linux-kernel - Re: [RESEND] sched/rt: Skip currently executing CPU in rto_next

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Message-ID: <20260106104911.54fac7f4@gandalf.local.home>
Date: Tue, 6 Jan 2026 10:49:11 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: Chen Jinghuang <chenjinghuang2@...wei.com>, <mingo@...hat.com>,
 <peterz@...radead.org>, <juri.lelli@...hat.com>,
 <vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
 <bsegall@...gle.com>, <mgorman@...e.de>, <vschneid@...hat.com>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND] sched/rt: Skip currently executing CPU in
 rto_next_cpu()

On Tue, 6 Jan 2026 14:11:57 +0530
K Prateek Nayak <kprateek.nayak@....com> wrote:

> Hello Chen, Steve,
> 
> On 1/5/2026 9:10 AM, Chen Jinghuang wrote:
> > @@ -2118,10 +2119,13 @@ static int rto_next_cpu(struct root_domain *rd)
> >  	 */
> >  	for (;;) {
> >  
> > -		/* When rto_cpu is -1 this acts like cpumask_first() */
> > -		cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
> > +		do {
> > +			/* When rto_cpu is -1 this acts like cpumask_first() */
> > +			cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);
> >  
> > -		rd->rto_cpu = cpu;
> > +			rd->rto_cpu = cpu;
> > +			/* Do not send IPI to self */
> > +		} while (cpu == this_cpu);  
> 
> nit.
> 
> Since we are already within an infinite for-loop, can't we simply do:
> 
>     /* Do not send IPI to self */
>     if (cpu == this_cpu)
>         continue;
> 
> here and go evaluate cpumask_next() again instead of adding another
> do-while? Was the nested loop intentional to highlight these bits
> explicitly?

Hmm, yeah, I think I added the loop to express this issue. But I think your
suggestion could work too. I originally had something like that, but for
some reason I thought it added an extra branch (over the added do { } while).

> 
> >  
> >  		if (cpu < nr_cpu_ids)
> >  			return cpu;  
> 


	for (;;) {

		/* When rto_cpu is -1 this acts like cpumask_first() */
		cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);

		rd->rto_cpu = cpu;

+		/* Do not send IPI to self */
+		if (cpu == this_cpu)
+			continue;
+
		if (cpu < nr_cpu_ids)
			return cpu;

		rd->rto_cpu = -1;

		/*
		 * ACQUIRE ensures we see the @rto_mask changes
		 * made prior to the @next value observed.
		 *
		 * Matches WMB in rt_set_overload().
		 */
		next = atomic_read_acquire(&rd->rto_loop_next);

		if (rd->rto_loop == next)
			break;

		rd->rto_loop = next;
	}

Looks to be equivalent.

Chen, care to send a new version?

-- Steve