linux-kernel - Re: [PATCH] sched/fair: ignore cache hotness for SMT migration

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Message-ID: <20200804105619.GE2657@hirez.programming.kicks-ass.net>
Date:   Tue, 4 Aug 2020 12:56:19 +0200
From:   peterz@...radead.org
To:     Josh Don <joshdon@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: ignore cache hotness for SMT migration

On Mon, Aug 03, 2020 at 05:06:14PM -0700, Josh Don wrote:
> SMT siblings share caches, so cache hotness should be irrelevant for
> cross-sibling migration.
> 
> Proposed-by: Venkatesh Pallipadi <venki@...gle.com>
> Signed-off-by: Josh Don <joshdon@...gle.com>
> ---
>  kernel/sched/fair.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2ba8f230feb9..5b203b55bcb2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7402,6 +7402,9 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
>  	if (unlikely(task_has_idle_policy(p)))
>  		return 0;
>  
> +	if ((env->sd->flags & cpu_smt_flags()) == cpu_smt_flags())
> +		return 0;

I think that wants to be:

	if (env->sd->flags & SD_SHARE_CPUCAPACITY)

Also, perhaps stick a comment on top with the rationale for this.

> +
>  	/*
>  	 * Buddy candidates are cache hot:
>  	 */
> -- 
> 2.28.0.163.g6104cc2f0b6-goog
>