linux-kernel - Re: [PATCH v3 5/5] sched/fair: Fix use of find_idlest

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Message-ID: <CAKfTPtA6gA=BPwTr6NCyArwHupBfDpjkc+Nd9U4D18JhATP4Zg@mail.gmail.com>
Date:   Thu, 31 Aug 2017 18:10:12 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Brendan Jackman <brendan.jackman@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Josef Bacik <josef@...icpanda.com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH v3 5/5] sched/fair: Fix use of find_idlest_group when
 local group is idlest

On 31 August 2017 at 13:58, Brendan Jackman <brendan.jackman@....com> wrote:
> find_idlest_group returns NULL when the local group is idlest. The
> caller then continues the find_idlest_group search at a lower level
> of the current CPU's sched_domain hierarchy. find_idlest_group_cpu is
> not consulted and, crucially, @new_cpu is not updated. This means the
> search is pointless and we return @prev_cpu from select_task_rq_fair.
>
> This is fixed by initialising @new_cpu to @cpu instead of
> @prev_cpu.
>
> Signed-off-by: Brendan Jackman <brendan.jackman@....com>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Josef Bacik <josef@...icpanda.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Morten Rasmussen <morten.rasmussen@....com>
> Cc: Peter Zijlstra <peterz@...radead.org>

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2608091..f93cb97 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5567,7 +5567,7 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this
>  static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p,
>                                   int cpu, int prev_cpu, int sd_flag)
>  {
> -       int new_cpu = prev_cpu;
> +       int new_cpu = cpu;
>
>         if (!cpumask_intersects(sched_domain_span(sd), &p->cpus_allowed))
>                 return prev_cpu;
> --
> 2.7.4
>