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

[Will Deacon]

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