Messages in this thread | | | Date | Mon, 05 May 2014 20:06:06 +0200 | From | Denys Vlasenko <> | Subject | Re: [PATCH 4/4] nohz: Fix iowait overcounting if iowait task migrates |
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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?
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