linux-kernel - Re: [PATCH 6/8] tick/nohz: Only wakeup a single target cpu when kicking a task

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Date:   Mon, 10 May 2021 12:48:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Yunfeng Ye <yeyunfeng@...wei.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH 6/8] tick/nohz: Only wakeup a single target cpu when
 kicking a task

On Mon, May 10, 2021 at 12:39:01PM +0200, Frederic Weisbecker wrote:
> On Wed, May 05, 2021 at 03:43:28PM +0200, Peter Zijlstra wrote:
> > That had me looking at tick_nohz_task_switch(), does we want the below?
> > 
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 9143163fa678..ff45fc513ba7 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4207,6 +4207,7 @@ 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);
> > @@ -4252,7 +4253,6 @@ static struct rq *finish_task_switch(struct task_struct *prev)
> >  		put_task_struct_rcu_user(prev);
> >  	}
> >  
> > -	tick_nohz_task_switch();
> >  	return rq;
> >  }
> >  
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 828b091501ca..ea079be9097f 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -447,13 +447,10 @@ void tick_nohz_dep_clear_signal(struct signal_struct *sig, enum tick_dep_bits bi
> >   */
> >  void __tick_nohz_task_switch(void)
> >  {
> > -	unsigned long flags;
> >  	struct tick_sched *ts;
> >  
> > -	local_irq_save(flags);
> > -
> >  	if (!tick_nohz_full_cpu(smp_processor_id()))
> > -		goto out;
> > +		return;
> >  
> >  	ts = this_cpu_ptr(&tick_cpu_sched);
> >  
> > @@ -462,8 +459,6 @@ void __tick_nohz_task_switch(void)
> >  		    atomic_read(&current->signal->tick_dep_mask))
> >  			tick_nohz_full_kick();
> >  	}
> > -out:
> > -	local_irq_restore(flags);
> >  }
> >  
> >  /* Get the boot-time nohz CPU list from the kernel parameters. */
> 
> 
> Sure, I'll take your SoB on that too, ok?

OK, but please also test it :-) I didn't even ask a compiler it's
opinion on the thing.