linux-kernel - Re: [PATCH 12/14] sched: Add shared runqueue locking to __task_rq

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Message-ID: <20250912141156.GE1386988@noisy.programming.kicks-ass.net>
Date: Fri, 12 Sep 2025 16:11:56 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Tejun Heo <tj@...nel.org>
Cc: linux-kernel@...r.kernel.org, mingo@...hat.com, juri.lelli@...hat.com,
	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: [PATCH 12/14] sched: Add shared runqueue locking to
 __task_rq_lock()

On Fri, Sep 12, 2025 at 01:54:59PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 11, 2025 at 02:19:57PM -1000, Tejun Heo wrote:
> > Hello,
> > 
> > On Wed, Sep 10, 2025 at 05:44:21PM +0200, Peter Zijlstra wrote:
> > > @@ -703,17 +703,24 @@ void double_rq_lock(struct rq *rq1, stru
> > >  struct rq *__task_rq_lock(struct task_struct *p, struct rq_flags *rf)
> > >  	__acquires(rq->lock)
> > >  {
> > > +	raw_spinlock_t *slock;
> > >  	struct rq *rq;
> > >  
> > >  	lockdep_assert_held(&p->pi_lock);
> > >  
> > >  	for (;;) {
> > >  		rq = task_rq(p);
> > > +		slock = p->srq_lock;
> > >  		raw_spin_rq_lock(rq);
> > > -		if (likely(rq == task_rq(p) && !task_on_rq_migrating(p))) {
> > > +		if (slock)
> > > +			raw_spin_lock(slock);
> > > +		if (likely(rq == task_rq(p) && !task_on_rq_migrating(p) &&
> > > +			   (!slock || p->srq_lock == slock))) {
> > >  			rq_pin_lock(rq, rf);
> > >  			return rq;
> > >  		}
> 
> Yeah, I think that needs to change a little. Perhaps something like:
> 
> 	slock2 = p->srq_lock;
> 	if (... && (!slock2 || slock2 == slock))

I'm being stupid, all that wants is: && (p->srq_lock == slock). If there
is a mis-match, unlock and re-try.