From 849369d6c66d3054688672f97d31fceb8e8230fb Mon Sep 17 00:00:00 2001 From: root Date: Fri, 25 Dec 2015 04:40:36 +0000 Subject: initial_commit --- Documentation/power/freezing-of-tasks.txt | 178 ++++++++++++++++++++++++++++++ 1 file changed, 178 insertions(+) create mode 100644 Documentation/power/freezing-of-tasks.txt (limited to 'Documentation/power/freezing-of-tasks.txt') diff --git a/Documentation/power/freezing-of-tasks.txt b/Documentation/power/freezing-of-tasks.txt new file mode 100644 index 00000000..38b57248 --- /dev/null +++ b/Documentation/power/freezing-of-tasks.txt @@ -0,0 +1,178 @@ +Freezing of tasks + (C) 2007 Rafael J. Wysocki , GPL + +I. What is the freezing of tasks? + +The freezing of tasks is a mechanism by which user space processes and some +kernel threads are controlled during hibernation or system-wide suspend (on some +architectures). + +II. How does it work? + +There are four per-task flags used for that, PF_NOFREEZE, PF_FROZEN, TIF_FREEZE +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 +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. + +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: + + set_freezable(); + do { + hub_events(); + wait_event_freezable(khubd_wait, + !list_empty(&hub_event_list) || + kthread_should_stop()); + } while (!kthread_should_stop() || !list_empty(&hub_event_list)); + +(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 +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. + +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. + +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 +and must call try_to_freeze() in a suitable place. + +IV. Why do we do that? + +Generally speaking, there is a couple of reasons to use the freezing of tasks: + +1. The principal reason is to prevent filesystems from being damaged after +hibernation. At the moment we have no simple means of checkpointing +filesystems, so if there are any modifications made to filesystem data and/or +metadata on disks, we cannot bring them back to the state from before the +modifications. At the same time each hibernation image contains some +filesystem-related information that must be consistent with the state of the +on-disk data and metadata after the system memory state has been restored from +the image (otherwise the filesystems will be damaged in a nasty way, usually +making them almost impossible to repair). We therefore freeze tasks that might +cause the on-disk filesystems' data and metadata to be modified after the +hibernation image has been created and before the system is finally powered off. +The majority of these are user space processes, but if any of the kernel threads +may cause something like this to happen, they have to be freezable. + +2. Next, to create the hibernation image we need to free a sufficient amount of +memory (approximately 50% of available RAM) and we need to do that before +devices are deactivated, because we generally need them for swapping out. Then, +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.] + +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 +process running on a second CPU while we are suspending devices may, for +example, be troublesome and without the freezing of tasks we would need some +safeguards against race conditions that might occur in such a case. + +Although Linus Torvalds doesn't like the freezing of tasks, he said this in one +of the discussions on LKML (http://lkml.org/lkml/2007/4/27/608): + +"RJW:> Why we freeze tasks at all or why we freeze kernel threads? + +Linus: In many ways, 'at all'. + +I _do_ realize the IO request queue issues, and that we cannot actually do +s2ram with some devices in the middle of a DMA. So we want to be able to +avoid *that*, there's no question about that. And I suspect that stopping +user threads and then waiting for a sync is practically one of the easier +ways to do so. + +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 +thawed after the driver's .resume() callback has run, so it won't be accessing +the device while it's suspended. + +4. Another reason for freezing tasks is to prevent user space processes from +realizing that hibernation (or suspend) operation takes place. Ideally, user +space processes should not notice that such a system-wide operation has occurred +and should continue running without any problems after the restore (or resume +from suspend). Unfortunately, in the most general case this is quite difficult +to achieve without the freezing of tasks. Consider, for example, a process +that depends on all CPUs being online while it's running. Since we need to +disable nonboot CPUs during the hibernation, if this process is not frozen, it +may notice that the number of CPUs has changed and may start to work incorrectly +because of that. + +V. Are there any problems related to the freezing of tasks? + +Yes, there are. + +First of all, the freezing of kernel threads may be tricky if they depend one +on another. For example, if kernel thread A waits for a completion (in the +TASK_UNINTERRUPTIBLE state) that needs to be done by freezable kernel thread B +and B is frozen in the meantime, then A will be blocked until B is thawed, which +may be undesirable. That's why kernel threads are not freezable by default. + +Second, there are the following two problems related to the freezing of user +space processes: +1. Putting processes into an uninterruptible sleep distorts the load average. +2. Now that we have FUSE, plus the framework for doing device drivers in +userspace, it gets even more complicated because some userspace processes are +now doing the sorts of things that kernel threads do +(https://lists.linux-foundation.org/pipermail/linux-pm/2007-May/012309.html). + +The problem 1. seems to be fixable, although it hasn't been fixed so far. The +other one is more serious, but it seems that we can work around it by using +hibernation (and suspend) notifiers (in that case, though, we won't be able to +avoid the realization by the user space processes that the hibernation is taking +place). + +There are also problems that the freezing of tasks tends to expose, although +they are not directly related to it. For example, if request_firmware() is +called from a device driver's .resume() routine, it will timeout and eventually +fail, because the user land process that should respond to the request is frozen +at this point. So, seemingly, the failure is due to the freezing of tasks. +Suppose, however, that the firmware file is located on a filesystem accessible +only through another device that hasn't been resumed yet. In that case, +request_firmware() will fail regardless of whether or not the freezing of tasks +is used. Consequently, the problem is not really related to the freezing of +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. -- cgit v1.2.3