linux-kernel - Re: [patch 2/2] nohz: change signal tick dependency to wakeup CPUs of member tasks

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Date:   Thu, 8 Oct 2020 15:04:41 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Peter Xu <peterx@...hat.com>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [patch 2/2] nohz: change signal tick dependency to wakeup CPUs
 of member tasks

On Thu, Oct 08, 2020 at 02:35:44PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 07, 2020 at 03:01:53PM -0300, Marcelo Tosatti wrote:
> > Rather than waking up all nohz_full CPUs on the system, only wakeup 
> > the target CPUs of member threads of the signal.
> > 
> > Reduces interruptions to nohz_full CPUs.
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> > 
> > Index: linux-2.6/kernel/time/tick-sched.c
> > ===================================================================
> > --- linux-2.6.orig/kernel/time/tick-sched.c
> > +++ linux-2.6/kernel/time/tick-sched.c
> > @@ -398,7 +398,15 @@ EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_ta
> >   */
> >  void tick_nohz_dep_set_signal(struct signal_struct *sig, enum tick_dep_bits bit)
> >  {
> > -	tick_nohz_dep_set_all(&sig->tick_dep_mask, bit);
> > +	int prev;
> > +
> > +	prev = atomic_fetch_or(BIT(bit), &sig->tick_dep_mask);
> > +	if (!prev) {
> > +		rcu_read_lock();
> > +		for_each_thread(sig, t)
> > +			tick_nohz_kick_task(t);
> > +		rcu_read_unlock();
> > +	}
> >  }
> 
> AFAICT, and this makes perfect sense, this function is only ever used
> while holding sighand->siglock, which makes the RCU read lock
> superfluous.
> 
> Would it make sense to change the signal_struct argument to task_struct,
> such that we can write:
> 
> 	lockdep_assert_held(&p->sighand->siglock);
> 	for_each_thread(p->signal, t)
> 		tick_nohz_kick_task(t);
> 
> ?

Makes sense, resending -v3.