Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

From: Lai Jiangshan <jiangshanlai@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>, Qian Cai <cai@redhat.com>,
	Vincent Donnefort <vincent.donnefort@arm.com>,
	Dexuan Cui <decui@microsoft.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Steven Rostedt <rostedt@goodmis.org>, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU
Date: Sat, 16 Jan 2021 22:45:04 +0800	[thread overview]
Message-ID: <CAJhGHyDT2FWsn15-_DQ4b_bkrRi74MzNnWt7YWTO49cSv4yjbg@mail.gmail.com> (raw)
In-Reply-To: <YALf4xDwTKCERPbf@hirez.programming.kicks-ass.net>

On Sat, Jan 16, 2021 at 8:45 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Sat, Jan 16, 2021 at 02:27:09PM +0800, Lai Jiangshan wrote:
> > On Thu, Jan 14, 2021 at 11:35 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > >
> > > -void kthread_set_per_cpu(struct task_struct *k, bool set)
> > > +void kthread_set_per_cpu(struct task_struct *k, int cpu)
> > >  {
> > >         struct kthread *kthread = to_kthread(k);
> > >         if (!kthread)
> > >                 return;
> > >
> > > -       if (set) {
> > > -               WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY));
> > > -               WARN_ON_ONCE(k->nr_cpus_allowed != 1);
> > > -               set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
> > > -       } else {
> > > +       WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY));
> > > +
> > > +       if (cpu < 0) {
> > >                 clear_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
> > > +               return;
> > >         }
> > > +
> > > +       kthread->cpu = cpu;
> > > +       set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
> > >  }
> > >
> >
> > I don't see the code to set the mask of the cpu to the task
> > since set_cpus_allowed_ptr() is removed from rebind_worker().
> >
> > Is it somewhere I missed?
>
> kthread_unpark().
>
> > > @@ -4978,9 +4982,9 @@ static void rebind_workers(struct worker_pool *pool)
> > >          * from CPU_ONLINE, the following shouldn't fail.
> > >          */
> > >         for_each_pool_worker(worker, pool) {
> > > -               WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
> > > -                                                 pool->attrs->cpumask) < 0);
> > > -               kthread_set_per_cpu(worker->task, true);
> > > +               WARN_ON_ONCE(kthread_park(worker->task) < 0);
> > > +               kthread_set_per_cpu(worker->task, pool->cpu);
> > > +               kthread_unpark(worker->task);
> >
> > I feel nervous to use kthread_park() here and kthread_parkme() in
> > worker thread.  And adding kthread_should_park() to the fast path
> > also daunt me.
>
> Is that really such a hot path that an additional load is problematic?
>
> > How about using a new KTHREAD_XXXX instead of KTHREAD_IS_PER_CPU,
> > so that we can set and clear KTHREAD_XXXX freely, especially before
> > set_cpus_allowed_ptr().
>
> KTHREAD_IS_PER_CPU is exactly what we need, why make another flag?
>
> The above sequence is nice in that it restores both the
> KTHREAD_IS_PER_CPU flag and affinity while the task is frozen, so there
> are no races where one is observed and not the other.
>
> It is also the exact sequence normal per-cpu threads (smpboot) use to
> preserve affinity.

Other per-cpu threads normally do short-live works. wq's work can be
lengthy, cpu-intensive, heavy-lock-acquiring or even call
get_online_cpus() which might result in a deadlock with kthread_park().