Re: Cpuidle drivers,Suspend : Fix suspend/resume hang with intel_idle driver

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Friday, June 29, 2012, preeti wrote:
> On 06/29/2012 09:41 AM, 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@linux.vnet.ibm.com>
> >> [...]
> >>> 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.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.
> >>
> >>> 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.This means we need to disable
> > entry into deeper C-states even before dpm_suspend_start(),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.
> 
> To clarify this further,since we take action upon PM_SUSPEND_PREPARE
> notification,which is called before suspend begins,we avoid race
> condition between suspend operations and disabling entry into deeper
> c-states altogether.

Well, what about races between disabling deeper C-states and cpuidle?

Rafael