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204 lines
11 KiB
Plaintext
204 lines
11 KiB
Plaintext
Freezing of tasks
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(C) 2007 Rafael J. Wysocki <rjw@sisk.pl>, GPL
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I. What is the freezing of tasks?
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The freezing of tasks is a mechanism by which user space processes and some
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kernel threads are controlled during hibernation or system-wide suspend (on some
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architectures).
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II. How does it work?
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There are four per-task flags used for that, PF_NOFREEZE, PF_FROZEN, TIF_FREEZE
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and PF_FREEZER_SKIP (the last one is auxiliary). The tasks that have
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PF_NOFREEZE unset (all user space processes and some kernel threads) are
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regarded as 'freezable' and treated in a special way before the system enters a
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suspend state as well as before a hibernation image is created (in what follows
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we only consider hibernation, but the description also applies to suspend).
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Namely, as the first step of the hibernation procedure the function
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freeze_processes() (defined in kernel/power/process.c) is called. It executes
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try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and
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either wakes them up, if they are kernel threads, or sends fake signals to them,
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if they are user space processes. A task that has TIF_FREEZE set, should react
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to it by calling the function called __refrigerator() (defined in
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kernel/freezer.c), which sets the task's PF_FROZEN flag, changes its state
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to TASK_UNINTERRUPTIBLE and makes it loop until PF_FROZEN is cleared for it.
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Then, we say that the task is 'frozen' and therefore the set of functions
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handling this mechanism is referred to as 'the freezer' (these functions are
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defined in kernel/power/process.c, kernel/freezer.c & include/linux/freezer.h).
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User space processes are generally frozen before kernel threads.
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__refrigerator() must not be called directly. Instead, use the
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try_to_freeze() function (defined in include/linux/freezer.h), that checks
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the task's TIF_FREEZE flag and makes the task enter __refrigerator() if the
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flag is set.
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For user space processes try_to_freeze() is called automatically from the
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signal-handling code, but the freezable kernel threads need to call it
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explicitly in suitable places or use the wait_event_freezable() or
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wait_event_freezable_timeout() macros (defined in include/linux/freezer.h)
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that combine interruptible sleep with checking if TIF_FREEZE is set and calling
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try_to_freeze(). The main loop of a freezable kernel thread may look like the
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following one:
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set_freezable();
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do {
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hub_events();
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wait_event_freezable(khubd_wait,
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!list_empty(&hub_event_list) ||
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kthread_should_stop());
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} while (!kthread_should_stop() || !list_empty(&hub_event_list));
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(from drivers/usb/core/hub.c::hub_thread()).
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If a freezable kernel thread fails to call try_to_freeze() after the freezer has
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set TIF_FREEZE for it, the freezing of tasks will fail and the entire
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hibernation operation will be cancelled. For this reason, freezable kernel
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threads must call try_to_freeze() somewhere or use one of the
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wait_event_freezable() and wait_event_freezable_timeout() macros.
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After the system memory state has been restored from a hibernation image and
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devices have been reinitialized, the function thaw_processes() is called in
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order to clear the PF_FROZEN flag for each frozen task. Then, the tasks that
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have been frozen leave __refrigerator() and continue running.
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III. Which kernel threads are freezable?
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Kernel threads are not freezable by default. However, a kernel thread may clear
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PF_NOFREEZE for itself by calling set_freezable() (the resetting of PF_NOFREEZE
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directly is not allowed). From this point it is regarded as freezable
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and must call try_to_freeze() in a suitable place.
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IV. Why do we do that?
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Generally speaking, there is a couple of reasons to use the freezing of tasks:
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1. The principal reason is to prevent filesystems from being damaged after
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hibernation. At the moment we have no simple means of checkpointing
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filesystems, so if there are any modifications made to filesystem data and/or
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metadata on disks, we cannot bring them back to the state from before the
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modifications. At the same time each hibernation image contains some
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filesystem-related information that must be consistent with the state of the
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on-disk data and metadata after the system memory state has been restored from
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the image (otherwise the filesystems will be damaged in a nasty way, usually
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making them almost impossible to repair). We therefore freeze tasks that might
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cause the on-disk filesystems' data and metadata to be modified after the
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hibernation image has been created and before the system is finally powered off.
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The majority of these are user space processes, but if any of the kernel threads
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may cause something like this to happen, they have to be freezable.
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2. Next, to create the hibernation image we need to free a sufficient amount of
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memory (approximately 50% of available RAM) and we need to do that before
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devices are deactivated, because we generally need them for swapping out. Then,
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after the memory for the image has been freed, we don't want tasks to allocate
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additional memory and we prevent them from doing that by freezing them earlier.
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[Of course, this also means that device drivers should not allocate substantial
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amounts of memory from their .suspend() callbacks before hibernation, but this
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is a separate issue.]
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3. The third reason is to prevent user space processes and some kernel threads
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from interfering with the suspending and resuming of devices. A user space
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process running on a second CPU while we are suspending devices may, for
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example, be troublesome and without the freezing of tasks we would need some
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safeguards against race conditions that might occur in such a case.
