  

Check if Hibernation is Available We begin by confirming that we actually can perform hibernation, via the hibernation_available function. kernel/power/hibernate.c:742

if (!hibernation_available())  
	pm_pr_dbg("Hibernation not available.\n");
	return -EPERM;
 

kernel/power/hibernate.c:92

bool hibernation_available(void)
 
	return nohibernate == 0 &&
		!security_locked_down(LOCKDOWN_HIBERNATION) &&
		!secretmem_active() && !cxl_mem_active();
 

nohibernate is controlled by the kernel command line, it s set via either nohibernate or hibernate=no. security_locked_down is a hook for Linux Security Modules to prevent hibernation. This is used to prevent hibernating to an unencrypted storage device, as specified in the manual page kernel_lockdown(7). Interestingly, either level of lockdown, integrity or confidentiality, locks down hibernation because with the ability to hibernate you can extract bascially anything from memory and even reboot into a modified kernel image. secretmem_active checks whether there is any active use of memfd_secret, and if so it prevents hibernation. memfd_secret returns a file descriptor that can be mapped into a process but is specifically unmapped from the kernel s memory space. Hibernating with memory that not even the kernel is supposed to access would expose that memory to whoever could access the hibernation image. This particular feature of secret memory was apparently controversial, though not as controversial as performance concerns around fragmentation when unmapping kernel memory (which did not end up being a real problem). cxl_mem_active just checks whether any CXL memory is active. A full explanation is provided in the commit introducing this check but there s also a shortened explanation from cxl_mem_probe that sets the relevant flag when initializing a CXL memory device. drivers/cxl/mem.c:186

* The kernel may be operating out of CXL memory on this device,
* there is no spec defined way to determine whether this device
* preserves contents over suspend, and there is no simple way
* to arrange for the suspend image to avoid CXL memory which
* would setup a circular dependency between PCI resume and save
* state restoration.

Check Compression The next check is for whether compression support is enabled, and if so whether the requested algorithm is enabled. kernel/power/hibernate.c:747

/*
 * Query for the compression algorithm support if compression is enabled.
 */
if (!nocompress)  
	strscpy(hib_comp_algo, hibernate_compressor, sizeof(hib_comp_algo));
	if (crypto_has_comp(hib_comp_algo, 0, 0) != 1)  
		pr_err("%s compression is not available\n", hib_comp_algo);
		return -EOPNOTSUPP;
	 
 

The nocompress flag is set via the hibernate command line parameter, setting hibernate=nocompress. If compression is enabled, then hibernate_compressor is copied to hib_comp_algo. This synchronizes the current requested compression setting (hibernate_compressor) with the current compression setting (hib_comp_algo). Both values are character arrays of size CRYPTO_MAX_ALG_NAME (128 in this kernel). kernel/power/hibernate.c:50

static char hibernate_compressor[CRYPTO_MAX_ALG_NAME] = CONFIG_HIBERNATION_DEF_COMP;
/*
 * Compression/decompression algorithm to be used while saving/loading
 * image to/from disk. This would later be used in 'kernel/power/swap.c'
 * to allocate comp streams.
 */
char hib_comp_algo[CRYPTO_MAX_ALG_NAME];

hibernate_compressor defaults to lzo if that algorithm is enabled, otherwise to lz4 if enabled⁵. It can be overwritten using the hibernate.compressor setting to either lzo or lz4. kernel/power/Kconfig:95

choice
	prompt "Default compressor"
	default HIBERNATION_COMP_LZO
	depends on HIBERNATION
config HIBERNATION_COMP_LZO
	bool "lzo"
	depends on CRYPTO_LZO
config HIBERNATION_COMP_LZ4
	bool "lz4"
	depends on CRYPTO_LZ4
endchoice
config HIBERNATION_DEF_COMP
	string
	default "lzo" if HIBERNATION_COMP_LZO
	default "lz4" if HIBERNATION_COMP_LZ4
	help
	  Default compressor to be used for hibernation.

kernel/power/hibernate.c:1425

static const char * const comp_alg_enabled[] =  
#if IS_ENABLED(CONFIG_CRYPTO_LZO)
	COMPRESSION_ALGO_LZO,
#endif
#if IS_ENABLED(CONFIG_CRYPTO_LZ4)
	COMPRESSION_ALGO_LZ4,
#endif
 ;
static int hibernate_compressor_param_set(const char *compressor,
		const struct kernel_param *kp)
 
	unsigned int sleep_flags;
	int index, ret;
	sleep_flags = lock_system_sleep();
	index = sysfs_match_string(comp_alg_enabled, compressor);
	if (index >= 0)  
		ret = param_set_copystring(comp_alg_enabled[index], kp);
		if (!ret)
			strscpy(hib_comp_algo, comp_alg_enabled[index],
				sizeof(hib_comp_algo));
	  else  
		ret = index;
	 
	unlock_system_sleep(sleep_flags);
	if (ret)
		pr_debug("Cannot set specified compressor %s\n",
			 compressor);
	return ret;
 
static const struct kernel_param_ops hibernate_compressor_param_ops =  
	.set    = hibernate_compressor_param_set,
	.get    = param_get_string,
 ;
static struct kparam_string hibernate_compressor_param_string =  
	.maxlen = sizeof(hibernate_compressor),
	.string = hibernate_compressor,
 ;

We then check whether the requested algorithm is supported via crypto_has_comp. If not, we bail out of the whole operation with EOPNOTSUPP. As part of crypto_has_comp we perform any needed initialization of the algorithm, loading kernel modules and running initialization code as needed⁶.

