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

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Date:   Wed, 20 Dec 2017 16:01:10 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.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

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.

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.