linux-kernel - Re: [PATCH v2 1/2] sched: force update of blocked load of idle cpus

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Date:   Thu, 21 Dec 2017 08:59:34 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Brendan Jackman <brendan.jackman@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH v2 1/2] sched: force update of blocked load of idle cpus

Le Wednesday 20 Dec 2017 à 16:01:10 (+0100), Peter Zijlstra a écrit :
> On Wed, Dec 20, 2017 at 03:23:01PM +0100, Vincent Guittot wrote:
> > On 20 December 2017 at 15:03, Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Fri, Dec 01, 2017 at 06:01:56PM +0000, Brendan Jackman wrote:
> > >> @@ -8955,8 +8964,20 @@ static void nohz_balancer_kick(void)
> > >>       if (ilb_cpu >= nr_cpu_ids)
> > >>               return;
> > >>
> > >> -     if (test_and_set_bit(NOHZ_BALANCE_KICK, nohz_flags(ilb_cpu)))
> > >> +     if (test_and_set_bit(NOHZ_BALANCE_KICK, nohz_flags(ilb_cpu))) {
> > >> +             if (!only_update) {
> > >> +                     /*
> > >> +                      * There's a pending/ongoing nohz kick/balance. If it's
> > >> +                      * just for stats, convert it to a proper load balance.
> > >> +                      */
> > >> +                     clear_bit(NOHZ_STATS_KICK, nohz_flags(ilb_cpu));
> > >> +             }
> > >>               return;
> > >> +     }
> > >> +
> > >> +     if (only_update)
> > >> +             set_bit(NOHZ_STATS_KICK, nohz_flags(ilb_cpu));
> > >> +
> > >>       /*
> > >>        * Use smp_send_reschedule() instead of resched_cpu().
> > >>        * This way we generate a sched IPI on the target cpu which
> > >
> > > This looks racy.. if its not we don't need atomic ops, if it is but is
> > > still fine it needs a comment.
> > 
> > NOHZ_STATS_KICK modification is protected by test_and_set_bit(NOHZ_BALANCE_KICK.
> > You're right that we don't need atomics ops and __set_bit() is enough
> 
> Well, you shouldn't mix atomic and non-atomic ops to the same word,
> that's asking for trouble.

In fact, the atomic operation is needed, I forgot that set/clear of NOHZ_TICK_STOPPED
can happen simultaneously on the ilb_cpu so we must keep
set_bit(NOHZ_STATS_KICK, nohz_flags(ilb_cpu));

As comment we can add:
 /*
  * Update of NOHZ_STATS_KICK itself is protected by test_and_set_biti but
  * clear/set of NOHZ_TICK_STOPPED can happen simultaneously so we need
  * atomic operation
  */

> 
> But why don't you do something like:
> 
> nohz_kick()
> 
> 	flags = NOHZ_STAT;
> 	if (!only_update)
> 		flags |= NOHZ_BALANCE;
> 
> 	atomic_long_or(flags, &nohz_cpu(cpu));
> 
> 
> nohz_idle_balance()
> 
> 	unsigned long do_flags = atomic_long_fetch_andnot(NOHZ_BALANCE|NOHZ_STAT, &nohz_flags(cpu));
> 
> 	if (do_flags & NOHZ_STAT)
> 		update_blocked_stuff();
> 
> 	if (do_flags & NOHZ_BALANCE)
> 		rebalance_domains();
> 
> That way its far more readable.
>