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

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Date:	Wed, 15 Apr 2015 18:02:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:	rjw@...ysocki.net, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, nicolas.pitre@...aro.org,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 3/3] sched: fair: Fix wrong idle timestamp usage

On Wed, Apr 15, 2015 at 05:43:17PM +0200, 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.
> 
> Even if that shouldn't have a big impact on the performance and energy side,
> we are dealing with a wrong information preventing us to improve the energy
> side later (eg. prevent to wakeup a cpu which did not reach its target
> residency yet).

Right, I figured as much; but no tangible results or behavioural fail
observed.

> >Urgh, you made horrid code more horrible.
> >
> >And all without reason.
> 
> Ok. What is horrible ? The 'if then else' blocks or the algorithm itself ?

Yeah the amount and depth of branches. I briefly tried to see if it
could be fixed but came up empty. Maybe I should try harder :-)
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