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

[preeti <preeti@linux.vnet.ibm.com>]

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.
> 
> 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?
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.
> That would be much more deterministic than your patch I think.
> 
> Thanks,
> Rafael
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Regards
Preeti