Re: [PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity

From: Quentin Perret <qperret@google.com>
To: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Qais Yousef <qais.yousef@arm.com>,
	Suren Baghdasaryan <surenb@google.com>, Tejun Heo <tj@kernel.org>,
	Li Zefan <lizefan@huawei.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	kernel-team@android.com
Subject: Re: [PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity
Date: Thu, 19 Nov 2020 11:03:23 +0000	[thread overview]
Message-ID: <20201119110323.GA2432333@google.com> (raw)
In-Reply-To: <20201119091820.GA2416649@google.com>

On Thursday 19 Nov 2020 at 09:18:20 (+0000), Quentin Perret wrote:
> Hey Will,
> 
> On Friday 13 Nov 2020 at 09:37:12 (+0000), Will Deacon wrote:
> > -static int __set_cpus_allowed_ptr(struct task_struct *p,
> > -				  const struct cpumask *new_mask, bool check)
> > +static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
> > +					 const struct cpumask *new_mask,
> > +					 bool check,
> > +					 struct rq *rq,
> > +					 struct rq_flags *rf)
> >  {
> >  	const struct cpumask *cpu_valid_mask = cpu_active_mask;
> >  	unsigned int dest_cpu;
> > -	struct rq_flags rf;
> > -	struct rq *rq;
> >  	int ret = 0;
> 
> Should we have a lockdep assertion here?
> 
> > -	rq = task_rq_lock(p, &rf);
> >  	update_rq_clock(rq);
> >  
> >  	if (p->flags & PF_KTHREAD) {
> > @@ -1929,7 +1923,7 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
> >  	if (task_running(rq, p) || p->state == TASK_WAKING) {
> >  		struct migration_arg arg = { p, dest_cpu };
> >  		/* Need help from migration thread: drop lock and wait. */
> > -		task_rq_unlock(rq, p, &rf);
> > +		task_rq_unlock(rq, p, rf);
> >  		stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
> >  		return 0;
> >  	} else if (task_on_rq_queued(p)) {
> > @@ -1937,20 +1931,69 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
> >  		 * OK, since we're going to drop the lock immediately
> >  		 * afterwards anyway.
> >  		 */
> > -		rq = move_queued_task(rq, &rf, p, dest_cpu);
> > +		rq = move_queued_task(rq, rf, p, dest_cpu);
> >  	}
> >  out:
> > -	task_rq_unlock(rq, p, &rf);
> > +	task_rq_unlock(rq, p, rf);
> 
> And that's a little odd to have here no? Can we move it back on the
> caller's side?

Yeah, no, that obviously doesn't work for the stop_one_cpu() call above,
so feel free to ignore ...

Thanks,
Quentin