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

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Message-ID: <20180208160341.GD17775@arm.com>
Date:   Thu, 8 Feb 2018 16:03:41 +0000
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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 04:46:43PM +0100, Peter Zijlstra wrote:
> 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.

I don't know where it originates from, but the imfamous "test6.pdf" has it:

https://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test6.pdf

half way down the first page on the left. It says "needs sync+sync" which
is about as bad as it gets for Power (compare with "2+2w", which gets away
with lwsync+lwsync). See also:

http://materials.dagstuhl.de/files/16/16471/16471.DerekWilliams.Slides.pdf

for a light-hearted, yet technically accurate story about the latter.

Will