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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 is one per-task flag (PF_NOFREEZE) and three per-task states
19 The tasks that have PF_NOFREEZE unset (all user space tasks and some kernel
23 to system-wide suspend too).
26 freeze_processes() (defined in kernel/power/process.c) is called. A system-wide
27 static key freezer_active (as opposed to a per-task flag or state) is used to
30 try_to_freeze_tasks() that sends a fake signal to all user space processes, and
32 calling try_to_freeze(), which results in a call to __refrigerator() (defined
36 referred to as 'the freezer' (these functions are defined in
37 kernel/power/process.c, kernel/freezer.c & include/linux/freezer.h). User space
41 try_to_freeze() function (defined in include/linux/freezer.h), that checks
44 For user space processes try_to_freeze() is called automatically from the
45 signal-handling code, but the freezable kernel threads need to call it
47 wait_event_freezable_timeout() macros (defined in include/linux/wait.h)
61 oom_reaper_list = tsk->oom_reaper_list;
73 entire system-wide transition will be cancelled. For this reason, freezable
84 -------------------------------------------------------------------------
87 - freezes only userspace tasks
90 - freezes all tasks (including kernel threads) because we can't freeze
94 - thaws only kernel threads; this is particularly useful if we need to do
99 - thaws all tasks (including kernel threads) because we can't thaw userspace
122 filesystem-related information that must be consistent with the state of the
123 on-disk data and metadata after the system memory state has been restored
126 tasks that might cause the on-disk filesystems' data and metadata to be
128 system is finally powered off. The majority of these are user space
141 3. The third reason is to prevent user space processes and some kernel threads
142 from interfering with the suspending and resuming of devices. A user space
148 … on LKML (https://lore.kernel.org/r/alpine.LFD.0.98.0704271801020.9964@woody.linux-foundation.org):
157 user threads and then waiting for a sync is practically one of the easier
161 freezing user threads I don't find really objectionable."
171 4. Another reason for freezing tasks is to prevent user space processes from
172 realizing that hibernation (or suspend) operation takes place. Ideally, user
173 space processes should not notice that such a system-wide operation has
193 Second, there are the following two problems related to the freezing of user
200 (https://lists.linux-foundation.org/pipermail/linux-pm/2007-May/012309.html).
205 avoid the realization by the user space processes that the hibernation is taking
211 fail, because the user land process that should respond to the request is frozen
222 Documentation/driver-api/pm/notifiers.rst.
230 piece of code from system-wide sleep such as suspend/hibernation is not
233 CPU-Hotplug code (kernel/cpu.c) for an example.
255 all user space processes or all freezable kernel threads, in unit of