linux-kernel - Re: [PATCH] sched: Optimize core cookie matching check

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Message-ID: <f7169de1-76f4-4bde-8d9a-cc62b7e3157a@amd.com>
Date: Wed, 5 Nov 2025 22:04:01 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Fernand Sieber <sieberf@...zon.com>, <peterz@...radead.org>
CC: <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	<juri.lelli@...hat.com>, <vincent.guittot@...aro.org>,
	<dietmar.eggemann@....com>, <rostedt@...dmis.org>, <bsegall@...gle.com>,
	<mgorman@...e.de>, <bristot@...hat.com>, <vschneid@...hat.com>,
	<dwmw@...zon.co.uk>, <jschoenh@...zon.de>, <liuyuxua@...zon.com>
Subject: Re: [PATCH] sched: Optimize core cookie matching check

Hello Fernand,

On 11/5/2025 8:55 PM, Fernand Sieber wrote:
> Early return true if the core cookie matches. This avoids the SMT mask
> loop to check for an idle core, which might be more expensive on wide
> platforms.
> 
> Signed-off-by: Fernand Sieber <sieberf@...zon.com>
> ---
>  kernel/sched/sched.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index adfb6e3409d7..381cd561e99b 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1432,6 +1432,9 @@ static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
>  	if (!sched_core_enabled(rq))
>  		return true;
>  
> +	if (rq->core->core_cookie == p->core_cookie)
> +		return true;

nit. We can use sched_cpu_cookie_match(rq, p) to check for the
above two conditions. but even this is good.

Apart from that, I think this optimization makes sense.

> +
>  	for_each_cpu(cpu, cpu_smt_mask(cpu_of(rq))) {
>  		if (!available_idle_cpu(cpu)) {
>  			idle_core = false;
> @@ -1443,7 +1446,7 @@ static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
>  	 * A CPU in an idle core is always the best choice for tasks with
>  	 * cookies.
>  	 */
> -	return idle_core || rq->core->core_cookie == p->core_cookie;
> +	return idle_core;

Peter, do we care about checking the core_cookie again before
returning just in case the task cookie was selected between the
check above an here?

If not, then this looks good to me. Feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@....com>

>  }
>  
>  static inline bool sched_group_cookie_match(struct rq *rq,

-- 
Thanks and Regards,
Prateek