Messages in this thread | | | Date | Tue, 20 Aug 2013 17:29:12 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock |
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On Tue, Aug 20, 2013 at 10:44:05AM +0200, Peter Zijlstra wrote: > On Tue, Aug 20, 2013 at 03:59:36PM +0900, Fernando Luis Vázquez Cao wrote: > > That said, if deemed acceptable, option A is the one I would > > choose. > > Right, so I think we can do A without much extra cost mostly because we > already have 2 atomics in the io_schedule() path. If we replace those > two atomic operations with locks and modify nr_iowait and the other > stats under the same lock, and ensure all those variables (including the > lock) live in the same cacheline we should have the same cost we have > now.
I can try that :-)
> > Of course, if we can get away with completely removing all of that > (which I think Arjan suggested was a real possibility) then that would > be ever so much better still :-)
Would be lovely. But I don't know much about cpufreq, I hope somebody who's familiar with that code can handle this. Then once there are no more users of get_cpu_iowait_sleep_time() I can simply zap and clean the tick/time related code.
Surely the overhead that this mess brings to io_schedule() (and it's going to be worth with a seqlock, whether in the same cacheline than nr_iowait or not) may be a good motivation to poke at that cpufreq code part.
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