Grab Locks The next step is to grab the sleep and hibernation locks via lock_system_sleep and hibernate_acquire. kernel/power/hibernate.c:758

sleep_flags = lock_system_sleep();
/* The snapshot device should not be opened while we're running */
if (!hibernate_acquire())  
	error = -EBUSY;
	goto Unlock;
 

First, lock_system_sleep marks the current thread as not freezable, which will be important later⁷. It then grabs the system_transistion_mutex, which locks taking snapshots or modifying how they are taken, resuming from a hibernation image, entering any suspend state, or rebooting.

The GFP Mask The kernel also issues a warning if the gfp mask is changed via either pm_restore_gfp_mask or pm_restrict_gfp_mask without holding the system_transistion_mutex. GFP flags tell the kernel how it is permitted to handle a request for memory. include/linux/gfp_types.h:12

 * GFP flags are commonly used throughout Linux to indicate how memory
 * should be allocated.  The GFP acronym stands for get_free_pages(),
 * the underlying memory allocation function.  Not every GFP flag is
 * supported by every function which may allocate memory.

In the case of hibernation specifically we care about the IO and FS flags, which are reclaim operators, ways the system is permitted to attempt to free up memory in order to satisfy a specific request for memory. include/linux/gfp_types.h:176

 * Reclaim modifiers
 * -----------------
 * Please note that all the following flags are only applicable to sleepable
 * allocations (e.g. %GFP_NOWAIT and %GFP_ATOMIC will ignore them).
 *
 * %__GFP_IO can start physical IO.
 *
 * %__GFP_FS can call down to the low-level FS. Clearing the flag avoids the
 * allocator recursing into the filesystem which might already be holding
 * locks.

gfp_allowed_mask sets which flags are permitted to be set at the current time. As the comment below outlines, preventing these flags from being set avoids situations where the kernel needs to do I/O to allocate memory (e.g. read/writing swap⁸) but the devices it needs to read/write to/from are not currently available. kernel/power/main.c:24

/*
 * The following functions are used by the suspend/hibernate code to temporarily
 * change gfp_allowed_mask in order to avoid using I/O during memory allocations
 * while devices are suspended.  To avoid races with the suspend/hibernate code,
 * they should always be called with system_transition_mutex held
 * (gfp_allowed_mask also should only be modified with system_transition_mutex
 * held, unless the suspend/hibernate code is guaranteed not to run in parallel
 * with that modification).
 */
static gfp_t saved_gfp_mask;
void pm_restore_gfp_mask(void)
 
	WARN_ON(!mutex_is_locked(&system_transition_mutex));
	if (saved_gfp_mask)  
		gfp_allowed_mask = saved_gfp_mask;
		saved_gfp_mask = 0;
	 
 
void pm_restrict_gfp_mask(void)
 
	WARN_ON(!mutex_is_locked(&system_transition_mutex));
	WARN_ON(saved_gfp_mask);
	saved_gfp_mask = gfp_allowed_mask;
	gfp_allowed_mask &= ~(__GFP_IO   __GFP_FS);
 

Sleep Flags After grabbing the system_transition_mutex the kernel then returns and captures the previous state of the threads flags in sleep_flags. This is used later to remove PF_NOFREEZE if it wasn t previously set on the current thread. kernel/power/main.c:52

unsigned int lock_system_sleep(void)
 
	unsigned int flags = current->flags;
	current->flags  = PF_NOFREEZE;
	mutex_lock(&system_transition_mutex);
	return flags;
 
EXPORT_SYMBOL_GPL(lock_system_sleep);

include/linux/sched.h:1633

#define PF_NOFREEZE		0x00008000	/* This thread should not be frozen */

Then we grab the hibernate-specific semaphore to ensure no one can open a snapshot or resume from it while we perform hibernation. Additionally this lock is used to prevent hibernate_quiet_exec, which is used by the nvdimm driver to active its firmware with all processes and devices frozen, ensuring it is the only thing running at that time⁹. kernel/power/hibernate.c:82

bool hibernate_acquire(void)
 
	return atomic_add_unless(&hibernate_atomic, -1, 0);
 

Prepare Console The kernel next calls pm_prepare_console. This function only does anything if CONFIG_VT_CONSOLE_SLEEP has been set. This prepares the virtual terminal for a suspend state, switching away to a console used only for the suspend state if needed. kernel/power/console.c:130

void pm_prepare_console(void)
 
	if (!pm_vt_switch())
		return;
	orig_fgconsole = vt_move_to_console(SUSPEND_CONSOLE, 1);
	if (orig_fgconsole < 0)
		return;
	orig_kmsg = vt_kmsg_redirect(SUSPEND_CONSOLE);
	return;
 

The first thing is to check whether we actually need to switch the VT kernel/power/console.c:94

