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From: Peter Zijlstra <peterz@infradead.org>
To: Xie XiuQi <xiexiuqi@huawei.com>
Cc: mingo@redhat.com, linux-kernel@vger.kernel.org, cj.chengjian@huawei.com
Subject: Re: [PATCH] sched: fix a potential divide error
Date: Thu, 25 Apr 2019 10:00:16 +0200
Message-ID: <20190425080016.GX11158@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <14cdc45e-7a6f-5e1f-e877-10503180ac4e@huawei.com>

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?

---
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;

  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-20  8:34 Xie XiuQi
2019-04-23 18:44 ` Peter Zijlstra
2019-04-25  3:52   ` Xie XiuQi
2019-04-25  8:00     ` Peter Zijlstra [this message]
2019-04-25  8:16       ` Xie XiuQi
2019-04-25 18:01       ` [tip:sched/urgent] sched/numa: Fix a possible divide-by-zero tip-bot for Xie XiuQi
2019-04-25 18:04       ` tip-bot for Xie XiuQi
2019-04-25 17:59 ` [PATCH] sched: fix a potential divide error Ingo Molnar

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