linux-kernel - Re: [PATCH] sched: fix a potential divide error

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Date:   Thu, 25 Apr 2019 10:00:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Xie XiuQi <xiexiuqi@...wei.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        cj.chengjian@...wei.com
Subject: Re: [PATCH] sched: fix a potential divide error

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@...wei.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@...hat.com
Cc: cj.chengjian@...wei.com
Signed-off-by: Xie XiuQi <xiexiuqi@...wei.com>
[peterz: simplified changelog]
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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;