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From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Thu, 9 Oct 2014 16:59:36 +0200
Message-ID: <CAKfTPtABoQFQfufYK+hTd96ufQT83uypedzuZOeWjnUd51AbaQ@mail.gmail.com>
Subject: Re: [PATCH v7 2/7] sched: move cfs task on a CPU with higher capacity
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Preeti U Murthy <preeti@linux.vnet.ibm.com>,
        Morten Rasmussen <Morten.Rasmussen@arm.com>,
        Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        LAK <linux-arm-kernel@lists.infradead.org>,
        Rik van Riel <riel@redhat.com>, Mike Galbraith <efault@gmx.de>,
        Nicolas Pitre <nicolas.pitre@linaro.org>,
        "linaro-kernel@lists.linaro.org" <linaro-kernel@lists.linaro.org>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Paul Turner <pjt@google.com>, Benjamin Segall <bsegall@google.com>
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On 9 October 2014 13:23, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Oct 07, 2014 at 02:13:32PM +0200, Vincent Guittot wrote:
>> +++ b/kernel/sched/fair.c
>> @@ -5896,6 +5896,18 @@ fix_small_capacity(struct sched_domain *sd, struct sched_group *group)
>>  }
>>
>>  /*
>> + * Check whether the capacity of the rq has been noticeably reduced by side
>> + * activity. The imbalance_pct is used for the threshold.
>> + * Return true is the capacity is reduced
>> + */
>> +static inline int
>> +check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
>> +{
>> +     return ((rq->cpu_capacity * sd->imbalance_pct) <
>> +                             (rq->cpu_capacity_orig * 100));
>> +}
>> +
>> +/*
>>   * Group imbalance indicates (and tries to solve) the problem where balancing
>>   * groups is inadequate due to tsk_cpus_allowed() constraints.
>>   *
>> @@ -6567,6 +6579,14 @@ static int need_active_balance(struct lb_env *env)
>>                */
>>               if ((sd->flags & SD_ASYM_PACKING) && env->src_cpu > env->dst_cpu)
>>                       return 1;
>> +
>> +             /*
>> +              * The src_cpu's capacity is reduced because of other
>> +              * sched_class or IRQs, we trig an active balance to move the
>> +              * task
>> +              */
>> +             if (check_cpu_capacity(env->src_rq, sd))
>> +                     return 1;
>>       }
>
> So does it make sense to first check if there's a better candidate at
> all? By this time we've already iterated the current SD while trying
> regular load balancing, so we could know this.

i'm not sure to completely catch your point.
Normally, f_b_g and f_b_q have already looked at the best candidate
when we call need_active_balance. And src_cpu has been elected.
Or i have missed your point ?


>
>