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From: Peter Zijlstra <peterz@infradead.org>
To: Qais Yousef <qais.yousef@arm.com>
Cc: Dave Jones <davej@codemonkey.org.uk>,
	Mel Gorman <mgorman@techsingularity.net>,
	Linux Kernel <linux-kernel@vger.kernel.org>,
	mingo@kernel.org, Linus Torvalds <torvalds@linux-foundation.org>,
	paul.gortmaker@windriver.com, valentin.schneider@arm.com
Subject: Re: weird loadavg on idle machine post 5.7
Date: Tue, 7 Jul 2020 11:47:18 +0200	[thread overview]
Message-ID: <20200707094718.GL4800@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200707092004.jviycpvzb4l5ytiz@e107158-lin.cambridge.arm.com>

On Tue, Jul 07, 2020 at 10:20:05AM +0100, Qais Yousef wrote:
> On 07/06/20 16:59, Peter Zijlstra wrote:

> > +	if (!preempt && prev_state && prev_state == prev->state) {
> 
> I think the compiler won't optimize `prev_state == prev->state` out because of
> the smp_mb__after_spinlock() which implies a compiler barrier. Still not sure
> if it's worth making prev->state accesses a READ_ONCE()?

task_struct::state is one of the very rare (and ancient) variables
that's declared volatile.

We should probably clean that up some day, but so far I've not attempted
to do such a thing.

      reply	other threads:[~2020-07-07  9:47 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-02 17:15 weird loadavg on idle machine post 5.7 Dave Jones
2020-07-02 19:46 ` Dave Jones
2020-07-02 21:15 ` Paul Gortmaker
2020-07-03 13:23   ` Paul Gortmaker
2020-07-02 21:36 ` Mel Gorman
2020-07-02 23:11   ` Michal Kubecek
2020-07-02 23:24   ` Dave Jones
2020-07-03  9:02   ` Peter Zijlstra
2020-07-03 10:40     ` Peter Zijlstra
2020-07-03 20:51       ` Dave Jones
2020-07-06 14:59         ` Peter Zijlstra
2020-07-06 21:20           ` Dave Jones
2020-07-07  7:48             ` Peter Zijlstra
2020-07-06 23:56           ` Valentin Schneider
2020-07-07  8:17             ` Peter Zijlstra
2020-07-07 10:20               ` Valentin Schneider
2020-07-07 10:29               ` Peter Zijlstra
2020-07-08  9:46                 ` [tip: sched/urgent] sched: Fix loadavg accounting race tip-bot2 for Peter Zijlstra
2020-07-07  9:20           ` weird loadavg on idle machine post 5.7 Qais Yousef
2020-07-07  9:47             ` Peter Zijlstra [this message]

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