From: Will Deacon <will@kernel.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Peter Zijlstra <peterz@infradead.org>,
Davidlohr Bueso <dave@stgolabs.net>,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org
Subject: Re: Loadavg accounting error on arm64
Date: Mon, 16 Nov 2020 17:41:20 +0000 [thread overview]
Message-ID: <20201116174119.GB30723@willie-the-truck> (raw)
In-Reply-To: <20201116172444.GV3371@techsingularity.net>
On Mon, Nov 16, 2020 at 05:24:44PM +0000, Mel Gorman wrote:
> On Mon, Nov 16, 2020 at 05:49:28PM +0100, Peter Zijlstra wrote:
> > > So while we might be able to avoid a smp_rmb() before the read of
> > > sched_contributes_to_load and rely on p->on_cpu ordering there,
> > > we may still need a smp_wmb() after nr_interruptible() increments
> > > instead of waiting until the smp_store_release() is hit while a task
> > > is scheduling. That would be a real memory barrier on arm64 and a plain
> > > compiler barrier on x86-64.
> >
>
> Wish I read this before sending the changelog
>
> > I'm mighty confused by your words here; and the patch below. What actual
> > scenario are you worried about?
> >
>
> The wrong one apparently. Even if the IRQ is released, the IPI would
> deliver to the CPU that should observe the correct value or take the
> other path when smp_cond_load_acquire(&p->on_cpu, !VAL) waits for the
> schedule to finish so I'm now both confused and wondering why smp_wmb
> made a difference at all.
Probably still worth trying Peter's hack to pad the bitfields though, as I
think that's still a plausible issue (and one which would appear to be
fixed by that smp_wmb() too).
Will
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next prev parent reply other threads:[~2020-11-16 17:43 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-16 9:10 Loadavg accounting error on arm64 Mel Gorman
2020-11-16 11:49 ` Mel Gorman
2020-11-16 12:00 ` Mel Gorman
2020-11-16 12:53 ` Peter Zijlstra
2020-11-16 12:58 ` Peter Zijlstra
2020-11-16 15:29 ` Mel Gorman
2020-11-16 16:42 ` Mel Gorman
2020-11-16 16:49 ` Peter Zijlstra
2020-11-16 17:24 ` Mel Gorman
2020-11-16 17:41 ` Will Deacon [this message]
2020-11-16 12:46 ` Peter Zijlstra
2020-11-16 12:58 ` Mel Gorman
2020-11-16 13:11 ` Will Deacon
2020-11-16 13:37 ` Mel Gorman
2020-11-16 14:20 ` Peter Zijlstra
2020-11-16 15:52 ` Mel Gorman
2020-11-16 16:54 ` Peter Zijlstra
2020-11-16 17:16 ` Mel Gorman
2020-11-16 19:31 ` Mel Gorman
2020-11-17 8:30 ` [PATCH] sched: Fix data-race in wakeup Peter Zijlstra
2020-11-17 9:15 ` Will Deacon
2020-11-17 9:29 ` Peter Zijlstra
2020-11-17 9:46 ` Peter Zijlstra
2020-11-17 10:36 ` Will Deacon
2020-11-17 12:52 ` Valentin Schneider
2020-11-17 15:37 ` Valentin Schneider
2020-11-17 16:13 ` Peter Zijlstra
2020-11-17 19:32 ` Valentin Schneider
2020-11-18 8:05 ` Peter Zijlstra
2020-11-18 9:51 ` Valentin Schneider
2020-11-18 13:33 ` Marco Elver
2020-11-17 9:38 ` [PATCH] sched: Fix rq->nr_iowait ordering Peter Zijlstra
2020-11-17 11:43 ` Mel Gorman
2020-11-17 12:40 ` [PATCH] sched: Fix data-race in wakeup Mel Gorman
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