linux-kernel - Re: [PATCH 3/3] sched: Spare idle load balancing on nohz

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Date:   Tue, 20 Jun 2017 13:42:27 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Frederic Weisbecker <fweisbec@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 3/3] sched: Spare idle load balancing on nohz_full CPUs

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@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.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?

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