From: Peter Zijlstra <peterz@infradead.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Will Deacon <will@kernel.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 13:53:55 +0100 [thread overview]
Message-ID: <20201116125355.GB3121392@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20201116114938.GN3371@techsingularity.net>
On Mon, Nov 16, 2020 at 11:49:38AM +0000, Mel Gorman wrote:
> On Mon, Nov 16, 2020 at 09:10:54AM +0000, Mel Gorman wrote:
> > I'll be looking again today to see can I find a mistake in the ordering for
> > how sched_contributes_to_load is handled but again, the lack of knowledge
> > on the arm64 memory model means I'm a bit stuck and a second set of eyes
> > would be nice :(
> >
>
> This morning, it's not particularly clear what orders the visibility of
> sched_contributes_to_load exactly like other task fields in the schedule
> vs try_to_wake_up paths. I thought the rq lock would have ordered them but
> something is clearly off or loadavg would not be getting screwed. It could
> be done with an rmb and wmb (testing and hasn't blown up so far) but that's
> far too heavy. smp_load_acquire/smp_store_release might be sufficient
> on it although less clear if the arm64 gives the necessary guarantees.
>
> (This is still at the chucking out ideas as I haven't context switched
> back in all the memory barrier rules).
IIRC it should be so ordered by ->on_cpu.
We have:
schedule()
prev->sched_contributes_to_load = X;
smp_store_release(prev->on_cpu, 0);
on the one hand, and:
sched_ttwu_pending()
if (WARN_ON_ONCE(p->on_cpu))
smp_cond_load_acquire(&p->on_cpu)
ttwu_do_activate()
if (p->sched_contributes_to_load)
...
on the other (for the remote case, which is the only 'interesting' one).
WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
Will Deacon <will@kernel.org>,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org
Subject: Re: Loadavg accounting error on arm64
Date: Mon, 16 Nov 2020 13:53:55 +0100 [thread overview]
Message-ID: <20201116125355.GB3121392@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20201116114938.GN3371@techsingularity.net>
On Mon, Nov 16, 2020 at 11:49:38AM +0000, Mel Gorman wrote:
> On Mon, Nov 16, 2020 at 09:10:54AM +0000, Mel Gorman wrote:
> > I'll be looking again today to see can I find a mistake in the ordering for
> > how sched_contributes_to_load is handled but again, the lack of knowledge
> > on the arm64 memory model means I'm a bit stuck and a second set of eyes
> > would be nice :(
> >
>
> This morning, it's not particularly clear what orders the visibility of
> sched_contributes_to_load exactly like other task fields in the schedule
> vs try_to_wake_up paths. I thought the rq lock would have ordered them but
> something is clearly off or loadavg would not be getting screwed. It could
> be done with an rmb and wmb (testing and hasn't blown up so far) but that's
> far too heavy. smp_load_acquire/smp_store_release might be sufficient
> on it although less clear if the arm64 gives the necessary guarantees.
>
> (This is still at the chucking out ideas as I haven't context switched
> back in all the memory barrier rules).
IIRC it should be so ordered by ->on_cpu.
We have:
schedule()
prev->sched_contributes_to_load = X;
smp_store_release(prev->on_cpu, 0);
on the one hand, and:
sched_ttwu_pending()
if (WARN_ON_ONCE(p->on_cpu))
smp_cond_load_acquire(&p->on_cpu)
ttwu_do_activate()
if (p->sched_contributes_to_load)
...
on the other (for the remote case, which is the only 'interesting' one).
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next prev parent reply other threads:[~2020-11-16 12:54 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 [this message]
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
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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