Root/Documentation/power/freezing-of-tasks.txt

1Freezing of tasks
2    (C) 2007 Rafael J. Wysocki <rjw@sisk.pl>, GPL
3
4I. What is the freezing of tasks?
5
6The freezing of tasks is a mechanism by which user space processes and some
7kernel threads are controlled during hibernation or system-wide suspend (on some
8architectures).
9
10II. How does it work?
11
12There are four per-task flags used for that, PF_NOFREEZE, PF_FROZEN, TIF_FREEZE
13and PF_FREEZER_SKIP (the last one is auxiliary). The tasks that have
14PF_NOFREEZE unset (all user space processes and some kernel threads) are
15regarded as 'freezable' and treated in a special way before the system enters a
16suspend state as well as before a hibernation image is created (in what follows
17we only consider hibernation, but the description also applies to suspend).
18
19Namely, as the first step of the hibernation procedure the function
20freeze_processes() (defined in kernel/power/process.c) is called. It executes
21try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and
22either wakes them up, if they are kernel threads, or sends fake signals to them,
23if they are user space processes. A task that has TIF_FREEZE set, should react
24to it by calling the function called refrigerator() (defined in
25kernel/power/process.c), which sets the task's PF_FROZEN flag, changes its state
26to TASK_UNINTERRUPTIBLE and makes it loop until PF_FROZEN is cleared for it.
27Then, we say that the task is 'frozen' and therefore the set of functions
28handling this mechanism is referred to as 'the freezer' (these functions are
29defined in kernel/power/process.c and include/linux/freezer.h). User space
30processes are generally frozen before kernel threads.
31
32It is not recommended to call refrigerator() directly. Instead, it is
33recommended to use the try_to_freeze() function (defined in
34include/linux/freezer.h), that checks the task's TIF_FREEZE flag and makes the
35task enter refrigerator() if the flag is set.
36
37For user space processes try_to_freeze() is called automatically from the
38signal-handling code, but the freezable kernel threads need to call it
39explicitly in suitable places or use the wait_event_freezable() or
40wait_event_freezable_timeout() macros (defined in include/linux/freezer.h)
41that combine interruptible sleep with checking if TIF_FREEZE is set and calling
42try_to_freeze(). The main loop of a freezable kernel thread may look like the
43following one:
44
45    set_freezable();
46    do {
47        hub_events();
48        wait_event_freezable(khubd_wait,
49                !list_empty(&hub_event_list) ||
50                kthread_should_stop());
51    } while (!kthread_should_stop() || !list_empty(&hub_event_list));
52
53(from drivers/usb/core/hub.c::hub_thread()).
54
55If a freezable kernel thread fails to call try_to_freeze() after the freezer has
56set TIF_FREEZE for it, the freezing of tasks will fail and the entire
57hibernation operation will be cancelled. For this reason, freezable kernel
58threads must call try_to_freeze() somewhere or use one of the
59wait_event_freezable() and wait_event_freezable_timeout() macros.
60
61After the system memory state has been restored from a hibernation image and
62devices have been reinitialized, the function thaw_processes() is called in
63order to clear the PF_FROZEN flag for each frozen task. Then, the tasks that
64have been frozen leave refrigerator() and continue running.
65
66III. Which kernel threads are freezable?
67
68Kernel threads are not freezable by default. However, a kernel thread may clear
69PF_NOFREEZE for itself by calling set_freezable() (the resetting of PF_NOFREEZE
70directly is strongly discouraged). From this point it is regarded as freezable
71and must call try_to_freeze() in a suitable place.
72
73IV. Why do we do that?
74
75Generally speaking, there is a couple of reasons to use the freezing of tasks:
76
771. The principal reason is to prevent filesystems from being damaged after
78hibernation. At the moment we have no simple means of checkpointing
79filesystems, so if there are any modifications made to filesystem data and/or
80metadata on disks, we cannot bring them back to the state from before the
81modifications. At the same time each hibernation image contains some
82filesystem-related information that must be consistent with the state of the
83on-disk data and metadata after the system memory state has been restored from
84the image (otherwise the filesystems will be damaged in a nasty way, usually
85making them almost impossible to repair). We therefore freeze tasks that might
86cause the on-disk filesystems' data and metadata to be modified after the
87hibernation image has been created and before the system is finally powered off.
88The majority of these are user space processes, but if any of the kernel threads
89may cause something like this to happen, they have to be freezable.
