Messages in this thread Patch in this message | | | Date | Tue, 23 Apr 2019 20:44:58 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: fix a potential divide error |
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On Sat, Apr 20, 2019 at 04:34:16PM +0800, Xie XiuQi wrote: > We meet a divide error on 3.10.0 kernel, the error message is bellow:
That is a _realllllllyyyy_ old kernel. I would urge you to upgrade.
> [499992.287996] divide error: 0000 [#1] SMP
> sched_clock_cpu may not be consistent bwtwen cpus. If a task migrate > to another cpu, the se.exec_start was set to that cpu's rq_clock_task > by update_stats_curr_start(). Which may not be monotonic. > > update_stats_curr_start > <- set_next_entity > <- set_curr_task_fair > <- sched_move_task
That is not in fact a cross-cpu migration path. But I see the point. Also many migration paths do in fact preserve monotonicity, even when the clock is busted, but you're right, not all of them.
> So, if now - p->last_task_numa_placement is -1, then (*period + 1) is > 0, and divide error was triggerred at the div operation: > task_numa_placement: > runtime = numa_get_avg_runtime(p, &period); > f_weight = div64_u64(runtime << 16, period + 1); // divide error here > > In this patch, we make sure period is not less than 0 to avoid this > divide error. > > Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com> > Cc: stable@vger.kernel.org > --- > kernel/sched/fair.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 40bd1e27b1b7..f2abb258fc85 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -2007,6 +2007,10 @@ static u64 numa_get_avg_runtime(struct task_struct *p, u64 *period) > if (p->last_task_numa_placement) { > delta = runtime - p->last_sum_exec_runtime; > *period = now - p->last_task_numa_placement; > + > + /* Avoid backward, and prevent potential divide error */ > + if ((s64)*period < 0) > + *period = 0; > } else { > delta = p->se.avg.load_sum; > *period = LOAD_AVG_MAX;
Yeah, I suppose that is indeed correct.
I'll try and come up with a better Changelog tomorrow.
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