Re: [PATCH] sched: fix a potential divide error

[Xie XiuQi]

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@xxxxxxxxxx>
> 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@xxxxxxxxxx
> Cc: cj.chengjian@xxxxxxxxxx
> Signed-off-by: Xie XiuQi <xiexiuqi@xxxxxxxxxx>
> [peterz: simplified changelog]
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Link: https://lkml.kernel.org/r/20190420083416.170446-1-xiexiuqi@xxxxxxxxxx
> ---
>  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