Messages in this thread | | | From | Suren Baghdasaryan <> | Date | Tue, 12 Nov 2019 09:27:09 -0800 | Subject | Re: [PATCH] psi:fix divide by zero in psi_update_stats |
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On Tue, Nov 12, 2019 at 8:08 AM Johannes Weiner <hannes@cmpxchg.org> wrote: > > 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 */
Both fixes for group_init() and window_update() make sense to me. window_update() division would be reproducible because win->size is set during trigger setup and does not change afterwards. Since userspace defines the window size in usecs this would require doing some math and finding a value that yields zeros in 32 LSBs after conversion into nsecs (see: t->win.size = window_us * NSEC_PER_USEC in psi_trigger_create()). Haven't seen this issue because all my test cases (1, 2, 10secs) had non-zero LSBs.
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