linux-kernel - Re: [PATCH v5 08/12] sched: move cfs task on a CPU with higher capacity

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Date:	Thu, 11 Sep 2014 14:14:52 +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>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Rik van Riel <riel@...hat.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>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v5 08/12] sched: move cfs task on a CPU with higher capacity

On 11 September 2014 12:13, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Aug 26, 2014 at 01:06:51PM +0200, Vincent Guittot wrote:
>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 18db43e..60ae1ce 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6049,6 +6049,14 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>>                       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->sgc->capacity_orig * 100) > (sgs->group_capacity *
>> +                             env->sd->imbalance_pct))
>> +             return true;
>> +
>>       return false;
>>  }
>>
>> @@ -6534,13 +6542,23 @@ static int need_active_balance(struct lb_env *env)
>>       struct sched_domain *sd = env->sd;
>>
>>       if (env->idle == CPU_NEWLY_IDLE) {
>> +             int src_cpu = env->src_cpu;
>>
>>               /*
>>                * ASYM_PACKING needs to force migrate tasks from busy but
>>                * higher numbered CPUs in order to pack all tasks in the
>>                * lowest numbered CPUs.
>>                */
>> -             if ((sd->flags & SD_ASYM_PACKING) && env->src_cpu > env->dst_cpu)
>> +             if ((sd->flags & SD_ASYM_PACKING) && src_cpu > env->dst_cpu)
>> +                     return 1;
>> +
>> +             /*
>> +              * If the CPUs share their cache and the src_cpu's capacity is
>> +              * reduced because of other sched_class or IRQs, we trig an
>> +              * active balance to move the task
>> +              */
>> +             if ((capacity_orig_of(src_cpu) * 100) > (capacity_of(src_cpu) *
>> +                             sd->imbalance_pct))
>>                       return 1;
>>       }
>
> Should you not also check -- in both cases -- that the destination is
> any better?

The case should have been solved earlier when calculating the
imbalance which should be null if the destination is worse than the
source.

But i haven't formally check that calculate_imbalance correctly
handles that case

>
> Also, there's some obvious repetition going on there, maybe add a
> helper?

yes

>
> Also, both sites should probably ensure they're operating in the
> non-saturated/overloaded scenario. Because as soon as we're completely
> saturated we want SMP nice etc. and that all already works right
> (presumably).

If both are overloaded, calculated_imbalance will cap the max load
that can be pulled so the busiest_group will not become idle
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