linux-kernel - Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed

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Message-ID: <ZwEfw3og4WuzXI7l@pavilion.home>
Date: Sat, 5 Oct 2024 13:15:15 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Leonardo Bras Soares Passos <leobras@...hat.com>
Cc: Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
	linux-kernel@...r.kernel.org, Junyao Zhao <junzhao@...hat.com>
Subject: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on
 queue_delayed_work

Le Fri, Oct 04, 2024 at 07:43:34PM -0300, Leonardo Bras Soares Passos a écrit :
> Well, back when it was merged, I could see add_timer() using an
> isolated CPU, and that's why I sent this change.
> Did this change recently?

That was (and still is) not supposed to happen because get_nohz_timer_target()
forbids that. Since then we have merged tmigr that also takes care of it.

So something looks broken...unless you had kernel.timer_migration=0 ?

> 
> Also, previously add_timer() was only called if (cpu ==
> WORK_CPU_UNBOUND), and now we make sure that for any value of 'cpu'
> the timer to be put in a housekeeping cpu.

I suppose it makes the situation slightly less worse when a work is bound
to an isolated CPU. You get the workqueue without the timer. But the context
switch/workqueue is the most unwanted part :-)

Thanks.

> 
> Thanks!
> Leo
> 
> > Thanks.
> >
> > >               add_timer_on(timer, cpu);
> > > -     else
> > > -             add_timer(timer);
> > > +     } else {
> > > +             if (likely(cpu == WORK_CPU_UNBOUND))
> > > +                     add_timer(timer);
> > > +             else
> > > +                     add_timer_on(timer, cpu);
> > > +     }
> >
> > >  }
> > >
> > >  /**
> > > --
> > > 2.43.0
> > >
> > >
> >
>