Messages in this thread | | | Date | Tue, 02 Jul 2013 19:39:08 +0900 | From | Fernando Luis Vazquez Cao <> | Subject | Re: [PATCH] proc: Add workaround for idle/iowait decreasing problem. |
| |
On 2013年07月02日 12:56, 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)
A possible solution to the races above would be to add a per-cpu variable such ->iowait_sleeptime_user which shadows ->iowait_sleeptime but is maintained in get_cpu_iowait_time_us() and kept monotonic, the former being the one we would export to user space.
Another approach would be updating ->nr_iowait of the source and destination CPUs during task migration, but this may be overkill.
What do you think?
Thanks, Fernando -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |