From: Rik van Riel <riel@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.com>,
Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 3/3] sched: Spare idle load balancing on nohz_full CPUs
Date: Tue, 20 Jun 2017 13:42:27 -0400 [thread overview]
Message-ID: <1497980547.20270.106.camel@redhat.com> (raw)
In-Reply-To: <1497838322-10913-4-git-send-email-fweisbec@gmail.com>
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On Mon, 2017-06-19 at 04:12 +0200, Frederic Weisbecker wrote:
> Although idle load balancing obviously only concern idle CPUs, it can
> be a disturbance on a busy nohz_full CPU. Indeed a CPU can only get
> rid
> of an idle load balancing duty once a tick fires while it runs a task
> and this can take a while in a nohz_full CPU.
>
> We could fix that and escape the idle load balancing duty from the
> very
> idle exit path but that would bring unecessary overhead. Lets just
> not
> bother and leave that job to housekeeping CPUs (those outside
> nohz_full
> range). The nohz_full CPUs simply don't want any disturbance.
>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> ---
> kernel/sched/fair.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d711093..cfca960 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8659,6 +8659,10 @@ void nohz_balance_enter_idle(int cpu)
> if (!cpu_active(cpu))
> return;
>
> + /* Spare idle load balancing on CPUs that don't want to be
> disturbed */
> + if (!is_housekeeping_cpu(cpu))
> + return;
> +
> if (test_bit(NOHZ_TICK_STOPPED, nohz_flags(cpu)))
> return;
I am not entirely convinced on this one.
Doesn't the if (on_null_domain(cpu_rq(cpu)) test
a few lines down take care of this already?
Do we want nohz_full to always automatically
imply that no idle balancing will happen, like
on isolated CPUs?
--
All rights reversed
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next prev parent reply other threads:[~2017-06-20 17:42 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-19 2:11 [PATCH 0/3] sched: A few nohz_full improvements Frederic Weisbecker
2017-06-19 2:12 ` [PATCH 1/3] sched/loadavg: Generalize idle naming to nohz Frederic Weisbecker
2017-06-20 17:38 ` Rik van Riel
2017-06-22 11:10 ` [tip:sched/core] sched/loadavg: Generalize "_idle" naming to "_nohz" tip-bot for Frederic Weisbecker
2017-06-19 2:12 ` [PATCH 2/3] nohz: Move idle balancer registration to idle path Frederic Weisbecker
2017-06-20 17:39 ` Rik van Riel
2017-06-22 11:11 ` [tip:sched/core] nohz: Move idle balancer registration to the " tip-bot for Frederic Weisbecker
2017-06-19 2:12 ` [PATCH 3/3] sched: Spare idle load balancing on nohz_full CPUs Frederic Weisbecker
2017-06-20 17:42 ` Rik van Riel [this message]
2017-06-20 19:06 ` Mike Galbraith
2017-06-21 13:23 ` Frederic Weisbecker
2017-06-20 20:26 ` Frederic Weisbecker
2017-06-22 11:11 ` [tip:sched/core] sched/fair: " tip-bot for Frederic Weisbecker
2017-06-22 13:52 ` Frederic Weisbecker
2017-06-22 19:47 ` Ingo Molnar
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