Documentation/power/freezing-of-tasks.rst

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.
37 frozen before kernel threads.
44 signal-handling code, but the freezable kernel threads need to call it
64 threads must call try_to_freeze() somewhere or use one of the
74 -------------------------------------------------------------------------
77   - freezes only userspace tasks
80   - freezes all tasks (including kernel threads) because we can't freeze
81     kernel threads without freezing userspace tasks
84   - thaws only kernel threads; this is particularly useful if we need to do
85     anything special in between thawing of kernel threads and thawing of
89   - thaws all tasks (including kernel threads) because we can't thaw userspace
90     tasks without thawing kernel threads
93 III. Which kernel threads are freezable?
96 Kernel threads are not freezable by default.  However, a kernel thread may clear
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
118    processes, but if any of the kernel threads may cause something like this
130 3. The third reason is to prevent user space processes and some kernel threads
139 "RJW:> Why we freeze tasks at all or why we freeze kernel threads?
146 user threads and then waiting for a sync is practically one of the easier
149 So in practice, the 'at all' may become a 'why freeze kernel threads?' and
150 freezing user threads I don't find really objectionable."
152 Still, there are kernel threads that may want to be freezable.  For example, if
162    space processes should not notice that such a system-wide operation has
176 First of all, the freezing of kernel threads may be tricky if they depend one
180 may be undesirable.  That's why kernel threads are not freezable by default.
188    now doing the sorts of things that kernel threads do
189    (https://lists.linux-foundation.org/pipermail/linux-pm/2007-May/012309.html).
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