Messages in this thread | | | Date | Thu, 18 Jul 2013 16:24:51 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH RFC nohz_full 6/7] nohz_full: Add full-system-idle state machine |
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On Wed, Jul 17, 2013 at 08:39:21PM -0700, Paul E. McKenney wrote: > On Thu, Jul 18, 2013 at 03:33:01AM +0200, Frederic Weisbecker wrote: > > So it's like: > > > > CPU 0 CPU 1 > > > > read I write I > > smp_mb() smp_mb() > > cmpxchg S read S > > > > I still can't find what guarantees we don't read a value in CPU 1 that is way below > > what we want. > > One key point is that there is a second cycle from LONG to FULL. > > (Not saying that there is not a bug -- there might well be. In fact, > I am starting to think that I need to do another Promela model...
Now I'm very confused :)
I'm far from being a specialist on these matters but I would really love to understand this patchset. Is there any documentation somewhere I can read that could help, something about cycles of committed memory or something?
> > > > Unfortunately, the reasoning in #2 above does not hold in the small-CPU > > > case because there is the possibility of both the timekeeping CPU and > > > the RCU grace-period kthread concurrently advancing the state machine. > > > This would be bad, good catch!!! > > > > It's not like I spotted anything myself but you're welcome :) > > I will take them any way I can get them. ;-) > > > > The patch below (untested) is an attempt to fix this. If it actually > > > works, I will merge it in with 6/7. > > > > > > Anything else I missed? ;-) > > > > Well I guess I'll wait one more night before trying to understand > > the below ;) > > The key point is that the added check means that either the timekeeping > CPU is advancing the state machine (if there are few CPUs) or the > RCU grace-period kthread is (if there are many CPUs), but never both. > Or that is the intent, anyway!
Yeah got that.
Thanks!
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