90
912. Next, to create the hibernation image we need to free a sufficient amount of
92memory (approximately 50% of available RAM) and we need to do that before
93devices are deactivated, because we generally need them for swapping out. Then,
94after the memory for the image has been freed, we don't want tasks to allocate
95additional memory and we prevent them from doing that by freezing them earlier.
96[Of course, this also means that device drivers should not allocate substantial
97amounts of memory from their .suspend() callbacks before hibernation, but this
98is e separate issue.]
99
1003. The third reason is to prevent user space processes and some kernel threads
101from interfering with the suspending and resuming of devices. A user space
102process running on a second CPU while we are suspending devices may, for
103example, be troublesome and without the freezing of tasks we would need some
104safeguards against race conditions that might occur in such a case.
105
106Although Linus Torvalds doesn't like the freezing of tasks, he said this in one
107of the discussions on LKML (http://lkml.org/lkml/2007/4/27/608):
108
109"RJW:> Why we freeze tasks at all or why we freeze kernel threads?
110
111Linus: In many ways, 'at all'.
112
113I _do_ realize the IO request queue issues, and that we cannot actually do
114s2ram with some devices in the middle of a DMA. So we want to be able to
115avoid *that*, there's no question about that. And I suspect that stopping
116user threads and then waiting for a sync is practically one of the easier
117ways to do so.
118
119So in practice, the 'at all' may become a 'why freeze kernel threads?' and
120freezing user threads I don't find really objectionable."
121
122Still, there are kernel threads that may want to be freezable. For example, if
123a kernel that belongs to a device driver accesses the device directly, it in
124principle needs to know when the device is suspended, so that it doesn't try to
125access it at that time. However, if the kernel thread is freezable, it will be
126frozen before the driver's .suspend() callback is executed and it will be
127thawed after the driver's .resume() callback has run, so it won't be accessing
128the device while it's suspended.
129
1304. Another reason for freezing tasks is to prevent user space processes from
131realizing that hibernation (or suspend) operation takes place. Ideally, user
132space processes should not notice that such a system-wide operation has occurred
133and should continue running without any problems after the restore (or resume
134from suspend). Unfortunately, in the most general case this is quite difficult
135to achieve without the freezing of tasks. Consider, for example, a process
136that depends on all CPUs being online while it's running. Since we need to
137disable nonboot CPUs during the hibernation, if this process is not frozen, it
138may notice that the number of CPUs has changed and may start to work incorrectly
139because of that.
140
141V. Are there any problems related to the freezing of tasks?
142
143Yes, there are.
144
145First of all, the freezing of kernel threads may be tricky if they depend one
146on another. For example, if kernel thread A waits for a completion (in the
147TASK_UNINTERRUPTIBLE state) that needs to be done by freezable kernel thread B
148and B is frozen in the meantime, then A will be blocked until B is thawed, which
149may be undesirable. That's why kernel threads are not freezable by default.
150
151Second, there are the following two problems related to the freezing of user
152space processes:
1531. Putting processes into an uninterruptible sleep distorts the load average.
1542. Now that we have FUSE, plus the framework for doing device drivers in
155userspace, it gets even more complicated because some userspace processes are
156now doing the sorts of things that kernel threads do
157(https://lists.linux-foundation.org/pipermail/linux-pm/2007-May/012309.html).
158
159The problem 1. seems to be fixable, although it hasn't been fixed so far. The
160other one is more serious, but it seems that we can work around it by using
161hibernation (and suspend) notifiers (in that case, though, we won't be able to
162avoid the realization by the user space processes that the hibernation is taking
163place).
164
165There are also problems that the freezing of tasks tends to expose, although
166they are not directly related to it. For example, if request_firmware() is
167called from a device driver's .resume() routine, it will timeout and eventually
168fail, because the user land process that should respond to the request is frozen
169at this point. So, seemingly, the failure is due to the freezing of tasks.
170Suppose, however, that the firmware file is located on a filesystem accessible
171only through another device that hasn't been resumed yet. In that case,
172request_firmware() will fail regardless of whether or not the freezing of tasks
173is used. Consequently, the problem is not really related to the freezing of
174tasks, since it generally exists anyway.
175
176A driver must have all firmwares it may need in RAM before suspend() is called.
177If keeping them is not practical, for example due to their size, they must be
178requested early enough using the suspend notifier API described in notifiers.txt.
179

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