/*
 * There are three cases when a VT switch on suspend/resume are required:
 *   1) no driver has indicated a requirement one way or another, so preserve
 *      the old behavior
 *   2) console suspend is disabled, we want to see debug messages across
 *      suspend/resume
 *   3) any registered driver indicates it needs a VT switch
 *
 * If none of these conditions is present, meaning we have at least one driver
 * that doesn't need the switch, and none that do, we can avoid it to make
 * resume look a little prettier (and suspend too, but that's usually hidden,
 * e.g. when closing the lid on a laptop).
 */
static bool pm_vt_switch(void)
 
	struct pm_vt_switch *entry;
	bool ret = true;
	mutex_lock(&vt_switch_mutex);
	if (list_empty(&pm_vt_switch_list))
		goto out;
	if (!console_suspend_enabled)
		goto out;
	list_for_each_entry(entry, &pm_vt_switch_list, head)  
		if (entry->required)
			goto out;
	 
	ret = false;
out:
	mutex_unlock(&vt_switch_mutex);
	return ret;
 

There is an explanation of the conditions under which a switch is performed in the comment above the function, but we ll also walk through the steps here. Firstly we grab the vt_switch_mutex to ensure nothing will modify the list while we re looking at it. We then examine the pm_vt_switch_list. This list is used to indicate the drivers that require a switch during suspend. They register this requirement, or the lack thereof, via pm_vt_switch_required. kernel/power/console.c:31

/**
 * pm_vt_switch_required - indicate VT switch at suspend requirements
 * @dev: device
 * @required: if true, caller needs VT switch at suspend/resume time
 *
 * The different console drivers may or may not require VT switches across
 * suspend/resume, depending on how they handle restoring video state and
 * what may be running.
 *
 * Drivers can indicate support for switchless suspend/resume, which can
 * save time and flicker, by using this routine and passing 'false' as
 * the argument.  If any loaded driver needs VT switching, or the
 * no_console_suspend argument has been passed on the command line, VT
 * switches will occur.
 */
void pm_vt_switch_required(struct device *dev, bool required)

Next, we check console_suspend_enabled. This is set to false by the kernel parameter no_console_suspend, but defaults to true. Finally, if there are any entries in the pm_vt_switch_list, then we check to see if any of them require a VT switch. Only if none of these conditions apply, then we return false. If a VT switch is in fact required, then we move first the currently active virtual terminal/console¹⁰ (vt_move_to_console) and then the current location of kernel messages (vt_kmsg_redirect) to the SUSPEND_CONSOLE. The SUSPEND_CONSOLE is the last entry in the list of possible consoles, and appears to just be a black hole to throw away messages. kernel/power/console.c:16

#define SUSPEND_CONSOLE	(MAX_NR_CONSOLES-1)

Interestingly, these are separate functions because you can use TIOCL_SETKMSGREDIRECT (an ioctl¹¹) to send kernel messages to a specific virtual terminal, but by default its the same as the currently active console. The locations of the previously active console and the previous kernel messages location are stored in orig_fgconsole and orig_kmsg, to restore the state of the console and kernel messages after the machine wakes up again. Interestingly, this means orig_fgconsole also ends up storing any errors, so has to be checked to ensure it s not less than zero before we try to do anything with the kernel messages on both suspend and resume. drivers/tty/vt/vt_ioctl.c:1268

/* Perform a kernel triggered VT switch for suspend/resume */
static int disable_vt_switch;
int vt_move_to_console(unsigned int vt, int alloc)
 
	int prev;
	console_lock();
	/* Graphics mode - up to X */
	if (disable_vt_switch)  
		console_unlock();
		return 0;
	 
	prev = fg_console;
	if (alloc && vc_allocate(vt))  
		/* we can't have a free VC for now. Too bad,
		 * we don't want to mess the screen for now. */
		console_unlock();
		return -ENOSPC;
	 
	if (set_console(vt))  
		/*
		 * We're unable to switch to the SUSPEND_CONSOLE.
		 * Let the calling function know so it can decide
		 * what to do.
		 */
		console_unlock();
		return -EIO;
	 
	console_unlock();
	if (vt_waitactive(vt + 1))  
		pr_debug("Suspend: Can't switch VCs.");
		return -EINTR;
	 
	return prev;
 

Unlike most other locking functions we ve seen so far, console_lock needs to be careful to ensure nothing else is panicking and needs to dump to the console before grabbing the semaphore for the console and setting a couple flags.

Panics Panics are tracked via an atomic integer set to the id of the processor currently panicking. kernel/printk/printk.c:2649

/**
 * console_lock - block the console subsystem from printing
 *
 * Acquires a lock which guarantees that no consoles will
 * be in or enter their write() callback.
 *
 * Can sleep, returns nothing.
 */
void console_lock(void)
 
	might_sleep();
	/* On panic, the console_lock must be left to the panic cpu. */
	while (other_cpu_in_panic())
		msleep(1000);
	down_console_sem();
	console_locked = 1;
	console_may_schedule = 1;
 
EXPORT_SYMBOL(console_lock);

kernel/printk/printk.c:362

/*
 * Return true if a panic is in progress on a remote CPU.
 *
 * On true, the local CPU should immediately release any printing resources
 * that may be needed by the panic CPU.
 */
bool other_cpu_in_panic(void)
 
	return (panic_in_progress() && !this_cpu_in_panic());
 

kernel/printk/printk.c:345

static bool panic_in_progress(void)
 
	return unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID);
 

kernel/printk/printk.c:350

/* Return true if a panic is in progress on the current CPU. */
bool this_cpu_in_panic(void)
 
