linux-kernel - Re: [PATCH 3/3] sched: fair: Fix wrong idle timestamp usage

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Date:	Thu, 16 Apr 2015 10:46:47 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 3/3] sched: fair: Fix wrong idle timestamp usage

On 04/15/2015 07:10 PM, Morten Rasmussen wrote:
> On Wed, Apr 15, 2015 at 04:43:17PM +0100, Daniel Lezcano wrote:
>> On 04/15/2015 02:18 PM, Peter Zijlstra wrote:
>>> On Wed, Apr 15, 2015 at 12:00:24PM +0200, Daniel Lezcano wrote:
>>>> The find_idlest_cpu is assuming the rq->idle_stamp information reflects when
>>>> the cpu entered the idle state. This is wrong as the cpu may exit and enter
>>>> the idle state several times without the rq->idle_stamp being updated.
>>>
>>> Sure, but you forgot to tell us why it matters.
>>
>> Yes, right. Thanks for pointing this out.
>>
>> Assuming we are in the situation where there are several idle cpus in
>> the same idle state.
>>
>> With the current code, the function find_idlest_cpu will choose a cpu
>> with the shortest idle duration. This information is based on the
>> rq->idle_stamp variable and is correct until one of the idle cpu is
>> exiting the cpuidle_enter function and re-entering it again. As soon as
>> this happen, the rq->idle_stamp value is no longer a reliable information.
>>
>> Example:
>>
>>    * CPU0 and CPU1 are running
>>    * CPU2 and CPU3 are in the C3 state.
>>    * CPU2 entered idle at T2
>>    * CPU3 entered idle at T3
>>    * T2 < T3
>>
>> The function find_idlest_cpu will choose CPU3 because it has a shorter
>> idle duration.
>>
>> Then CPU3 is woken up by an interrupt, process it and re-enter idle C3.
>>
>> The information will still give the out to date information T2 < T3 and
>> find_idlest_cpu will choose CPU2 instead of CPU3.
>
> I can't get the example to match your description of how
> find_idlest_cpu() is supposed to work :-(
>
> Did you mean CPU2 (not CPU3) getting woken up by an interrupt and
> find_busiest_cpu() choosing CPU3 instead of CPU2 after the interrupt?
>
> In your example find_busiest_cpu() should return CPU3 before the
> interrupt as it went to sleep last and the interrupt on CPU3 should not
> affect that choice as CPU3 is still the last cpu to go to sleep
> (regardless of your patch). No?

Yes you are right. I meant CPU2 is woken up by the interrupt.


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