Re: [PATCH 3/4 v2] nohz: Fix idle/iowait counts going backwards

[Denys Vlasenko <dvlasenk@redhat.com>]

On 05/07/2014 04:23 PM, Peter Zijlstra wrote:
> On Wed, May 07, 2014 at 03:41:33PM +0200, Denys Vlasenko wrote:
>> With this change, "iowait-ness" of every idle period is decided
>> at the moment it starts:
>> if this CPU's run-queue had tasks waiting on I/O, then this idle
>> period's duration will be added to iowait_sleeptime.
>>
>> This fixes the bug where iowait and/or idle counts could go backwards,
>> but iowait accounting is not precise (it can show more iowait
>> that there really is).
>>
> 
> NAK on this, the thing going backwards is a symptom of the bug, not an
> actual bug itself.

This patch does fix that bug.

The bug is that in nohz_stop_idle(),
we base the decision to add accumulated time to
ts->iowait_sleeptime or to ts->idle_sleeptime
on nr_iowait_cpu(cpu):

        if (nr_iowait_cpu(smp_processor_id()) > 0)
                ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
        else
                ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);

and we use the same nr_iowait_cpu(cpu)
to calculate result of get_cpu_iowait_time_us():

               if (ts->idle_active && nr_iowait_cpu(cpu) > 0) {
                       delta = ktime_sub(now, ts->idle_entrytime);
                       iowait = ktime_add(ts->iowait_sleeptime, delta);
               }

This is wrong because nr_iowait_cpu(cpu) is not stable.
It could be != 0 in get_cpu_iowait_time_us() but later
become 0 when we are in nohz_stop_idle().

We must use consistent logic in these two functions.
If nr_iowait_cpu() added something to iowait counter,
the same should be done in nohz_stop_idle().