linux-kernel - Re: [RFC PATCH 7/9] sched: search SMT before LLC domain

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Message-ID: <20190905093127.GI2349@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Sep 2019 11:31:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     subhra mazumdar <subhra.mazumdar@...cle.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
        steven.sistare@...cle.com, dhaval.giani@...cle.com,
        daniel.lezcano@...aro.org, vincent.guittot@...aro.org,
        viresh.kumar@...aro.org, tim.c.chen@...ux.intel.com,
        mgorman@...hsingularity.net, parth@...ux.ibm.com,
        patrick.bellasi@....com
Subject: Re: [RFC PATCH 7/9] sched: search SMT before LLC domain

On Fri, Aug 30, 2019 at 10:49:42AM -0700, subhra mazumdar wrote:
> Search SMT siblings before all CPUs in LLC domain for idle CPU. This helps
> in L1 cache locality.
> ---
>  kernel/sched/fair.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8856503..94dd4a32 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6274,11 +6274,11 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  			return i;
>  	}
>  
> -	i = select_idle_cpu(p, sd, target);
> +	i = select_idle_smt(p, target);
>  	if ((unsigned)i < nr_cpumask_bits)
>  		return i;
>  
> -	i = select_idle_smt(p, target);
> +	i = select_idle_cpu(p, sd, target);
>  	if ((unsigned)i < nr_cpumask_bits)
>  		return i;
>  

But it is absolutely conceptually wrong. An idle core is a much better
target than an idle sibling.