linux-kernel - Re: [PATCH 2/3] sched/fair: Fix newidle

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Date:   Mon, 20 Dec 2021 17:17:28 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Donnefort <vincent.donnefort@....com>,
        peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org
Cc:     linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        morten.rasmussen@....com, qperret@...gle.com,
        Vincent Donnefort <vincent.donnefort@....com>
Subject: Re: [PATCH 2/3] sched/fair: Fix newidle_balance() for overutilized systems

On 20/12/21 12:43, Vincent Donnefort wrote:
> On Energy-Aware Scheduling systems, load balancing is disabled in favor of
> energy based placement, until one of the CPU is identified as being
> overutilized. Once the overutilization is resolved, two paths can lead to
> marking the system as non overutilized again:
>
>   * load_balance() triggered from newidle_balance().
>   * load_balance() triggered from the scheduler tick.
>
> However, small caveat for each of those paths. newidle_balance() needs
> rd->overload set to run load_balance(), while the load_balance() triggered
> by the scheduler tick needs to run from the first idle CPU of the root
> domain (see should_we_balance()).
>
> Overutilized can be triggered without setting overload (this can happen
> for a CPU which had a misfit task but didn't had its util_avg updated
> yet). Then, only the scheduler tick could help to reset overutilized...
> but if most of the CPUs are idle, it is very unlikely load_balance() would
> run on the only CPU which can reset the flag. This means the root domain
> can spuriously maintain overutilized for a long period of time.
>
> We then need newidle_balance() to proceed with balancing if the system is
> overutilized.
>
> Fixes: 2802bf3cd936 ("sched/fair: Add over-utilization/tipping point indicator")
> Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
>

Reviewed-by: Valentin Schneider <valentin.schneider@....com>

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e2f6fa14e5e7..51f6f55abb37 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10849,7 +10849,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>       rcu_read_lock();
>       sd = rcu_dereference_check_sched_domain(this_rq->sd);
>
> -	if (!READ_ONCE(this_rq->rd->overload) ||
> +	if ((!READ_ONCE(this_rq->rd->overload) &&
> +	    !READ_ONCE(this_rq->rd->overutilized)) ||
>           (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
>
>               if (sd)
> --
> 2.25.1