Re: [PATCH 4/4] nohz: Fix iowait overcounting if iowait task migrates

[Denys Vlasenko]

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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