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

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Date:   Mon, 26 Oct 2020 15:42:53 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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 12:12:35PM +0200, Peter Zijlstra wrote:
> 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)

Right. Well I'm not that worried about rq->lock though. If you're ok
with it I can just move it before finish_lock_switch().

Thanks.