Messages in this thread | | | Date | Thu, 8 Feb 2018 16:03:41 +0000 | From | Will Deacon <> | Subject | Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed |
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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
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