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SubjectRe: [PATCH] sched: fix a potential divide error
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Hi Peter,

On 2019/4/25 16:00, Peter Zijlstra wrote:
> On Thu, Apr 25, 2019 at 11:52:28AM +0800, Xie XiuQi wrote:
>> On 2019/4/24 2:44, Peter Zijlstra wrote:
>
>>> I'll try and come up with a better Changelog tomorrow.
>
> I actually did, but forgot to send out. I have the below.
> Does that work for you?

It works for me, thank you very much.

>
> ---
> Subject: sched/numa: Fix a possible divide-by-zero
> From: Xie XiuQi <xiexiuqi@huawei.com>
> Date: Sat, 20 Apr 2019 16:34:16 +0800
>
> sched_clock_cpu() may not be consistent between CPUs. If a task
> migrates to another CPU, then se.exec_start is set to that CPU's
> rq_clock_task() by update_stats_curr_start(). Specifically, the new
> value might be before the old value due to clock skew.
>
> So then if in numa_get_avg_runtime() the expression:
>
> 'now - p->last_task_numa_placement'
>
> ends up as -1, then the divider '*period + 1' in task_numa_placement()
> is 0 and things go bang. Similar to update_curr(), check if time goes
> backwards to avoid this.
>
> Cc: mingo@redhat.com
> Cc: cj.chengjian@huawei.com
> Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com>
> [peterz: simplified changelog]
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20190420083416.170446-1-xiexiuqi@huawei.com
> ---
> kernel/sched/fair.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2007,6 +2007,10 @@ static u64 numa_get_avg_runtime(struct t
> 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 (unlikely((s64)*period < 0))
> + *period = 0;
> } else {
> delta = p->se.avg.load_sum;
> *period = LOAD_AVG_MAX;
>
> .
>

--
Thanks,
Xie XiuQi

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