1.介绍Linux 休眠提供了一种类似于Windows 的休眠方式，使用户能够通过休眠操作，保存系统当前的内存数据到硬盘，即s w a p 分区中。当计算机重新启动后，系统重新装载保存的内存数据，包括进程数据，寄存器数值等，并恢复到关机前的状态。由于不需要重新装载文档，应用程序也不用重新打开，因此休眠启动方式要比正常的启动过程快得多。
2.Linux 休眠原理要实现操作系统的休眠，首先要理解linux 的内存管理机制。标准L i n u x 的分页是三级页表结构：页目录、中间页目录和页。i 3 8 6 采用的是两级页表结构：页目录和页，不支持中间页目录。4 G 的线性地址空间，只有一个页目录，它最多有1024 个目录项，每个目录项又含有1024 个页面项，每个页面有4 K 字节。分页机制通过把线性地址空间中的页，重新定位到物理地址空间来进行管理，因为每个页面的整个4K 字节作为一个单位进行映射，并且每个页面都对齐4K 字节的边界，因此，线性地址的低12 位经过分页机制直接地作为物理地址的低1 2 位使用。下图所示是x86 下线性地址映射为物理地址的过程：休眠过程可以分为两个阶段, 一是SUSPEND 阶段, 二是R E S U M E 阶段, R E S U M E 过程是S U S P E N D 的逆过程。S U S P E N D 阶段保存进程数据到硬盘中, 并关机;RESUME 阶段, 从硬盘中读取保存的进程数据, 并恢复到关机前的原始状态。休眠需要解决的问题中,最重要的部分是内存数据的保存和如何恢复保存的内存数据。我们可以很容易获取内存页面数据,SUSPEND 的过程中,主要任务就是要保存这些需要保存的页面,但是,作为存储页面地址的页表也需要保存下来，因为页表仅仅是一个中间转换作用的链表, 所以，可以在S U S P E N D 的过程中, 临时建立, 然后将内存页面地址记录在页表中。RESUME 的阶段,将保存的页面和页表写到内存页中，完成后,只要重新修改页目录数据, 就完成内存数据还原动作了。经过以上分析，可以得到休眠的大体原理图，如下所示：如图所示,实现S U S P E N D需要完成三个主要步骤：冻结系统中的活动进程, 准备保存内存数据, 写内存数据到硬盘。冻结活动进程：包括三类主要的活动源，即，用户空间进程和内核线程，设备驱动和活动的计时器；准备保存数据：计算需要保存的内存页数，分配内存以保存进程数据，复制进程数据到分配的内存中；保存数据到硬盘：写需要保存的内存页到硬盘中。RESUME 是SUSPEND 的逆过程，要完成分配内存以读取硬盘中的进程数据，读取硬盘数据，重新映射页表地址，更新段描述符表等。
3 Linux 软件休眠实现休眠以模块方式实现，用户可以根据自己的需要选择是否装载此模块。但是，因为休眠在R E S U M E 的过程中，需要恢复关机前的内存数据，以及c p u 状态等，所以，此模块的装载应该通过ramdisk 的init 自动装载，并且要在mount root文件系统之前。
3.1 SUSPEND 阶段3.1.1 冻结活动进程进程执行时，它会根据具体情况改变状态。Linux 中的进程状态主要有以下几种：T A S K _ R U N N I N G 可运行T A S K _ I N T E R R U P T I B L E 可中断的等待状态T A S K _ U N I N T E R R U P T I B L E 不可中断的等待状态T A S K _ Z O M B I E 僵死T A S K _ S T O P P E D 暂停T A S K _ S W A P P I N G 换入/ 换出操作系统在运行过程中，一般有十几个，甚至几十个进程在运行。S U S P E N D 进程获得执行的资源而执行，即当前进程（current），是不能被冻结和中止执行，否则后续的操作会得不到完全执行；另外，进程标志为P F _ N O F R E E E Z E 和P F _ F R O Z E N 的；以及进程状态为T A S K _ Z O M B I E 、T A S K _ D E A D、T A S K _ S T O P P E D，这些进程是不能冻结的或者不需要冻结的。除此之外，其余的进程需要冻结，也就是改变进程标志为P F _ F R E E Z E 。进程标志改为P F _ F R E E Z E后，相应的进程会因为获不到资源，从而处于静止状态。3.1.2 准备保存数据检测所有内存页，如果页面标识不是PG_reserved，则需要保存的页面数加1 。内存检测完成后，得到需要保存的页面数目，即nr_copy_pages。for (pfn = 0; pfn < max_pfn; pfn++){page = pfn_to_page(pfn);if (!PageReserved(page)){ ⋯ .nr_copy_pages ++⋯ .}⋯由nr_copy_pages 数目，得到内存中对应数目的空闲页面作为页表目录数，同时分配nr_copy_pages 个空闲页，页地址由页表目录记录管理。除了进程数据外，当前寄存器的数据，包括描述符表，段寄存器，控制寄存器，以及通用寄存器的值，都作为全局变量保存下来。复制需要保存的内存页面到新分配的空闲页中。for (pfn = 0; pfn < max_pfn; pfn++) {⋯ .if (pagedir_p) {pagedir_p->orig_address =ADDRESS(pfn);copy_page((void *) pagedir_p->address,(void *) pagedir_p->orig_address);pagedir_p++;}⋯ .}3.1.3 保存数据到swap 分区
摘 要:休眠操作通过保存当前系统进程数据和cpu 状态数据到硬盘中，当系统断电并重新启动后，又自动读取保存的数据并恢复到原始系统状态，如此大大减少了系统的启动时间。内存管理，进程管理和swap 操作等方面是休眠实现的主要涉及范围，因此对于深入理解linux 操作系统有所帮助。
关键词:Linux; 内核; 休眠; swap__
Freezing of tasks
(C) 2007 Rafael J. Wysocki <>, GPL
I. What is the freezing of tasks?
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).
II. How does it work?
There are four per-task flags used for that, PF_NOFREEZE, PF_FROZEN, TIF_FREEZE
