linux-imx

Freezing of tasks

    (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

kernel threads are controlled during hibernation or system-wide suspend (on some

architectures).

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II. How does it work?

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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

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).

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Namely, as the first step of the hibernation procedure the function

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.

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__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().

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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 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:

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    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));

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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

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.

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After the system memory state has been restored from a hibernation image and

devices have been reinitialized, the function thaw_processes() is called in