	/*
	 * We can use raw_smp_processor_id() here because it is impossible for
	 * the task to be migrated to the panic_cpu, or away from it. If
	 * panic_cpu has already been set, and we're not currently executing on
	 * that CPU, then we never will be.
	 */
	return unlikely(atomic_read(&panic_cpu) == raw_smp_processor_id());
 

console_locked is a debug value, used to indicate that the lock should be held, and our first indication that this whole virtual terminal system is more complex than might initially be expected. kernel/printk/printk.c:373

/*
 * This is used for debugging the mess that is the VT code by
 * keeping track if we have the console semaphore held. It's
 * definitely not the perfect debug tool (we don't know if _WE_
 * hold it and are racing, but it helps tracking those weird code
 * paths in the console code where we end up in places I want
 * locked without the console semaphore held).
 */
static int console_locked;

console_may_schedule is used to see if we are permitted to sleep and schedule other work while we hold this lock. As we ll see later, the virtual terminal subsystem is not re-entrant, so there s all sorts of hacks in here to ensure we don t leave important code sections that can t be safely resumed.

Disable VT Switch As the comment below lays out, when another program is handling graphical display anyway, there s no need to do any of this, so the kernel provides a switch to turn the whole thing off. Interestingly, this appears to only be used by three drivers, so the specific hardware support required must not be particularly common.

drivers/gpu/drm/omapdrm/dss
drivers/video/fbdev/geode
drivers/video/fbdev/omap2

drivers/tty/vt/vt_ioctl.c:1308

/*
 * Normally during a suspend, we allocate a new console and switch to it.
 * When we resume, we switch back to the original console.  This switch
 * can be slow, so on systems where the framebuffer can handle restoration
 * of video registers anyways, there's little point in doing the console
 * switch.  This function allows you to disable it by passing it '0'.
 */
void pm_set_vt_switch(int do_switch)
 
	console_lock();
	disable_vt_switch = !do_switch;
	console_unlock();
 
EXPORT_SYMBOL(pm_set_vt_switch);

The rest of the vt_switch_console function is pretty normal, however, simply allocating space if needed to create the requested virtual terminal and then setting the current virtual terminal via set_console.

Virtual Terminal Set Console With set_console, we begin (as if we haven t been already) to enter the madness that is the virtual terminal subsystem. As mentioned previously, modifications to its state must be made very carefully, as other stuff happening at the same time could create complete messes. All this to say, calling set_console does not actually perform any work to change the state of the current console. Instead it indicates what changes it wants and then schedules that work. drivers/tty/vt/vt.c:3153

int set_console(int nr)
 
	struct vc_data *vc = vc_cons[fg_console].d;
	if (!vc_cons_allocated(nr)   vt_dont_switch  
		(vc->vt_mode.mode == VT_AUTO && vc->vc_mode == KD_GRAPHICS))  
		/*
		 * Console switch will fail in console_callback() or
		 * change_console() so there is no point scheduling
		 * the callback
		 *
		 * Existing set_console() users don't check the return
		 * value so this shouldn't break anything
		 */
		return -EINVAL;
	 
	want_console = nr;
	schedule_console_callback();
	return 0;
 

The check for vc->vc_mode == KD_GRAPHICS is where most end-user graphical desktops will bail out of this change, as they re in graphics mode and don t need to switch away to the suspend console. vt_dont_switch is a flag used by the ioctls¹¹ VT_LOCKSWITCH and VT_UNLOCKSWITCH to prevent the system from switching virtual terminal devices when the user has explicitly locked it. VT_AUTO is a flag indicating that automatic virtual terminal switching is enabled¹², and thus deliberate switching to a suspend terminal is not required. However, if you do run your machine from a virtual terminal, then we indicate to the system that we want to change to the requested virtual terminal via the want_console variable and schedule a callback via schedule_console_callback. drivers/tty/vt/vt.c:315

void schedule_console_callback(void)
 
	schedule_work(&console_work);
 

console_work is a workqueue² that will execute the given task asynchronously.

Console Callback drivers/tty/vt/vt.c:3109

/*
 * This is the console switching callback.
 *
 * Doing console switching in a process context allows
 * us to do the switches asynchronously (needed when we want
 * to switch due to a keyboard interrupt).  Synchronization
 * with other console code and prevention of re-entrancy is
 * ensured with console_lock.
 */
static void console_callback(struct work_struct *ignored)
 
	console_lock();
	if (want_console >= 0)  
		if (want_console != fg_console &&
		    vc_cons_allocated(want_console))  
			hide_cursor(vc_cons[fg_console].d);
			change_console(vc_cons[want_console].d);
			/* we only changed when the console had already
			   been allocated - a new console is not created
			   in an interrupt routine */
		 
		want_console = -1;
	 
...

console_callback first looks to see if there is a console change wanted via want_console and then changes to it if it s not the current console and has been allocated already. We do first remove any cursor state with hide_cursor. drivers/tty/vt/vt.c:841

static void hide_cursor(struct vc_data *vc)
 
	if (vc_is_sel(vc))
		clear_selection();
	vc->vc_sw->con_cursor(vc, false);
	hide_softcursor(vc);
 

A full dive into the tty driver is a task for another time, but this should give a general sense of how this system interacts with hibernation.

