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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 26 Mar 2020 17:03:10 +0100
Message-ID: <CAKfTPtBWQDkmkuxPTEC8QY1PXicZ51w7tjDPRm2rYM+8QN5rLQ@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: Fix negative imbalance in imbalance calculation
To:     Aubrey Li <aubrey.li@intel.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vineeth Pillai <vpillai@digitalocean.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Aubrey Li <aubrey.li@linux.intel.com>,
        Phil Auld <pauld@redhat.com>
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On Thu, 26 Mar 2020 at 06:53, Aubrey Li <aubrey.li@intel.com> wrote:
>
> A negative imbalance value was observed after imbalance calculation,
> this happens when the local sched group type is group_fully_busy,
> and the average load of local group is greater than the selected
> busiest group. Fix this problem by comparing the average load of the
> local and busiest group before imbalance calculation formula.
>
> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
> Cc: Phil Auld <pauld@redhat.com>

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

> ---
>  kernel/sched/fair.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c1217bf..4a2ba3f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8761,6 +8761,14 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>
>                 sds->avg_load = (sds->total_load * SCHED_CAPACITY_SCALE) /
>                                 sds->total_capacity;
> +               /*
> +                * If the local group is more loaded than the selected
> +                * busiest group don't try to pull any tasks.
> +                */
> +               if (local->avg_load >= busiest->avg_load) {
> +                       env->imbalance = 0;
> +                       return;
> +               }
>         }
>
>         /*
> --
> 2.7.4
>