Documentation/power/freezing-of-tasks.rst

10 The freezing of tasks is a mechanism by which user space processes and some
11 kernel threads are controlled during hibernation or system-wide suspend (on some
17 There are three per-task flags used for that, PF_NOFREEZE, PF_FROZEN
19 PF_NOFREEZE unset (all user space processes and some kernel threads) are
25 freeze_processes() (defined in kernel/power/process.c) is called.  A system-wide
26 variable system_freezing_cnt (as opposed to a per-task flag) is used to indicate
29 fake signal to all user space processes, and wakes up all the kernel threads.
31 results in a call to __refrigerator() (defined in kernel/freezer.c), which sets
35 to as 'the freezer' (these functions are defined in kernel/power/process.c,
36 kernel/freezer.c & include/linux/freezer.h). User space processes are generally
40 try_to_freeze() function (defined in include/linux/freezer.h), that checks
43 For user space processes try_to_freeze() is called automatically from the
44 signal-handling code, but the freezable kernel threads need to call it
46 wait_event_freezable_timeout() macros (defined in include/linux/freezer.h)
74 -------------------------------------------------------------------------
77   - freezes only userspace tasks
80   - freezes all tasks (including kernel threads) because we can't freeze
84   - thaws only kernel threads; this is particularly useful if we need to do
89   - thaws all tasks (including kernel threads) because we can't thaw userspace
111    filesystem-related information that must be consistent with the state of the
112    on-disk data and metadata after the system memory state has been restored
115    tasks that might cause the on-disk filesystems' data and metadata to be
117    system is finally powered off. The majority of these are user space
130 3. The third reason is to prevent user space processes and some kernel threads
131    from interfering with the suspending and resuming of devices.  A user space
146 user threads and then waiting for a sync is practically one of the easier
150 freezing user threads I don't find really objectionable."
160 4. Another reason for freezing tasks is to prevent user space processes from
161    realizing that hibernation (or suspend) operation takes place.  Ideally, user
162    space processes should not notice that such a system-wide operation has
182 Second, there are the following two problems related to the freezing of user
189    (https://lists.linux-foundation.org/pipermail/linux-pm/2007-May/012309.html).
194 avoid the realization by the user space processes that the hibernation is taking
200 fail, because the user land process that should respond to the request is frozen
211 Documentation/driver-api/pm/notifiers.rst.
219 piece of code from system-wide sleep such as suspend/hibernation is not
222 CPU-Hotplug code (kernel/cpu.c) for an example.
244 all user space processes or all freezable kernel threads, in unit of