linux-kernel - Re: [PATCH v2 10/11] sched: move cfs task on a CPU with higher capacity

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Date:	Fri, 30 May 2014 22:05:17 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v2 10/11] sched: move cfs task on a CPU with higher capacity

On 30 May 2014 08:29, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, May 29, 2014 at 09:37:39PM +0200, Vincent Guittot wrote:
>> On 29 May 2014 11:50, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Fri, May 23, 2014 at 05:53:04PM +0200, Vincent Guittot wrote:
>> >> If the CPU is used for handling lot of IRQs, trig a load balance to check if
>> >> it's worth moving its tasks on another CPU that has more capacity
>> >>
>> >> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>> >> ---
>> >>  kernel/sched/fair.c | 13 +++++++++++++
>> >>  1 file changed, 13 insertions(+)
>> >>
>> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> >> index e8a30f9..2501e49 100644
>> >> --- a/kernel/sched/fair.c
>> >> +++ b/kernel/sched/fair.c
>> >> @@ -5948,6 +5948,13 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>> >>       if (sgs->sum_nr_running > sgs->group_capacity)
>> >>               return true;
>> >>
>> >> +     /*
>> >> +      * The group capacity is reduced probably because of activity from other
>> >> +      * sched class or interrupts which use part of the available capacity
>> >> +      */
>> >> +     if ((sg->sgp->power_orig * 100) > (sgs->group_power * env->sd->imbalance_pct))
>> >> +             return true;
>> >> +
>> >>       if (sgs->group_imb)
>> >>               return true;
>> >>
>> >
>> > But we should already do this because the load numbers are scaled with
>> > the power/capacity figures. If one CPU gets significant less time to run
>> > fair tasks, its effective load would spike and it'd get to be selected
>> > here anyway.
>> >
>> > Or am I missing something?
>>
>> The CPU could have been picked when the capacity becomes null (which
>> occurred when the cpu_power goes below half the default
>> SCHED_POWER_SCALE). And even after that, there were some conditions in
>> find_busiest_group that was bypassing this busiest group
>
> Could you detail those conditions? FWIW those make excellent Changelog
> material.

I need to go back to my traces and use case to be sure that I point
the right culprit but IIRC, the imbalance was null. I will come back
with more details once i'll be back in front of the boards.
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