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Diffstat (limited to 'Documentation/power/freezing-of-tasks.txt')
| -rw-r--r-- | Documentation/power/freezing-of-tasks.txt | 108 |
1 files changed, 80 insertions, 28 deletions
diff --git a/Documentation/power/freezing-of-tasks.txt b/Documentation/power/freezing-of-tasks.txt index 38b57248fd6..85894d83b35 100644 --- a/Documentation/power/freezing-of-tasks.txt +++ b/Documentation/power/freezing-of-tasks.txt @@ -9,7 +9,7 @@ architectures). II. How does it work? -There are four per-task flags used for that, PF_NOFREEZE, PF_FROZEN, TIF_FREEZE +There are three per-task flags used for that, PF_NOFREEZE, PF_FROZEN and PF_FREEZER_SKIP (the last one is auxiliary). The tasks that have PF_NOFREEZE unset (all user space processes and some kernel threads) are regarded as 'freezable' and treated in a special way before the system enters a @@ -17,30 +17,31 @@ suspend state as well as before a hibernation image is created (in what follows we only consider hibernation, but the description also applies to suspend). Namely, as the first step of the hibernation procedure the function -freeze_processes() (defined in kernel/power/process.c) is called. It executes -try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and -either wakes them up, if they are kernel threads, or sends fake signals to them, -if they are user space processes. A task that has TIF_FREEZE set, should react -to it by calling the function called refrigerator() (defined in -kernel/power/process.c), which sets the task's PF_FROZEN flag, changes its state -to TASK_UNINTERRUPTIBLE and makes it loop until PF_FROZEN is cleared for it. -Then, we say that the task is 'frozen' and therefore the set of functions -handling this mechanism is referred to as 'the freezer' (these functions are -defined in kernel/power/process.c and include/linux/freezer.h). User space -processes are generally frozen before kernel threads. - -It is not recommended to call refrigerator() directly. Instead, it is -recommended to use the try_to_freeze() function (defined in -include/linux/freezer.h), that checks the task's TIF_FREEZE flag and makes the -task enter refrigerator() if the flag is set. +freeze_processes() (defined in kernel/power/process.c) is called. A system-wide +variable system_freezing_cnt (as opposed to a per-task flag) is used to indicate +whether the system is to undergo a freezing operation. And freeze_processes() +sets this variable. After this, it executes try_to_freeze_tasks() that sends a +fake signal to all user space processes, and wakes up all the kernel threads. +All freezable tasks must react to that by calling try_to_freeze(), which +results in a call to __refrigerator() (defined in kernel/freezer.c), which sets +the task's PF_FROZEN flag, changes its state to TASK_UNINTERRUPTIBLE and makes +it loop until PF_FROZEN is cleared for it. Then, we say that the task is +'frozen' and therefore the set of functions handling this mechanism is referred +to as 'the freezer' (these functions are defined in kernel/power/process.c, +kernel/freezer.c & include/linux/freezer.h). User space processes are generally +frozen before kernel threads. + +__refrigerator() must not be called directly. Instead, use the +try_to_freeze() function (defined in include/linux/freezer.h), that checks +if the task is to be frozen and makes the task enter __refrigerator(). For user space processes try_to_freeze() is called automatically from the signal-handling code, but the freezable kernel threads need to call it explicitly in suitable places or use the wait_event_freezable() or wait_event_freezable_timeout() macros (defined in include/linux/freezer.h) -that combine interruptible sleep with checking if TIF_FREEZE is set and calling -try_to_freeze(). The main loop of a freezable kernel thread may look like the -following one: +that combine interruptible sleep with checking if the task is to be frozen and +calling try_to_freeze(). The main loop of a freezable kernel thread may look +like the following one: set_freezable(); do { @@ -53,7 +54,7 @@ following one: (from drivers/usb/core/hub.c::hub_thread()). If a freezable kernel thread fails to call try_to_freeze() after the freezer has -set TIF_FREEZE for it, the freezing of tasks will fail and the entire +initiated a freezing operation, the freezing of tasks will fail and the entire hibernation operation will be cancelled. For this reason, freezable kernel threads must call try_to_freeze() somewhere or use one of the wait_event_freezable() and wait_event_freezable_timeout() macros. @@ -61,13 +62,34 @@ wait_event_freezable() and wait_event_freezable_timeout() macros. After the system memory state has been restored from a hibernation image and devices have been reinitialized, the function thaw_processes() is