linux-kernel - Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a task

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Message-ID: <20201015101235.GA2611@hirez.programming.kicks-ass.net>
Date:   Thu, 15 Oct 2020 12:12:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        linux-kernel@...r.kernel.org,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a
 task

On Thu, Oct 15, 2020 at 01:40:53AM +0200, Frederic Weisbecker wrote:
> > re tick_nohz_task_switch() being placed wrong, it should probably be
> > placed before finish_lock_switch(). Something like so.
> > 
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index cf044580683c..5c92c959824f 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4084,6 +4084,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
> >  	vtime_task_switch(prev);
> >  	perf_event_task_sched_in(prev, current);
> >  	finish_task(prev);
> > +	tick_nohz_task_switch();
> >  	finish_lock_switch(rq);
> >  	finish_arch_post_lock_switch();
> >  	kcov_finish_switch(current);
> > @@ -4121,7 +4122,6 @@ static struct rq *finish_task_switch(struct task_struct *prev)
> >  		put_task_struct_rcu_user(prev);
> >  	}
> >  
> > -	tick_nohz_task_switch();
> 
> IIRC, we wanted to keep it outside rq lock because it shouldn't it...

But now you've created a window with IRQs on and cause additional IRQ
state changes.

If you're really worried about rq->lock, I suppose we can do:

	rq_unlock(rq->lock);
	tick_nohz_task_switch();
	local_irq_enable();

(much like we do at the beginning of __schedule for RCU)