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

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Message-ID: <20251106110943.GP3245006@noisy.programming.kicks-ass.net>
Date: Thu, 6 Nov 2025 12:09:43 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: Fernand Sieber <sieberf@...zon.com>, 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

On Wed, Nov 05, 2025 at 10:04:01PM +0530, K Prateek Nayak wrote:
> 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?

I don't think it really matters, but someone what runs this stuff would
probably know better than me ;-)

> If not, then this looks good to me. Feel free to include:
> 
> Reviewed-by: K Prateek Nayak <kprateek.nayak@....com>

Thanks all!