linux-kernel - Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed

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Message-ID: <20180208154643.GB25181@hirez.programming.kicks-ass.net>
Date:   Thu, 8 Feb 2018 16:46:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, valentin.schneider@....com,
        morten.rasmussen@...s.arm.com, brendan.jackman@....com,
        dietmar.eggemann@....com
Subject: Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed

On Thu, Feb 08, 2018 at 03:30:31PM +0000, Will Deacon wrote:
> On Thu, Feb 08, 2018 at 03:00:05PM +0100, Peter Zijlstra wrote:

> > Without this ordering I think it would be possible to loose has_blocked
> > and not observe the CPU either.
> 
> I had a quick look at this, and I think you're right. This looks very much
> like an 'R'-shaped test, which means it's smp_mb() all round otherwise Power
> will go wrong. That also means the smp_mb__after_atomic() in
> nohz_balance_enter_idle *cannot* be an smp_wmb(), so you might want a
> comment stating that explicitly.

Thanks Will. BTW, where does that 'R' shape nomenclature come from?
This is the first I've heard of it.