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).
Namely, as the first step of the hibernation procedure the function
freeze_processes() (defined in kernel/power/process.c) is called. It executes
try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and
either wakes them up, if they are kernel threads, or sends fake signals to them,
if they are user space processes. A task that has TIF_FREEZE set, should react
to it by calling the function called refrigerator() (defined in
kernel/power/process.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 and include/linux/freezer.h). User space
processes are generally frozen before kernel threads.
It is not recommended to call refrigerator() directly. Instead, it is
recommended to use the try_to_freeze() function (defined in
include/linux/freezer.h), that checks the task's TIF_FREEZE flag and makes the
task enter refrigerator() if the flag is set.
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 TIF_FREEZE is set and calling
try_to_freeze(). The main loop of a freezable kernel thread may look like the
following one:
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));
(from drivers/usb/core/hub.c::hub_thread()).
If a freezable kernel thread fails to call try_to_freeze() after the freezer has
set TIF_FREEZE for it, 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.
After the system memory state has been restored from a hibernation image and
devices have been reinitialized, the function thaw_processes() is called in
order to clear the PF_FROZEN flag for each frozen task. Then, the tasks that
have been frozen leave refrigerator() and continue running.
III. Which kernel threads are freezable?
Kernel threads are not freezable by default. However, a kernel thread may clear
PF_NOFREEZE for itself by calling set_freezable() (the resetting of PF_NOFREEZE
directly is strongly discouraged). From this point it is regarded as freezable
and must call try_to_freeze() in a suitable place.
IV. Why do we do that?
Generally speaking, there is a couple of reasons to use the freezing of tasks:
1. The principal reason is to prevent filesystems from being damaged after
hibernation. At the moment we have no simple means of checkpointing
filesystems, so if there are any modifications made to filesystem data and/or
metadata on disks, we cannot bring them back to the state from before the
modifications. At the same time each hibernation image contains some
filesystem-related information that must be consistent with the state of the
on-disk data and metadata after the system memory state has been restored from
the image (otherwise the filesystems will be damaged in a nasty way, usually
making them almost impossible to repair). We therefore freeze tasks that might
cause the on-disk filesystems' data and metadata to be modified after the
hibernation image has been created and before the system is finally powered off.