Notify Power Management Call Chain kernel/power/hibernate.c:767

pm_notifier_call_chain_robust(PM_HIBERNATION_PREPARE, PM_POST_HIBERNATION)

This will call a chain of power management callbacks, passing first PM_HIBERNATION_PREPARE and then PM_POST_HIBERNATION on startup or on error with another callback. kernel/power/main.c:98

int pm_notifier_call_chain_robust(unsigned long val_up, unsigned long val_down)
 
	int ret;
	ret = blocking_notifier_call_chain_robust(&pm_chain_head, val_up, val_down, NULL);
	return notifier_to_errno(ret);
 

The power management notifier is a blocking notifier chain, which means it has the following properties. include/linux/notifier.h:23

 *	Blocking notifier chains: Chain callbacks run in process context.
 *		Callouts are allowed to block.

The callback chain is a linked list with each entry containing a priority and a function to call. The function technically takes in a data value, but it is always NULL for the power management chain. include/linux/notifier.h:49

struct notifier_block;
typedef	int (*notifier_fn_t)(struct notifier_block *nb,
			unsigned long action, void *data);
struct notifier_block  
	notifier_fn_t notifier_call;
	struct notifier_block __rcu *next;
	int priority;
 ;

The head of the linked list is protected by a read-write semaphore. include/linux/notifier.h:65

struct blocking_notifier_head  
	struct rw_semaphore rwsem;
	struct notifier_block __rcu *head;
 ;

Because it is prioritized, appending to the list requires walking it until an item with lower¹³ priority is found to insert the current item before. kernel/notifier.c:252

/*
 *	Blocking notifier chain routines.  All access to the chain is
 *	synchronized by an rwsem.
 */
static int __blocking_notifier_chain_register(struct blocking_notifier_head *nh,
					      struct notifier_block *n,
					      bool unique_priority)
 
	int ret;
	/*
	 * This code gets used during boot-up, when task switching is
	 * not yet working and interrupts must remain disabled.  At
	 * such times we must not call down_write().
	 */
	if (unlikely(system_state == SYSTEM_BOOTING))
		return notifier_chain_register(&nh->head, n, unique_priority);
	down_write(&nh->rwsem);
	ret = notifier_chain_register(&nh->head, n, unique_priority);
	up_write(&nh->rwsem);
	return ret;
 

kernel/notifier.c:20

/*
 *	Notifier chain core routines.  The exported routines below
 *	are layered on top of these, with appropriate locking added.
 */
static int notifier_chain_register(struct notifier_block **nl,
				   struct notifier_block *n,
				   bool unique_priority)
 
	while ((*nl) != NULL)  
		if (unlikely((*nl) == n))  
			WARN(1, "notifier callback %ps already registered",
			     n->notifier_call);
			return -EEXIST;
		 
		if (n->priority > (*nl)->priority)
			break;
		if (n->priority == (*nl)->priority && unique_priority)
			return -EBUSY;
		nl = &((*nl)->next);
	 
	n->next = *nl;
	rcu_assign_pointer(*nl, n);
	trace_notifier_register((void *)n->notifier_call);
	return 0;
 

Each callback can return one of a series of options. include/linux/notifier.h:18

#define NOTIFY_DONE		0x0000		/* Don't care */
#define NOTIFY_OK		0x0001		/* Suits me */
#define NOTIFY_STOP_MASK	0x8000		/* Don't call further */
#define NOTIFY_BAD		(NOTIFY_STOP_MASK 0x0002)
						/* Bad/Veto action */

When notifying the chain, if a function returns STOP or BAD then the previous parts of the chain are called again with PM_POST_HIBERNATION¹⁴ and an error is returned. kernel/notifier.c:107

/**
 * notifier_call_chain_robust - Inform the registered notifiers about an event
 *                              and rollback on error.
 * @nl:		Pointer to head of the blocking notifier chain
 * @val_up:	Value passed unmodified to the notifier function
 * @val_down:	Value passed unmodified to the notifier function when recovering
 *              from an error on @val_up
 * @v:		Pointer passed unmodified to the notifier function
 *
 * NOTE:	It is important the @nl chain doesn't change between the two
 *		invocations of notifier_call_chain() such that we visit the
 *		exact same notifier callbacks; this rules out any RCU usage.
 *
 * Return:	the return value of the @val_up call.
 */
static int notifier_call_chain_robust(struct notifier_block **nl,
				     unsigned long val_up, unsigned long val_down,
				     void *v)
 
	int ret, nr = 0;
	ret = notifier_call_chain(nl, val_up, v, -1, &nr);
	if (ret & NOTIFY_STOP_MASK)
		notifier_call_chain(nl, val_down, v, nr-1, NULL);
	return ret;
 

Each of these callbacks tends to be quite driver-specific, so we ll cease discussion of this here.

