Messages in this thread Patches in this message | | | Date | Tue, 12 Nov 2019 11:08:21 -0500 | From | Johannes Weiner <> | Subject | Re: [PATCH] psi:fix divide by zero in psi_update_stats |
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On Tue, Nov 12, 2019 at 10:48:46AM -0500, Johannes Weiner wrote: > On Tue, Nov 12, 2019 at 10:41:46AM -0500, Johannes Weiner wrote: > > On Fri, Nov 08, 2019 at 03:33:24PM +0800, tim wrote: > > > In psi_update_stats, it is possible that period has value like > > > 0xXXXXXXXX00000000 where the lower 32 bit is 0, then it calls div_u64 which > > > truncates u64 period to u32, results in zero divisor. > > > Use div64_u64() instead of div_u64() if the divisor is u64 to avoid > > > truncation to 32-bit on 64-bit platforms. > > > > > > Signed-off-by: xiejingfeng <xiejingfeng@linux.alibaba.com> > > > > This is legit. When we stop the periodic averaging worker due to an > > idle CPU, the period after restart can be much longer than the ~4 sec > > in the lower 32 bits. See the missed_periods logic in update_averages. > > Argh, that's not right. Of course I notice right after hitting send. > > missed_periods are subtracted out of the difference between now and > the last update, so period should be not much bigger than 2s. > > Something else is going on here.
Tim, does this happen right after boot? I wonder if it's because we're not initializing avg_last_update, and the initial delta between the last update (0) and the first scheduled update (sched_clock() + 2s) ends up bigger than 4 seconds somehow. Later on, the delta between the last and the scheduled update should always be ~2s. But for that to happen, it would require a pretty slow boot, or a sched_clock() that does not start at 0.
Tim, if you have a coredump, can you extract the value of the other variables printed in the following patch?
diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c index 84af7aa158bf..1b6836d23091 100644 --- a/kernel/sched/psi.c +++ b/kernel/sched/psi.c @@ -374,6 +374,10 @@ static u64 update_averages(struct psi_group *group, u64 now) */ avg_next_update = expires + ((1 + missed_periods) * psi_period); period = now - (group->avg_last_update + (missed_periods * psi_period)); + + WARN(period >> 32, "period=%ld now=%ld expires=%ld last=%ld missed=%ld\n", + period, now, expires, group->avg_last_update, missed_periods); + group->avg_last_update = now; for (s = 0; s < NR_PSI_STATES - 1; s++) { And we may need something like this to make the tick initialization more robust regardless of the reported bug here:
diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c index 84af7aa158bf..ce8f6748678a 100644 --- a/kernel/sched/psi.c +++ b/kernel/sched/psi.c @@ -185,7 +185,8 @@ static void group_init(struct psi_group *group) for_each_possible_cpu(cpu) seqcount_init(&per_cpu_ptr(group->pcpu, cpu)->seq); - group->avg_next_update = sched_clock() + psi_period; + group->avg_last_update = sched_clock(); + group->avg_next_update = group->avg_last_update + psi_period; INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work); mutex_init(&group->avgs_lock); /* Init trigger-related members */
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