Re: [PATCH] proc: Add workaround for idle/iowait decreasing problem.

[Frederic Weisbecker]

On Tue, Jul 02, 2013 at 12:56:04PM +0900, Fernando Luis Vazquez Cao wrote:
> Hi Frederic,
> 
> I'm sorry it's taken me so long to respond; I got sidetracked for
> a while. Comments follow below.
> 
> On 2013/04/28 09:49, Frederic Weisbecker wrote:
> >On Tue, Apr 23, 2013 at 09:45:23PM +0900, Tetsuo Handa wrote:
> >>CONFIG_NO_HZ=y can cause idle/iowait values to decrease.
> [...]
> >It's not clear in the changelog why you see non-monotonic idle/iowait values.
> >
> >Looking at the previous patch from Fernando, it seems that's because we can
> >race with concurrent updates from the CPU target when it wakes up from idle?
> >(could be updated by drivers/cpufreq/cpufreq_governor.c as well).
> >
> >If so the bug has another symptom: we may also report a wrong iowait/idle time
> >by accounting the last idle time twice.
> >
> >In this case we should fix the bug from the source, for example we can force
> >the given ordering:
> >
> >= Write side =                          = Read side =
> >
> >// tick_nohz_start_idle()
> >write_seqcount_begin(ts->seq)
> >ts->idle_entrytime = now
> >ts->idle_active = 1
> >write_seqcount_end(ts->seq)
> >
> >// tick_nohz_stop_idle()
> >write_seqcount_begin(ts->seq)
> >ts->iowait_sleeptime += now - ts->idle_entrytime
> >t->idle_active = 0
> >write_seqcount_end(ts->seq)
> >
> >                                         // get_cpu_iowait_time_us()
> >                                         do {
> >                                             seq = read_seqcount_begin(ts->seq)
> >                                             if (t->idle_active) {
> >                                                 time = now - ts->idle_entrytime
> >                                                 time += ts->iowait_sleeptime
> >                                             } else {
> >                                                 time = ts->iowait_sleeptime
> >                                             }
> >                                         } while (read_seqcount_retry(ts->seq, seq));
> >
> >Right? seqcount should be enough to make sure we are getting a consistent result.
> >I doubt we need harder locking.
> 
> I tried that and it doesn't suffice. The problem that causes the most
> serious skews is related to the CPU scheduler: the per-run queue
> counter nr_iowait can be updated not only from the CPU it belongs
> to but also from any other CPU if tasks are migrated out while
> waiting on I/O.
> 
> The race looks like this:
> 
> CPU0                            CPU1
>                                 [ CPU1_rq->nr_iowait == 0 ]
>                                 Task foo: io_schedule()
>                                             schedule()
>                                 [ CPU1_rq->nr_iowait == 1) ]
>                                 Task foo migrated to CPU0
>                                 Goes to sleep
> 
> // get_cpu_iowait_time_us(1, NULL)
> [ CPU1_ts->idle_active == 1, CPU1_rq->nr_iowait == 1         ]
> [ CPU1_ts->iowait_sleeptime = 4, CPU1_ts->idle_entrytime = 3 ]
> now = 5
> delta = 5 - 3 = 2
> iowait = 4 + 2 = 6
> 
> Task foo wakes up
> [ CPU1_rq->nr_iowait == 0 ]
> 
>                                 CPU1 comes out of sleep state
>                                 tick_nohz_stop_idle()
>                                   update_ts_time_stats()
>                                     [ CPU1_ts->idle_active == 1, CPU1_rq->nr_iowait == 0         ]
>                                     [ CPU1_ts->iowait_sleeptime = 4, CPU1_ts->idle_entrytime = 3 ]
>                                     now = 6
>                                     delta = 6 - 3 = 3
>                                     (CPU1_ts->iowait_sleeptime is not updated)
>                                     CPU1_ts->idle_entrytime = now = 6
>                                   CPU1_ts->idle_active = 0
> 
> // get_cpu_iowait_time_us(1, NULL)
> [ CPU1_ts->idle_active == 0, CPU1_rq->nr_iowait == 0         ]
> [ CPU1_ts->iowait_sleeptime = 4, CPU1_ts->idle_entrytime = 6 ]
> iowait = CPU1_ts->iowait_sleeptime = 4
> (iowait decreased from 6 to 4)

Yeah, that's why we need to allow updates of ts->idle/iowait_sleeptime only from the local CPU
when it exits idle.

> 
> 
> >Another thing while at it. It seems that an update done from drivers/cpufreq/cpufreq_governor.c
> >(calling get_cpu_iowait_time_us() -> update_ts_time_stats()) can randomly race with a CPU
> >entering/exiting idle. I have no idea why drivers/cpufreq/cpufreq_governor.c does the update
> >itself. It can just compute the delta like any reader. May be we could remove that and only
> >ever call update_ts_time_stats() from the CPU that exit idle.
> >
> >What do you think?
> 
> I am all for it. We just need to make sure that CPU governors
> can cope with non-monotonic idle and iowait times. I'll take
> a closer look at the code but I wouldn't mind if Arjan (CCed)
> beat me at that.

I'm not sure what you mean. Only allowing the update from local idle exit won't break
monotonicity.

I'll try to write some patches about that.

> 
> Thanks,
> Fernando
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