linux-kernel - Re: [PATCH 08/10] sched/balancing: Update run_rebalance

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Date: Fri, 8 Mar 2024 13:02:10 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, 
	Shrikanth Hegde <sshegde@...ux.ibm.com>, Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH 08/10] sched/balancing: Update run_rebalance_domains() comments

On Fri, 8 Mar 2024 at 11:59, Ingo Molnar <mingo@...nel.org> wrote:
>
> The first sentence of the comment explaining run_rebalance_domains()
> is historic and not true anymore:
>
>     * run_rebalance_domains is triggered when needed from the scheduler tick.
>
> ... contradicted/modified by the second sentence:
>
>     * Also triggered for NOHZ idle balancing (with NOHZ_BALANCE_KICK set).
>
> Avoid that kind of confusion straight away and explain from what
> places sched_balance_softirq() is triggered.
>
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Shrikanth Hegde <sshegde@...ux.ibm.com>
> Cc: Valentin Schneider <vschneid@...hat.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>

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

> ---
>  kernel/sched/fair.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f3c03c6db3c8..b567c0790f44 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12409,9 +12409,12 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>  }
>
>  /*
> - * This softirq may be triggered from the scheduler tick, or by
> - * any of the flags in NOHZ_KICK_MASK: NOHZ_BALANCE_KICK,
> - * NOHZ_STATS_KICK or NOHZ_NEXT_KICK.
> + * This softirq handler is triggered via SCHED_SOFTIRQ from two places:
> + *
> + * - directly from the local scheduler_tick() for periodic load balancing
> + *
> + * - indirectly from a remote scheduler_tick() for NOHZ idle balancing
> + *   through the SMP cross-call nohz_csd_func()
>   */
>  static __latent_entropy void run_rebalance_domains(struct softirq_action *h)
>  {
> --
> 2.40.1
>