On Sun, Aug 18, 2013 at 06:49:37PM +0200, Oleg Nesterov wrote:
> On 08/16, Frederic Weisbecker wrote:
> >
> > To fix this, lets only update the sleeptime stats locally when the CPU
> > exits from idle.
>
> I am in no position to ack the changes in this area, but I like this
> change very much. Because, as a code reader, I was totally confused by
>
> if (last_update_time)
> update_ts_time_stats()
>
> code and it looks "obviously wrong".
>
> I added more cc's. It seems to me that 9366d840 "cpufreq: governors:
> Calculate iowait time only when necessary" doesn't realize what
>
> - u64 idle_time = get_cpu_idle_time_us(cpu, NULL);
> + u64 idle_time = get_cpu_idle_time_us(cpu, io_busy ? wall :
> NULL);
>
> actually means. OTOH, get_cpu_iowait_time_us() was called with
> last_update_time != NULL even before this patch...
>
> In short. This looks like the clear fix to me, but I do not understand
> this code enough, and I think that cpufreq should know about this change.
Good point, and this time I'm really adding the Cc :)
>
> > static void tick_nohz_stop_idle(int cpu, ktime_t now)
> > {
> > struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> > + ktime_t delta;
> >
> > - update_ts_time_stats(cpu, ts, now, NULL);
> > + /* Updates the per cpu time idle statistics counters */
> > + delta = ktime_sub(now, ts->idle_entrytime);
> > + if (nr_iowait_cpu(cpu) > 0)
> > + ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
> > + else
> > + ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
> > + ts->idle_entrytime = now;
> > ts->idle_active = 0;
>
> With or without this change, why we update ->idle_entrytime in this case?
> Looks harmless, but a bit confusing.
Oh indeed I missed that. It's a leftover from the copy-paste of
update_ts_time_stats()
content.
Well spotted, I'll fix.
>
> While this doesn't really matter, we could probably even kill ->idle_active
> and use !!ts->idle_entrytime instead.
We could but it would be slightly more overhead in the irq entry path (cf:
tick_check_nohz())
and it makes the code also a little bit harder to review IMHO.
>
> > @@ -473,17 +458,14 @@ u64 get_cpu_idle_time_us(int cpu, u64
> > *last_update_time)
>
> And I think that we should kill this last_update_time argument later.
Agreed.
Thanks.
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