The majority of these are user space processes, but if any of the kernel threads
may cause something like this to happen, they have to be freezable.
2. Next, to create the hibernation image we need to free a sufficient amount of
memory (approximately 50% of available RAM) and we need to do that before
devices are deactivated, because we generally need them for swapping out. Then,
after the memory for the image has been freed, we don't want tasks to allocate
additional memory and we prevent them from doing that by freezing them earlier.
[Of course, this also means that device drivers should not allocate substantial
amounts of memory from their .suspend() callbacks before hibernation, but this
is e separate issue.]
3. The third reason is to prevent user space processes and some kernel threads
from interfering with the suspending and resuming of devices. A user space
process running on a second CPU while we are suspending devices may, for
example, be troublesome and without the freezing of tasks we would need some
safeguards against race conditions that might occur in such a case.
Although Linus Torvalds doesn't like the freezing of tasks, he said this in one
of the discussions on LKML ():
"RJW:> Why we freeze tasks at all or why we freeze kernel threads?
Linus: In many ways, 'at all'.
I _do_ realize the IO request queue issues, and that we cannot actually do
s2ram with some devices in the middle of a DMA. So we want to be able to
avoid *that*, there's no question about that. And I suspect that stopping
user threads and then waiting for a sync is practically one of the easier
ways to do so.
So in practice, the 'at all' may become a 'why freeze kernel threads?' and
freezing user threads I don't find really objectionable."
Still, there are kernel threads that may want to be freezable. For example, if
a kernel that belongs to a device driver accesses the device directly, it in
principle needs to know when the device is suspended, so that it doesn't try to
access it at that time. However, if the kernel thread is freezable, it will be
frozen before the driver's .suspend() callback is executed and it will be
thawed after the driver's .resume() callback has run, so it won't be accessing
the device while it's suspended.
4. Another reason for freezing tasks is to prevent user space processes from
realizing that hibernation (or suspend) operation takes place. Ideally, user
space processes should not notice that such a system-wide operation has occurred
and should continue running without any problems after the restore (or resume
from suspend). Unfortunately, in the most general case this is quite difficult
to achieve without the freezing of tasks. Consider, for example, a process
that depends on all CPUs being online while it's running. Since we need to
disable nonboot CPUs during the hibernation, if this process is not frozen, it
may notice that the number of CPUs has changed and may start to work incorrectly
because of that.
V. Are there any problems related to the freezing of tasks?
Yes, there are.
First of all, the freezing of kernel threads may be tricky if they depend one
on another. For example, if kernel thread A waits for a completion (in the
TASK_UNINTERRUPTIBLE state) that needs to be done by freezable kernel thread B
and B is frozen in the meantime, then A will be blocked until B is thawed, which
may be undesirable. That's why kernel threads are not freezable by default.
Second, there are the following two problems related to the freezing of user
space processes:
1. Putting processes into an uninterruptible sleep distorts the load average.
2. Now that we have FUSE, plus the framework for doing device drivers in
userspace, it gets even more complicated because some userspace processes are
now doing the sorts of things that kernel threads do
().
The problem 1. seems to be fixable, although it hasn't been fixed so far. The
other one is more serious, but it seems that we can work around it by using
hibernation (and suspend) notifiers (in that case, though, we won't be able to
avoid the realization by the user space processes that the hibernation is taking
place).
There are also problems that the freezing of tasks tends to expose, although
they are not directly related to it. For example, if request_firmware() is
called from a device driver's .resume() routine, it will timeout and eventually
fail, because the user land process that should respond to the request is frozen
at this point. So, seemingly, the failure is due to the freezing of tasks.
Suppose, however, that the firmware file is located on a filesystem accessible
only through another device that hasn't been resumed yet. In that case,
request_firmware() will fail regardless of whether or not the freezing of tasks
is used. Consequently, the problem is not really related to the freezing of
tasks, since it generally exists anyway.
A driver must have all firmwares it may need in RAM before suspend() is called.
If keeping them is not practical, for example due to their size, they must be
requested early enough using the suspend notifier API described in notifiers.txt.
