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

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Date:   Thu, 25 Apr 2019 11:52:28 +0800
From:   Xie XiuQi <xiexiuqi@...wei.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
        <cj.chengjian@...wei.com>
Subject: Re: [PATCH] sched: fix a potential divide error

Hi Peter,

Thanks for your comments.

On 2019/4/24 2:44, Peter Zijlstra wrote:
> 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.

I will.

> 
>> [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@...wei.com>
>> Cc: stable@...r.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.

Thanks.

-- 
Thanks,
Xie XiuQi