Sync Filesystems The next step is to ensure all filesystems have been synchronized to disk. This is performed via a simple helper function that times how long the full synchronize operation, ksys_sync takes. kernel/power/main.c:69

void ksys_sync_helper(void)
 
	ktime_t start;
	long elapsed_msecs;
	start = ktime_get();
	ksys_sync();
	elapsed_msecs = ktime_to_ms(ktime_sub(ktime_get(), start));
	pr_info("Filesystems sync: %ld.%03ld seconds\n",
		elapsed_msecs / MSEC_PER_SEC, elapsed_msecs % MSEC_PER_SEC);
 
EXPORT_SYMBOL_GPL(ksys_sync_helper);

ksys_sync wakes and instructs a set of flusher threads to write out every filesystem, first their inodes¹⁵, then the full filesystem, and then finally all block devices, to ensure all pages are written out to disk. fs/sync.c:87

/*
 * Sync everything. We start by waking flusher threads so that most of
 * writeback runs on all devices in parallel. Then we sync all inodes reliably
 * which effectively also waits for all flusher threads to finish doing
 * writeback. At this point all data is on disk so metadata should be stable
 * and we tell filesystems to sync their metadata via ->sync_fs() calls.
 * Finally, we writeout all block devices because some filesystems (e.g. ext2)
 * just write metadata (such as inodes or bitmaps) to block device page cache
 * and do not sync it on their own in ->sync_fs().
 */
void ksys_sync(void)
 
	int nowait = 0, wait = 1;
	wakeup_flusher_threads(WB_REASON_SYNC);
	iterate_supers(sync_inodes_one_sb, NULL);
	iterate_supers(sync_fs_one_sb, &nowait);
	iterate_supers(sync_fs_one_sb, &wait);
	sync_bdevs(false);
	sync_bdevs(true);
	if (unlikely(laptop_mode))
		laptop_sync_completion();
 

It follows an interesting pattern of using iterate_supers to run both sync_inodes_one_sb and then sync_fs_one_sb on each known filesystem¹⁶. It also calls both sync_fs_one_sb and sync_bdevs twice, first without waiting for any operations to complete and then again waiting for completion¹⁷. When laptop_mode is enabled the system runs additional filesystem synchronization operations after the specified delay without any writes. mm/page-writeback.c:111

/*
 * Flag that puts the machine in "laptop mode". Doubles as a timeout in jiffies:
 * a full sync is triggered after this time elapses without any disk activity.
 */
int laptop_mode;
EXPORT_SYMBOL(laptop_mode);

However, when running a filesystem synchronization operation, the system will add an additional timer to schedule more writes after the laptop_mode delay. We don t want the state of the system to change at all while performing hibernation, so we cancel those timers. mm/page-writeback.c:2198

/*
 * We're in laptop mode and we've just synced. The sync's writes will have
 * caused another writeback to be scheduled by laptop_io_completion.
 * Nothing needs to be written back anymore, so we unschedule the writeback.
 */
void laptop_sync_completion(void)
 
	struct backing_dev_info *bdi;
	rcu_read_lock();
	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list)
		del_timer(&bdi->laptop_mode_wb_timer);
	rcu_read_unlock();
 

As a side note, the ksys_sync function is simply called when the system call sync is used. fs/sync.c:111

SYSCALL_DEFINE0(sync)
 
	ksys_sync();
	return 0;
 

The End of Preparation With that the system has finished preparations for hibernation. This is a somewhat arbitrary cutoff, but next the system will begin a full freeze of userspace to then dump memory out to an image and finally to perform hibernation. All this will be covered in future articles!

1. Hibernation modes are outside of scope for this article, see the previous article for a high-level description of the different types of hibernation.
2. Workqueues are a mechanism for running asynchronous tasks. A full description of them is a task for another time, but the kernel documentation on them is available here: https://www.kernel.org/doc/html/v6.9/core-api/workqueue.html ²
3. This is a bit of an oversimplification, but since this isn t the main focus of this article this description has been kept to a higher level.
4. Kconfig is Linux s build configuration system that sets many different macros to enable/disable various features.
5. Kconfig defaults to the first default found
6. Including checking whether the algorithm is larval? Which appears to indicate that it requires additional setup, but is an interesting choice of name for such a state.
7. Specifically when we get to process freezing, which we ll get to in the next article in this series.
8. Swap space is outside the scope of this article, but in short it is a buffer on disk that the kernel uses to store memory not current in use to free up space for other things. See Swap Management for more details.
9. The code for this is lengthy and tangential, thus it has not been included here. If you re curious about the details of this, see kernel/power/hibernate.c:858 for the details of hibernate_quiet_exec, and drivers/nvdimm/core.c:451 for how it is used in nvdimm.
10. Annoyingly this code appears to use the terms console and virtual terminal interchangeably.
11. ioctls are special device-specific I/O operations that permit performing actions outside of the standard file interactions of read/write/seek/etc. ²
12. I m not entirely clear on how this flag works, this subsystem is particularly complex.
13. In this case a higher number is higher priority.
14. Or whatever the caller passes as val_down, but in this case we re specifically looking at how this is used in hibernation.
15. An inode refers to a particular file or directory within the filesystem. See Wikipedia for more details.
16. Each active filesystem is registed with the kernel through a structure known as a superblock, which contains references to all the inodes contained within the filesystem, as well as function pointers to perform the various required operations, like sync.
17. I m including minimal code in this section, as I m not looking to deep dive into the filesystem code at this time.

