Re: [PATCH] Documentation/power: Update docs about suspend and CPU hotplug

[Rafael J. Wysocki]

On Wednesday, October 12, 2011, Srivatsa S. Bhat wrote:
> On 10/12/2011 03:32 AM, Rafael J. Wysocki wrote:
> > On Tuesday, October 11, 2011, Srivatsa S. Bhat wrote:
> >> Update the documentation about the interaction between the suspend (S3) call
> >> path and the CPU hotplug infrastructure.
> >> This patch focusses only on the activities of the freezer, cpu hotplug and
> >> the notifications involved. It outlines how regular CPU hotplug differs from
> >> the way it is invoked during suspend and also tries to explain the locking
> >> involved.
> >>
> >> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> >> ---
> >>
> >>  Documentation/power/00-INDEX                   |    2 
> >>  Documentation/power/suspend-and-cpuhotplug.txt |  113 ++++++++++++++++++++++++
> >>  2 files changed, 115 insertions(+), 0 deletions(-)
> >>  create mode 100644 Documentation/power/suspend-and-cpuhotplug.txt
> >>
> >> diff --git a/Documentation/power/00-INDEX b/Documentation/power/00-INDEX
> >> index 45e9d4a..a4d682f 100644
> >> --- a/Documentation/power/00-INDEX
> >> +++ b/Documentation/power/00-INDEX
> >> @@ -26,6 +26,8 @@ s2ram.txt
> >>  	- How to get suspend to ram working (and debug it when it isn't)
> >>  states.txt
> >>  	- System power management states
> >> +suspend-and-cpuhotplug.txt
> >> +	- Explains the interaction between Suspend-to-RAM (S3) and CPU hotplug
> >>  swsusp-and-swap-files.txt
> >>  	- Using swap files with software suspend (to disk)
> >>  swsusp-dmcrypt.txt
> >> diff --git a/Documentation/power/suspend-and-cpuhotplug.txt b/Documentation/power/suspend-and-cpuhotplug.txt
> >> new file mode 100644
> >> index 0000000..d0ba411
> >> --- /dev/null
> >> +++ b/Documentation/power/suspend-and-cpuhotplug.txt
> >> @@ -0,0 +1,113 @@
> >> +Interaction of Suspend code (S3) with the CPU hotplug infrastructure
> >> +   (C) 2011 Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>, GPL
> >> +
> >> +
> >> +I. How does the Suspend-to-RAM code interact with CPU hotplug infrastructure?
> >> +
> >> +Well, a picture speaks more than a thousand words... So ASCII art follows :-)
> >> +
> >> +[This depicts the current design in the kernel, and focusses only on the
> >> +interactions between suspend call paths involving the freezer and cpu hotplug
> >> +and also tries to explain the locking involved. It also outlines the
> >> +notifications involved.]
> >> +
> >> +On a high level, the suspend-resume cycle goes like this:
> >> +
> >> +|Freeze| -> |Disable nonboot| -> |Do suspend| -> |Enable nonboot| -> |Thaw |
> >> +|tasks |    |     cpus      |    |          |    |     cpus     |    |tasks|
> >> +
> >> +
> >> +More details follow:
> >> +
> >> +Regular CPU hotplug                                   Suspend call path
> >> +-------------------                              ---------------------------
> >> +
> >> +Write 0 (or 1) to                                     Write 'mem' to
> >> +/sys/devices/system/cpu/cpu*/online                   /sys/power/state
> >> +     sysfs file                                          syfs file
> >> +         |                                                   |
> >> +         |                                                   v
> >> +         |                                         Acquire pm_mutex lock
> >> +         |                                                   |
> >> +         |                                                   v
> >> +         |                                Send PM_SUSPEND_PREPARE notifications
> >> +         |                                                   |
> >> +         |                                                   v
> >> +         |                                              Freeze tasks
> > 
> > OK, so something appears to be missing here.  Namely, the task writing to
> > /sys/devices/system/cpu/cpu*/online should be frozen at this point or
> > suspend should be aborted.  I suppose neither of these happens and I wonder
> > why exactly.
> > 
> 
> I have a couple of clarifications to make here:
> * Firstly, this picture is not meant to represent what happens when regular
>   cpu hotplug and suspend run together. That race condition has not been
>   brought out here. What it does try to explain is, how the regular cpu
>   hotplug path is different from suspend, and where they share common code.
>   Please don't think about timing/race condition when reading it. Its just
>   meant to explain the call path and locking involved.

Well, I didn't understand this part.  And the question above is:

> I. How does the Suspend-to-RAM code interact with CPU hotplug infrastructure?

which kind of suggests something different from what you're saying.  Care to
clarify that in the document?

> * Secondly, this picture explains the *current* design, and *not* the mutual
>   exclusion design I have proposed between regular cpu hotplug and suspend.
>   The reason being, this doc was written to help everyone understand the
>   current locking schemes, to help evaluate my proposal for a different
>   scheme (mutual exclusion).

I understand that.

> Now, coming to your point, if that task writing to the sysfs file has not
> been frozen, then the current kernel doesn't abort suspend, which is why we are
> encountering problems, and which is exactly what my patchset tries to solve.
> Link to my patchset:
> http://thread.gmane.org/gmane.linux.documentation/3414/focus=3414

This isn't my point, actually.  My point is that the task writing to
/sys/devices/system/cpu/cpu*/online should be frozen by the freezer.
If it is not frozen, then the freezer should fail.  If that doesn't
happen, there's a bug that has to be fixed and it is _not_ the lack
of mutual exclusion.  The bug is that, apparently, suspend continues
even though there is an unfrozen user space process in the system.

Do you have any idea why that happens?

Rafael
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