Messages in this thread | | | Date | Wed, 07 May 2014 15:38:15 +0200 | From | Denys Vlasenko <> | Subject | Re: [PATCH 4/4] nohz: Fix iowait overcounting if iowait task migrates |
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On 05/05/2014 08:06 PM, Denys Vlasenko wrote: > On 04/29/2014 04:47 PM, Frederic Weisbecker wrote: >> On Thu, Apr 24, 2014 at 08:45:58PM +0200, Denys Vlasenko wrote: >>> +void tick_nohz_iowait_to_idle(int cpu) >>> +{ >>> + struct tick_sched *ts = tick_get_tick_sched(cpu); >>> + ktime_t now = ktime_get(); >>> + >>> + write_seqcount_begin(&ts->idle_sleeptime_seq); >>> + ts->iowait_exittime = now; >>> + write_seqcount_end(&ts->idle_sleeptime_seq); >> >> So now you have two concurrent updaters using the seqcount, which is >> very dangerous as the counters aren't updated atomically. >> >> seqcount is only suitable when there is a single sequential updater. >> Once you deal with concurrent updaters you need seqlock. >> >> And once you add seqlock in the hot scheduler path, you're hitting >> a big scalability issue. > > What I need here is merely an atomic store. > The complication is, of course, that, ktime_t is not atomic[64]_t. > > How do you think I can do an atomic store?
Ok, I think a have a version which uses atomic64_t and is not looking ugly. Sending new patchset now...
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