• Rooms had heated floors. It seems to snow in Busan.
• Each area was had card based access. There was a separate card for laundry too.
• Rooms had announcement systems right inside the room, though we couldn t decipher any announcement as all of them were in Korean.
• Each room was provided with a dedicated access point and own SSID inside the room.

• Convenience stores were everywhere, one could see same brand stores less than a kilometer apart. We had even had two of them (GS25(s)), a road cross away too. These places were well stocked with almost everything, even alcohol.
• There were wide footpaths and pedestrian friendly policies.
• Public transport and intra modal transfer is convenient and easy to figure. Each metro station connects to multiple nearby buildings through underground walkways, and one never had to go out in the sun (in hot and humid weather). Also, Seoul and Busan metro networks were massive. The Same T money card worked for buses (almost hop on, tap and hop off at your destination), metros and even cabs.
• South Korea pays special attention to maintaining their historical and cultural buildings. These venues had informational brochures in Korea, English, Japanese and Chinese.
• We got a constant stream of Public safety alerts on our phones. Some phones even read them aloud for heatwaves and rains warnings, all in Korean.
• Trash was segregated at source everywhere.
• Public, high speed Wi-Fi was omnipresent in malls, public transport, airport etc. In metro, each coach had access points from all three telecom providers (SK Telecom, KT and LG U+) which also had almost similar voice and data plans.
• Police personals were quite helpful despite the language issue.
• Not many folks here are comfortable in English, but one can always make use of various mobile translation apps.
• Cards are accepted everywhere and there are too many of these cards ;)
• Food situation was a bit difficult for me as a vegetarian. We always have vegan/veg food in DebConf but outside, this whole concept doesn t seem to exist here.
• I couldn t find any public speedtest servers inside Korea. All my fast.com/speedtest.net servers were located either Hong Kong, Singapore, Japan and even in the United States. On the very last day, I got a speedtest servers in Seoul, inside SK Telecom.

• [1] https://tinyurl.com/ytwvpary
• [2] https://www.davidbrin.com/nonfiction/immortality.html
• [3] https://tinyurl.com/26czjpjl
• [4] https://sams-blog.com/?p=5615
• [5] https://worksinprogress.co/issue/the-maintenance-race/
• [6] https://www.youtube.com/watch?v=NhajAqI66nU
• [7] https://hashman.ca/me-cfs/
• [8] https://tinyurl.com/29b4h5oz
• [9] https://tinyurl.com/24sz7lwa
• [10] https://tinyurl.com/2c6dx8d4
• [11] https://tinyurl.com/26e3xxmn
• [12] https://tinyurl.com/26ykzgn4
• [13] https://www.youtube.com/watch?v=5V6Pez_Dbzs
• [14] https://tinyurl.com/27h5urv9

BGP routing loop using a confederation

router bgp 64502
 bgp confederation peers
  64500
  64501
  64503
 !
 bgp confederation identifier 64496
 bgp router-id 1.0.0.2
 address-family ipv6 unicast
 !
 neighbor 2001:db8::2:0
  remote-as 64501
  description R1
  address-family ipv6 unicast
  !
 !
 neighbor 2001:db8::3:1
  remote-as 64503
  advertisement-interval 0
  description R3
  address-family ipv6 unicast
   next-hop-self
   as-override
  !
 !
!

1. R0 sends the prefix to R1 with AS path (64500).³
2. R1 selects it as the best path, forwarding it to R2 with AS path (64501 64500).
3. R2 selects it as the best path, forwarding it to R3 with AS path (64500 64501 64502).
4. R3 selects it as the best path. It would forward it to R1 with AS path (64503 64502 64501 64500), but due to AS override, it substitutes R1 s ASN with its own, forwarding it with AS path (64503 64502 64503 64500).
5. R1 accepts the prefix, as its own ASN is not in the AS path. It compares this new prefix with the one from R0. Both (64500) and (64503 64502 64503 64500) have the same length because confederation sub-ASes don t contribute to AS path length. The first tie-breaker is the router ID. R0 s router ID (1.0.0.4) is higher than R3 s (1.0.0.3). The new prefix becomes the best path and is forwarded to R2 with AS path (64501 64503 64501 64503 64500).
6. R2 receives the new prefix, replacing the old one. It selects it as the best path and forwards it to R3 with AS path (64502 64501 64502 64501 64502 64500).
7. R3 receives the new prefix, replacing the old one. It selects it as the best path and forwards it to R0 with AS path (64503 64502 64503 64502 64503 64502 64500).
8. R1 receives the new prefix, replacing the old one. Again, it competes with the prefix from R0, and again the new prefix wins due to the lower router ID. The prefix is forwarded to R2 with AS path (64501 64503 64501 64503 64501 64503 64501 64500).