called in order to clear the PF_FROZEN flag for each frozen task. Then, the tasks that -have been frozen leave refrigerator() and continue running. +have been frozen leave __refrigerator() and continue running. + + +Rationale behind the functions dealing with freezing and thawing of tasks: +------------------------------------------------------------------------- + +freeze_processes(): + - freezes only userspace tasks + +freeze_kernel_threads(): + - freezes all tasks (including kernel threads) because we can't freeze + kernel threads without freezing userspace tasks + +thaw_kernel_threads(): + - thaws only kernel threads; this is particularly useful if we need to do + anything special in between thawing of kernel threads and thawing of + userspace tasks, or if we want to postpone the thawing of userspace tasks + +thaw_processes(): + - thaws all tasks (including kernel threads) because we can't thaw userspace + tasks without thawing kernel threads + III. Which kernel threads are freezable? Kernel threads are not freezable by default. However, a kernel thread may clear PF_NOFREEZE for itself by calling set_freezable() (the resetting of PF_NOFREEZE -directly is strongly discouraged). From this point it is regarded as freezable +directly is not allowed). From this point it is regarded as freezable and must call try_to_freeze() in a suitable place. IV. Why do we do that? @@ -95,7 +117,7 @@ after the memory for the image has been freed, we don't want tasks to allocate additional memory and we prevent them from doing that by freezing them earlier. [Of course, this also means that device drivers should not allocate substantial amounts of memory from their .suspend() callbacks before hibernation, but this -is e separate issue.] +is a separate issue.] 3. The third reason is to prevent user space processes and some kernel threads from interfering with the suspending and resuming of devices. A user space @@ -120,10 +142,10 @@ So in practice, the 'at all' may become a 'why freeze kernel threads?' and freezing user threads I don't find really objectionable." Still, there are kernel threads that may want to be freezable. For example, if -a kernel that belongs to a device driver accesses the device directly, it in -principle needs to know when the device is suspended, so that it doesn't try to -access it at that time. However, if the kernel thread is freezable, it will be -frozen before the driver's .suspend() callback is executed and it will be +a kernel thread that belongs to a device driver accesses the device directly, it +in principle needs to know when the device is suspended, so that it doesn't try +to access it at that time. However, if the kernel thread is freezable, it will +be frozen before the driver's .suspend() callback is executed and it will be thawed after the driver's .resume() callback has run, so it won't be accessing the device while it's suspended. @@ -176,3 +198,33 @@ tasks, since it generally exists anyway. A driver must have all firmwares it may need in RAM before suspend() is called. If keeping them is not practical, for example due to their size, they must be requested early enough using the suspend notifier API described in notifiers.txt. + +VI. Are there any precautions to be taken to prevent freezing failures? + +Yes, there are. + +First of all, grabbing the 'pm_mutex' lock to mutually exclude a piece of code +from system-wide sleep such as suspend/hibernation is not encouraged. +If possible, that piece of code must instead hook onto the suspend/hibernation +notifiers to achieve mutual exclusion. Look at the CPU-Hotplug code +(kernel/cpu.c) for an example. + +However, if that is not feasible, and grabbing 'pm_mutex' is deemed necessary, +it is strongly discouraged to directly call mutex_[un]lock(&pm_mutex) since +that could lead to freezing failures, because if the suspend/hibernate code +successfully acquired the 'pm_mutex' lock, and hence that other entity failed +to acquire the lock, then that task would get blocked in TASK_UNINTERRUPTIBLE +state. As a consequence, the freezer would not be able to freeze that task, +leading to freezing failure. + +However, the [un]lock_system_sleep() APIs are safe to use in this scenario, +since they ask the freezer to skip freezing this task, since it is anyway +"frozen enough" as it is blocked on 'pm_mutex', which will be released +only after the entire suspend/hibernation sequence is complete. +So, to summarize, use [un]lock_system_sleep() instead of directly using +mutex_[un]lock(&pm_mutex). That would prevent freezing failures. + +V. Miscellaneous +/sys/power/pm_freeze_timeout controls how long it will cost at most to freeze +all user space processes or all freezable kernel threads, in unit of millisecond. +The default value is 20000, with range of unsigned integer. |