From: Valentin Schneider <valentin.schneider@arm.com>
To: Vincent Donnefort <vincent.donnefort@arm.com>,
peterz@infradead.org, mingo@redhat.com,
vincent.guittot@linaro.org
Cc: linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com,
morten.rasmussen@arm.com, qperret@google.com,
Vincent Donnefort <vincent.donnefort@arm.com>
Subject: Re: [PATCH 2/3] sched/fair: Fix newidle_balance() for overutilized systems
Date: Mon, 20 Dec 2021 17:17:28 +0000 [thread overview]
Message-ID: <874k738ac7.mognet@arm.com> (raw)
In-Reply-To: <20211220114323.22811-3-vincent.donnefort@arm.com>
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@arm.com>
>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.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
next prev parent reply other threads:[~2021-12-20 17:17 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-20 11:43 [PATCH 0/3] Fix stuck overutilized Vincent Donnefort
2021-12-20 11:43 ` [PATCH 1/3] sched/fair: Make cpu_overutilized() EAS dependent Vincent Donnefort
2021-12-20 17:17 ` Valentin Schneider
2021-12-21 9:09 ` Vincent Donnefort
2021-12-20 11:43 ` [PATCH 2/3] sched/fair: Fix newidle_balance() for overutilized systems Vincent Donnefort
2021-12-20 17:17 ` Valentin Schneider [this message]
2021-12-22 8:14 ` Vincent Guittot
2022-01-10 16:29 ` Vincent Donnefort
2021-12-20 11:43 ` [PATCH 3/3] sched/fair: Do not raise overutilized for idle CPUs Vincent Donnefort
2021-12-20 17:17 ` Valentin Schneider
2021-12-22 8:20 ` Vincent Guittot
2022-01-10 16:40 ` Vincent Donnefort
2022-01-17 10:45 ` Vincent Guittot
2022-01-17 12:18 ` Vincent Donnefort
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