RP/0/RP0/CPU0:R1#show bgp ipv6 u 2001:db8::1/128 bestpath-compare
BGP routing table entry for 2001:db8::1/128
Last Modified: Jul 28 10:23:05.560 for 00:00:00
Paths: (2 available, best #2)
  Path #1: Received by speaker 0
  Not advertised to any peer
  (64500)
    2001:db8::1:0 from 2001:db8::1:0 (1.0.0.4), if-handle 0x00000000
      Origin IGP, metric 0, localpref 100, valid, confed-external
      Received Path ID 0, Local Path ID 0, version 0
      Higher router ID than best path (path #2)
  Path #2: Received by speaker 0
  Advertised IPv6 Unicast paths to peers (in unique update groups):
    2001:db8::2:1
  (64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64503 64502 64500)
    2001:db8::4:0 from 2001:db8::4:0 (1.0.0.3), if-handle 0x00000000
      Origin IGP, metric 0, localpref 100, valid, confed-external, best, group-best
      Received Path ID 0, Local Path ID 1, version 37
      best of AS 64503, Overall best

• never use BGP confederations under any circumstances, and
• be cautious of features that weaken BGP routing loop detection.

• Proxmox is the first Platinum sponsor. Proxmox provides powerful and user-friendly Open Source server software. Enterprises of all sizes and industries use Proxmox solutions to deploy efficient and simplified IT infrastructures, minimize total cost of ownership, and avoid vendor lock-in. Proxmox also offers commercial support, training services, and an extensive partner ecosystem to ensure business continuity for its customers. Proxmox Server Solutions GmbH was established in 2005 and is headquartered in Vienna, Austria. Proxmox builds its product offerings on top of the Debian operating system.
• Our second Platinum sponsor is Infomaniak. Infomaniak is an independent cloud service provider recognised throughout Europe for its commitment to privacy, the local economy and the environment. Recording growth of 18% in 2023, the company is developing a suite of online collaborative tools and cloud hosting, streaming, marketing and events solutions. Infomaniak uses exclusively renewable energy, builds its own data centers and develops its solutions in Switzerland, without relocating. The company powers the website of the Belgian radio and TV service (RTBF) and provides streaming for more than 3,000 TV and radio stations in Europe.
• Wind River is our third Platinum sponsor. For nearly 20 years, Wind River has led in commercial Open Source Linux solutions for mission-critical enterprise edge computing. With expertise across aerospace, automotive, industrial, telecom, and more, the company is committed to Open Source through initiatives like eLxr, Yocto, Zephyr, and StarlingX.

• Ubuntu, the Operating System delivered by Canonical.
• Freexian, a services company specialized in Free Software and in particular Debian GNU/Linux, covering consulting, custom developments, support and training. Freexian has a recognized Debian expertise thanks to the participation of Debian developers.
• Lenovo, a global technology leader manufacturing a wide portfolio of connected products including smartphones, tablets, PCs and workstations as well as AR/VR devices, smart home/office and data center solutions.
• Korea Tourism Organization, which purpose is to advance tourism as a key driver for national economic growth and enhancement of national welfare and intends to be a public organization that makes the Korean people happier; it promotes national wealth through tourism.
• Busan IT Industry Promotion Agency, an industry promotion organization that contributes to the innovation of the digital economy with the power of IT and CT and supports the ecosystem for innovative local startups and companies to grow.
• Microsoft, who enables digital transformation for the era of an intelligent cloud and an intelligent edge. Its mission is to empower every person and every organization on the planet to achieve more.
• doubleO, a company that specializes in consulting and developing empirical services using big data analysis and artificial intelligence. doubleO provides a variety of data-centered services together with small and medium-sized businesses in Busan/Gyeongnam.

• Roche, a major international pharmaceutical provider and research company dedicated to personalized healthcare.
• Two Sigma, rigorous inquiry, technology data science, and invention to bring science to finance and help solve the toughest challenges across financial services.
• Arm: leading technology provider of processor IP, Arm powered solutions have been supporting innovation for more than 30 years and are deployed in over 280 billion chips to date.
• The Bern University of Applied Sciences with around 7,800 students enrolled, located in the Swiss capital.
• Google, one of the largest technology companies in the world, providing a wide range of Internet-related services and products such as online advertising technologies, search, cloud computing, software, and hardware.
• FSIJ, the Free Software Initiative of Japan, a non-profit organization dedicated to supporting Free Software growth and development.
• Busan Tourism Organisation: leading public corporation that generates social and economic values in Busan tourism industry, developing tourism resources in accordance with government policies and invigorate tourism industry.
• Civil Infrastructure Platform, a collaborative project hosted by the Linux Foundation, establishing an open source base layer of industrial grade software.
• Collabora, a global consultancy delivering Open Source software solutions to the commercial world.
• Matanel Foundation, which operates in Israel, as its first concern is to preserve the cohesion of a society and a nation plagued by divisions.

• Altus Metrum,
• Loongson,
• NIPA,
• IPA,

• evolix,
• ISG,
• Linux Professional Institute.

# get display mode
ddcutil getvcp DC
# set standard mode
ddcutil setvcp DC 0
# set movie mode
ddcutil setvcp DC 03

• [1] https://etbe.coker.com.au/2024/05/18/kogan-5120 2160-40-monitor/
• [2] https://en.wikipedia.org/wiki/Anamorphic_format
• [3] https://etbe.coker.com.au/2022/07/19/ddc-switch/

Changes in version 0.1.3 (2024-07-18)

• Windows builds use localtime_s with LLVM (Tomas Kalibera in #16)
• Tests code has been adjusted for an upstream change in data.table tests for all.equal (Michael Chirico in #18 addressing #17)

This post by Dirk Eddelbuettel originated on his Thinking inside the box blog. Please report excessive re-aggregation in third-party for-profit settings.