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Message-Id: <201110160042.40869.rjw@sisk.pl> Date: Sun, 16 Oct 2011 00:42:40 +0200 From: "Rafael J. Wysocki" <rjw@...k.pl> To: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> Cc: rdunlap@...otime.net, len.brown@...el.com, pavel@....cz, a.p.zijlstra@...llo.nl, linux-pm@...r.kernel.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] Documentation/power: Update docs about suspend and CPU hotplug On Wednesday, October 12, 2011, Srivatsa S. Bhat wrote: > On 10/13/2011 12:49 AM, Rafael J. Wysocki wrote: > > 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@...ux.vnet.ibm.com> > >>>> --- > >>>> > >>>> 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@...ux.vnet.ibm.com>, 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? > > > > Ok, I get it. I'll clarify the question and post the next version. > > >> * 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? > > > > Sorry, I think I explained it wrong above. The freezer doesn't have any bugs > in this context. If it fails to freeze the tasks, suspend does get aborted. > > But the point here is, suppose the task writing to the 'online' sysfs > file has already entered the kernel, and _now_ the freezer started > freezing tasks, it might encounter trouble in freezing that cpu hotplug > operation that has already begun (because the cpu hotplug online operation > waits on the frozen userspace to get microcode). > > So, to clarify again, regular cpu hotplug and the cpu hotplug operations > carried out during suspend are properly serialized in the current kernel. > We have no problems there. > > The problem is with freezer and regular cpu hotplug racing with each other, > as illustrated in this scenario: > * the regular cpu online operation continues its journey from userspace > into the kernel, since the freezing has not yet begun. > * then freezer starts and freezes userspace. > * If cpu online has not yet completed the microcode update stuff by now, > it will now start waiting on the frozen userspace unfortunately. > * Now the freezer continues and tries to freeze the remaining tasks. But > due to this wait mentioned above, the freezer won't be able to freeze > the cpu online hotplug task and hence freezing of tasks fails. > [This race condition is where the whole problem lies.] > > And if freezing of tasks fails, then suspend gets aborted. So no problems > there again. That pretty much is how it is supposed to be. Do I understand correctly that you're attempting to make suspend always succeed if CPU hotplug stress test is run in parallel with it? Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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