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Although Linus Torvalds doesn't like the freezing of tasks, he said this in one
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of the discussions on LKML (http://lkml.org/lkml/2007/4/27/608):
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"RJW:> Why we freeze tasks at all or why we freeze kernel threads?
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Linus: In many ways, 'at all'.
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I _do_ realize the IO request queue issues, and that we cannot actually do
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s2ram with some devices in the middle of a DMA. So we want to be able to
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avoid *that*, there's no question about that. And I suspect that stopping
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user threads and then waiting for a sync is practically one of the easier
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ways to do so.
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So in practice, the 'at all' may become a 'why freeze kernel threads?' and
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freezing user threads I don't find really objectionable."
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Still, there are kernel threads that may want to be freezable. For example, if
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a kernel thread that belongs to a device driver accesses the device directly, it
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in principle needs to know when the device is suspended, so that it doesn't try
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to access it at that time. However, if the kernel thread is freezable, it will
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be frozen before the driver's .suspend() callback is executed and it will be
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thawed after the driver's .resume() callback has run, so it won't be accessing
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the device while it's suspended.
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4. Another reason for freezing tasks is to prevent user space processes from
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realizing that hibernation (or suspend) operation takes place. Ideally, user
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space processes should not notice that such a system-wide operation has occurred
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and should continue running without any problems after the restore (or resume
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from suspend). Unfortunately, in the most general case this is quite difficult
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to achieve without the freezing of tasks. Consider, for example, a process
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that depends on all CPUs being online while it's running. Since we need to
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disable nonboot CPUs during the hibernation, if this process is not frozen, it
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may notice that the number of CPUs has changed and may start to work incorrectly
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because of that.
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V. Are there any problems related to the freezing of tasks?
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Yes, there are.
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First of all, the freezing of kernel threads may be tricky if they depend one
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on another. For example, if kernel thread A waits for a completion (in the
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TASK_UNINTERRUPTIBLE state) that needs to be done by freezable kernel thread B
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and B is frozen in the meantime, then A will be blocked until B is thawed, which
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may be undesirable. That's why kernel threads are not freezable by default.
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Second, there are the following two problems related to the freezing of user
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space processes:
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1. Putting processes into an uninterruptible sleep distorts the load average.
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2. Now that we have FUSE, plus the framework for doing device drivers in
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userspace, it gets even more complicated because some userspace processes are
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now doing the sorts of things that kernel threads do
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(https://lists.linux-foundation.org/pipermail/linux-pm/2007-May/012309.html).
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The problem 1. seems to be fixable, although it hasn't been fixed so far. The
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other one is more serious, but it seems that we can work around it by using
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hibernation (and suspend) notifiers (in that case, though, we won't be able to
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avoid the realization by the user space processes that the hibernation is taking
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place).
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There are also problems that the freezing of tasks tends to expose, although
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they are not directly related to it. For example, if request_firmware() is
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called from a device driver's .resume() routine, it will timeout and eventually
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fail, because the user land process that should respond to the request is frozen
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at this point. So, seemingly, the failure is due to the freezing of tasks.
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Suppose, however, that the firmware file is located on a filesystem accessible
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only through another device that hasn't been resumed yet. In that case,
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request_firmware() will fail regardless of whether or not the freezing of tasks
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is used. Consequently, the problem is not really related to the freezing of
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tasks, since it generally exists anyway.
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A driver must have all firmwares it may need in RAM before suspend() is called.
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If keeping them is not practical, for example due to their size, they must be
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requested early enough using the suspend notifier API described in notifiers.txt.
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VI. Are there any precautions to be taken to prevent freezing failures?
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Yes, there are.
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First of all, grabbing the 'pm_mutex' lock to mutually exclude a piece of code
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from system-wide sleep such as suspend/hibernation is not encouraged.
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If possible, that piece of code must instead hook onto the suspend/hibernation
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notifiers to achieve mutual exclusion. Look at the CPU-Hotplug code
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(kernel/cpu.c) for an example.
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However, if that is not feasible, and grabbing 'pm_mutex' is deemed necessary,
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it is strongly discouraged to directly call mutex_[un]lock(&pm_mutex) since
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that could lead to freezing failures, because if the suspend/hibernate code
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successfully acquired the 'pm_mutex' lock, and hence that other entity failed
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to acquire the lock, then that task would get blocked in TASK_UNINTERRUPTIBLE
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state. As a consequence, the freezer would not be able to freeze that task,
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leading to freezing failure.
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However, the [un]lock_system_sleep() APIs are safe to use in this scenario,
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since they ask the freezer to skip freezing this task, since it is anyway
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"frozen enough" as it is blocked on 'pm_mutex', which will be released
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only after the entire suspend/hibernation sequence is complete.
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So, to summarize, use [un]lock_system_sleep() instead of directly using
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mutex_[un]lock(&pm_mutex). That would prevent freezing failures.
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