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

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Message-ID: <20180209095125.GA17357@andrea>
Date:   Fri, 9 Feb 2018 10:51:25 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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 04:03:41PM +0000, Will Deacon wrote:
> 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

Indeed.  As a curiosity: I've never _observed_ R+lwsync+sync (the lwsync
separating the two writes), and other people who tried found the same

  http://moscova.inria.fr/~maranget/cats7/linux/hard.html#unseen
  http://www.cl.cam.ac.uk/~pes20/ppc-supplemental/ppc051.html#toc8 .

It would be interesting to hear about different results ... ;-)

  Andrea


> 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