From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org, mingo@redhat.com,
tglx@linutronix.de
Subject: Re: [PATCH] time/tick-broadcast: Fix tick_broadcast_offline() lockdep complaint
Date: Mon, 24 Jun 2019 16:44:22 -0700 [thread overview]
Message-ID: <20190624234422.GP26519@linux.ibm.com> (raw)
In-Reply-To: <20190624231222.GA17497@lerouge>
On Tue, Jun 25, 2019 at 01:12:23AM +0200, Frederic Weisbecker wrote:
> On Fri, Jun 21, 2019 at 04:46:02PM -0700, Paul E. McKenney wrote:
> > @@ -3097,13 +3126,21 @@ static void sched_tick_remote(struct work_struct *work)
> > /*
> > * Run the remote tick once per second (1Hz). This arbitrary
> > * frequency is large enough to avoid overload but short enough
> > - * to keep scheduler internal stats reasonably up to date.
> > + * to keep scheduler internal stats reasonably up to date. But
> > + * first update state to reflect hotplug activity if required.
> > */
> > + os = atomic_read(&twork->state);
> > + if (os) {
> > + WARN_ON_ONCE(os != TICK_SCHED_REMOTE_OFFLINING);
> > + if (atomic_inc_not_zero(&twork->state))
> > + return;
>
> Using inc makes me a bit nervous here. If we do so, we should somewhow
> make sure that we never exceed a value higher than TICK_SCHED_REMOTE_OFFLINE
> by accident.
>
> atomic_xchg() is probably a bit costlier but also safer as it allows
> us to check both the old and the new value. That path shouldn't be critically fast
> after all.
It would need to be cmpxchg() to avoid messing with the state if
the state were somehow TICK_SCHED_REMOTE_RUNNING, right?
> > + }
> > queue_delayed_work(system_unbound_wq, dwork, HZ);
> > }
> >
> > static void sched_tick_start(int cpu)
> > {
> > + int os;
> > struct tick_work *twork;
> >
> > if (housekeeping_cpu(cpu, HK_FLAG_TICK))
> > @@ -3112,15 +3149,20 @@ static void sched_tick_start(int cpu)
> > WARN_ON_ONCE(!tick_work_cpu);
> >
> > twork = per_cpu_ptr(tick_work_cpu, cpu);
> > - twork->cpu = cpu;
> > - INIT_DELAYED_WORK(&twork->work, sched_tick_remote);
> > - queue_delayed_work(system_unbound_wq, &twork->work, HZ);
> > + os = atomic_xchg(&twork->state, TICK_SCHED_REMOTE_RUNNING);
> > + WARN_ON_ONCE(os == TICK_SCHED_REMOTE_RUNNING);
>
> See if we use atomic_inc(), we would need to also WARN(os > TICK_SCHED_REMOTE_OFFLINE).
How about if I put that WARN() between the atomic_inc_not_zero() and
the return, presumably also adding braces?
Thanx, Paul
> > + if (os == TICK_SCHED_REMOTE_OFFLINE) {
> > + twork->cpu = cpu;
> > + INIT_DELAYED_WORK(&twork->work, sched_tick_remote);
> > + queue_delayed_work(system_unbound_wq, &twork->work, HZ);
> > + }
> > }
>
> Thanks.
>
next prev parent reply other threads:[~2019-06-24 23:44 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-19 18:19 [PATCH] time/tick-broadcast: Fix tick_broadcast_offline() lockdep complaint Paul E. McKenney
2019-06-20 12:10 ` Peter Zijlstra
2019-06-20 16:01 ` Paul E. McKenney
2019-06-20 21:10 ` Peter Zijlstra
2019-06-20 22:13 ` Paul E. McKenney
2019-06-21 10:55 ` Peter Zijlstra
2019-06-21 12:16 ` Paul E. McKenney
2019-06-21 12:29 ` Peter Zijlstra
2019-06-21 13:34 ` Paul E. McKenney
2019-06-21 17:41 ` Paul E. McKenney
2019-06-21 17:50 ` Paul E. McKenney
2019-06-21 23:46 ` Paul E. McKenney
2019-06-24 23:12 ` Frederic Weisbecker
2019-06-24 23:44 ` Paul E. McKenney [this message]
2019-06-25 0:43 ` Frederic Weisbecker
2019-06-25 2:05 ` Paul E. McKenney
2019-06-25 7:51 ` Peter Zijlstra
2019-06-25 12:25 ` Frederic Weisbecker
2019-06-25 13:54 ` Paul E. McKenney
2019-06-25 14:05 ` Peter Zijlstra
2019-06-25 14:16 ` Paul E. McKenney
2019-06-25 16:20 ` Frederic Weisbecker
2019-06-25 16:52 ` Paul E. McKenney
2019-06-28 7:37 ` Peter Zijlstra
2019-06-28 12:17 ` Paul E. McKenney
2019-07-25 16:14 ` [tip:sched/core] " tip-bot for Paul E. McKenney
-- strict thread matches above, loose matches on Subject: below --
2019-05-27 14:39 [PATCH] " Paul E. McKenney
2019-05-28 20:07 ` Thomas Gleixner
2019-05-29 18:19 ` Paul E. McKenney
2019-05-30 12:58 ` Paul E. McKenney
2019-05-31 1:36 ` Frederic Weisbecker
2019-05-31 13:43 ` Paul E. McKenney
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