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From: Will Deacon <will@kernel.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Davidlohr Bueso <dave@stgolabs.net>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.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

WARNING: multiple messages have this Message-ID (diff)
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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  reply	other threads:[~2020-11-16 17:42 UTC|newest]

Thread overview: 70+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-16  9:10 Loadavg accounting error on arm64 Mel Gorman
2020-11-16  9:10 ` Mel Gorman
2020-11-16 11:49 ` Mel Gorman
2020-11-16 11:49   ` Mel Gorman
2020-11-16 12:00   ` Mel Gorman
2020-11-16 12:00     ` Mel Gorman
2020-11-16 12:53   ` Peter Zijlstra
2020-11-16 12:53     ` Peter Zijlstra
2020-11-16 12:58     ` Peter Zijlstra
2020-11-16 12:58       ` Peter Zijlstra
2020-11-16 15:29       ` Mel Gorman
2020-11-16 15:29         ` Mel Gorman
2020-11-16 16:42         ` Mel Gorman
2020-11-16 16:42           ` Mel Gorman
2020-11-16 16:49         ` Peter Zijlstra
2020-11-16 16:49           ` Peter Zijlstra
2020-11-16 17:24           ` Mel Gorman
2020-11-16 17:24             ` Mel Gorman
2020-11-16 17:41             ` Will Deacon [this message]
2020-11-16 17:41               ` Will Deacon
2020-11-16 12:46 ` Peter Zijlstra
2020-11-16 12:46   ` Peter Zijlstra
2020-11-16 12:58   ` Mel Gorman
2020-11-16 12:58     ` Mel Gorman
2020-11-16 13:11 ` Will Deacon
2020-11-16 13:11   ` Will Deacon
2020-11-16 13:37   ` Mel Gorman
2020-11-16 13:37     ` Mel Gorman
2020-11-16 14:20     ` Peter Zijlstra
2020-11-16 14:20       ` Peter Zijlstra
2020-11-16 15:52       ` Mel Gorman
2020-11-16 15:52         ` Mel Gorman
2020-11-16 16:54         ` Peter Zijlstra
2020-11-16 16:54           ` Peter Zijlstra
2020-11-16 17:16           ` Mel Gorman
2020-11-16 17:16             ` Mel Gorman
2020-11-16 19:31       ` 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  8:30           ` Peter Zijlstra
2020-11-17  9:15           ` Will Deacon
2020-11-17  9:15             ` Will Deacon
2020-11-17  9:29             ` Peter Zijlstra
2020-11-17  9:29               ` Peter Zijlstra
2020-11-17  9:46               ` Peter Zijlstra
2020-11-17  9:46                 ` Peter Zijlstra
2020-11-17 10:36                 ` Will Deacon
2020-11-17 10:36                   ` Will Deacon
2020-11-17 12:52                 ` Valentin Schneider
2020-11-17 12:52                   ` Valentin Schneider
2020-11-17 15:37                   ` Valentin Schneider
2020-11-17 15:37                     ` Valentin Schneider
2020-11-17 16:13                     ` Peter Zijlstra
2020-11-17 16:13                       ` Peter Zijlstra
2020-11-17 19:32                       ` Valentin Schneider
2020-11-17 19:32                         ` Valentin Schneider
2020-11-18  8:05                         ` Peter Zijlstra
2020-11-18  8:05                           ` Peter Zijlstra
2020-11-18  9:51                           ` Valentin Schneider
2020-11-18  9:51                             ` Valentin Schneider
2020-11-18 13:33               ` Marco Elver
2020-11-18 13:33                 ` Marco Elver
2020-11-17  9:38           ` [PATCH] sched: Fix rq->nr_iowait ordering Peter Zijlstra
2020-11-17  9:38             ` Peter Zijlstra
2020-11-17 11:43             ` Mel Gorman
2020-11-17 11:43               ` Mel Gorman
2020-11-19  9:55             ` [tip: sched/urgent] " tip-bot2 for Peter Zijlstra
2020-11-17 12:40           ` [PATCH] sched: Fix data-race in wakeup Mel Gorman
2020-11-17 12:40             ` Mel Gorman
2020-11-19  9:55           ` [tip: sched/urgent] " tip-bot2 for Peter Zijlstra

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