On 07/02/2012 11:06 AM, preeti wrote: > On 06/30/2012 03:37 AM, Rafael J. Wysocki wrote: >> On Friday, June 29, 2012, preeti wrote: >>> On 06/29/2012 12:41 AM, Rafael J. Wysocki wrote: >>>> On Thursday, June 28, 2012, preeti wrote: >>>>> On 06/28/2012 03:23 PM, Rafael J. Wysocki wrote: >>>>>> On Thursday, June 28, 2012, preeti wrote: >>>>>>> >>>>>>> From: Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx> >>>> [...] >>>>> cpuidle is an architecture independent part of the kernel code.Since >>>>> this patch aims at x86 architecture in specific,I considered it >>>>> inappropriate. >>>>> >>>>> In addition to this,suspend happens on x86 only if ACPI is configured. >>>> >>>> But that is not required for intel_idle, so if it hangs with intel_idle, >>>> then it is not dependent on ACPI after all. >>> True intel_idle does not need ACPI to be configured,but that also means >>> that the kernel will not provide you the means to suspend. >> >> Yes, it will. You can hibernate without ACPI in theory. > > True.You can suspend to disk without ACPI,but can suspend to RAM only > with ACPI. > This also highlights the fact that my patch has not taken care of > hibernate notifiers,which I should have done.This goes to say that the > callbacks during suspend better not be in ACPI specific code.I will look > into correcting the placement of the callbacks. > >> >>> There is no question of resume hang here at all as you cannot suspend in >>> the first place. >>> >>> The issue is when ACPI is configured,and intel_idle is chosen to be the >>> cpuidle driver.In this situation when the user suspends,cpus can enter >>> deep sleep states as intel_idle driver does not prevent then from doing so. >>> This is when resume hangs. >> >> I know that. >> >>>>> Therefore it seemed right to put the callback in ACPI specific code >>>>> which deals with ACPI sleep support. >>>> >>>> I wonder if we can address this issue correctly. That is, in a non-racy >>>> way and in a better place. >>>> >>>> First, I really don't think it is necessary to "suspend" cpuidle (be it >>>> ACPI or any other) when device drivers' suspend routines are being >>>> executed (which also is racy, because the cpuidle "suspend" may be running >>>> concurrently with cpuidle on another CPU) or earlier. We really may want >>>> to disable the deeper C-states when we're about to execute >>>> suspend_ops->prepare_late(), or hibernation_ops->prepare(), i.e. after >>>> we've run dpm_suspend_end() successfully. >>> >>> The commit "ACPI:disable lower idle C-states across suspend/resume" >>> states that device_suspend() calls ACPI suspend functions which cause >>> side effects on the lower idle C-states. >> >> I'd like to know the details, then. Which ACPI suspend functions those are, >> in particular, because I'm not aware of any in device_suspend(). >> >> Also, please note that this commit is 5 years old and some things have changed >> in the suspend code paths since that time. > > I agree.My view on this,as I have mentioned in one of my previous mails > is, in the patch with the acpi_idle_suspend workaround,we have not taken > precautions to check if there are cpus that have already entered deep > C-states before the suspend routine has begun. > > We take care of disabling entry into deep C-states only during suspend. > so if deep C-states are posing problems during suspend,then why dont > these cpus that have entered idle states before suspend cause a resume hang? > Because cpu hotplug kicks the cpu out of any idle state it was in, in order to execute the CPU_DYING_FROZEN callbacks. (See my other mail about this). >> >>> This means we need to disable entry into deeper C-states even before >>> dpm_suspend_start(), >> >> This most likely is wrong. >> >> We may need to "suspend" cpuidle before calling suspend_device_irqs(), >> but then again that shouldn't be made via a notifier I think. Why don't >> we simply make suspend_device_irqs() disable lower C-states to start with? >> >>> but how much before? >>> >>> The commit answers this too.It says removing the functionality of >>> entering deep C-states before suspend removed the side effects.Besides, >>> the commit title says'across suspend/resume'.So I think to address the >>> resume hang effectively,it is desirable to disable entry into deeper >>> C-states during suspend_prepare operations. >>>> >>>> So, it seems, it might be a good idea to place a cpuidle driver suspend >>>> (and/or hibernation) hook at the end of dpm_suspend_end() and make ACPI >>>> and intel_idle drivers implement that hook. >>>> >>> Dont you think this patch is meant to fix a very ACPI specific problem? >> >> No, I don't. >> >>> Which is why I think the call backs or any hook meant to fix this should >>> go into ACPI specific code. >>> Else it will seem irrelevant to all other architectures that implement >>> suspend. >> >> I don't honestly think it is irrelevant. The fact is that similar problems >> have not been reported on other architectures _yet_, which by no means can >> be taken as a proof that those architectures are not affected. >> >> Anyway, I think that the right way to address this is through a cpuidle driver >> callback executed from suspend_device_irqs() (and analogously for the resume >> code path) and not through some hacky-ugly ACPI changes. > > I agree with having a cpuidle driver callback as even hibernate > callbacks need to be taken care of which have nothing to do with ACPI. > > But on what basis is the placement of the call back suggested at > suspend_device_irqs()? What is the assurance that this placement will > not cause a resume hang considering we dont know what precisely is > causing this problem? > Any place before CPU hotplug should do, I think. Regards, Srivatsa S. 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