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 08:29:37 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.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 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.

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