linux-kernel - Re: [RFC][PATCH 1/3] sched: Detect per-class runqueue changes

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Message-ID: <aOTq19a_OIONFmTs@jlelli-thinkpadt14gen4.remote.csb>
Date: Tue, 7 Oct 2025 12:26:31 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: tj@...nel.org, linux-kernel@...r.kernel.org, mingo@...nel.org,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, longman@...hat.com, hannes@...xchg.org,
	mkoutny@...e.com, void@...ifault.com, arighi@...dia.com,
	changwoo@...lia.com, cgroups@...r.kernel.org,
	sched-ext@...ts.linux.dev, liuwenfang@...or.com, tglx@...utronix.de
Subject: Re: [RFC][PATCH 1/3] sched: Detect per-class runqueue changes

On 07/10/25 12:16, Peter Zijlstra wrote:
> On Tue, Oct 07, 2025 at 12:08:03PM +0200, Juri Lelli wrote:
> > Hi Peter,
> > 
> > On 06/10/25 12:46, Peter Zijlstra wrote:
> > > Have enqueue/dequeue set a per-class bit in rq->queue_mask. This then
> > > enables easy tracking of which runqueues are modified over a
> > > lock-break.
> > > 
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > ---
> > 
> > Nice.
> > 
> > > @@ -12887,8 +12888,8 @@ static int sched_balance_newidle(struct
> > >  	if (this_rq->cfs.h_nr_queued && !pulled_task)
> > >  		pulled_task = 1;
> > >  
> > > -	/* Is there a task of a high priority class? */
> > > -	if (this_rq->nr_running != this_rq->cfs.h_nr_queued)
> > > +	/* If a higher prio class was modified, restart the pick */
> > > +	if (this_rq->queue_mask & ~((fair_sched_class.queue_mask << 1)-1))
> > >  		pulled_task = -1;
> > 
> > Does this however want a self-documenting inline helper or macro to make
> > it even more clear? If this is always going to be the only caller maybe
> > not so much.
> 
> There's another one in patch 3. I suppose we can do that. Maybe
> something like:
> 
> static inline bool rq_modified_above(struct rq *rq, struct sched_class *class)
> {
> 	unsigned int mask = class->queue_mask;
> 	return rq->queue_mask & ~((mask << 1) - 1);
> }
> 
> This then writes the above like:
> 
> 	if (rq_modified_above(this_rq, &fair_sched_class))
> 

Yeah. Maybe also add a "check rq::queue_mask comment for additional
details" or something like this.

Thanks!
Juri