Re: [PATCH] sched: reset sysctl_sched_time_avg to default when

[Ethan Zhao]

Yep, that is the first place I considered to set the limit, but that would
break KABI ?

Thanks,
Ethan

On Fri, Sep 1, 2017 at 8:32 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Fri, Sep 01, 2017 at 07:31:54PM +0800, Ethan Zhao wrote:
>> System will hang if user set sysctl_sched_time_avg to 0 by
>>
>> [root@XXX ~]# sysctl kernel.sched_time_avg_ms=0
>>
>> Stack traceback for pid 0
>> 0xffff883f6406c600 0 0 1 3 R 0xffff883f6406cf50 *swapper/3
>> ffff883f7ccc3ae8 0000000000000018 ffffffff810c4dd0 0000000000000000
>> 0000000000017800 ffff883f7ccc3d78 0000000000000003 ffff883f7ccc3bf8
>> ffffffff810c4fc9 ffff883f7ccc3c08 00000000810c5043 ffff883f7ccc3c08
>> Call Trace:
>> <IRQ> [<ffffffff810c4dd0>] ? update_group_capacity+0x110/0x200
>> [<ffffffff810c4fc9>] ? update_sd_lb_stats+0x109/0x600
>> [<ffffffff810c5507>] ? find_busiest_group+0x47/0x530
>> [<ffffffff810c5b84>] ? load_balance+0x194/0x900
>> [<ffffffff810ad5ca>] ? update_rq_clock.part.83+0x1a/0xe0
>> [<ffffffff810c6d42>] ? rebalance_domains+0x152/0x290
>> [<ffffffff810c6f5c>] ? run_rebalance_domains+0xdc/0x1d0
>> [<ffffffff8108a75b>] ? __do_softirq+0xfb/0x320
>> [<ffffffff8108ac85>] ? irq_exit+0x125/0x130
>> [<ffffffff810b3a17>] ? scheduler_ipi+0x97/0x160
>> [<ffffffff81052709>] ? smp_reschedule_interrupt+0x29/0x30
>> [<ffffffff8173a1be>] ? reschedule_interrupt+0x6e/0x80
>> <EOI> [<ffffffff815bc83c>] ? cpuidle_enter_state+0xcc/0x230
>> [<ffffffff815bc80c>] ? cpuidle_enter_state+0x9c/0x230
>> [<ffffffff815bc9d7>] ? cpuidle_enter+0x17/0x20
>> [<ffffffff810cd6dc>] ? cpu_startup_entry+0x38c/0x420
>> [<ffffffff81053373>] ? start_secondary+0x173/0x1e0
>>
>> Because divide-by-zero error happens in function
>>
>> update_group_capacity()
>>   update_cpu_capacity()
>>     scale_rt_capacity()
>>       {
>>               ...
>>               total = sched_avg_period() + delta;
>>               used = div_u64(avg, total);
>>               ...
>>       }
>>
>> Seems this issue could be reproduced on all I tried stable 4.1 - last
>> kernel.
>>
>> To fix this issue, reset sysctl_sched_time_avg to default value
>> MSEC_PER_SEC if user input invalid value in function
>>
>> sched_avg_period()
>>
>> Reported-by: James Puthukattukaran <james.puthukattukaran@xxxxxxxxxx>
>> Signed-off-by: Ethan Zhao <ethan.zhao@xxxxxxxxxx>
>> ---
>>  Tested on stable 4.1, compiled on stable 4.13-rc5
>>  kernel/sched/sched.h | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index eeef1a3..b398560 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -1620,6 +1620,9 @@ static inline void rq_last_tick_reset(struct rq *rq)
>>
>>  static inline u64 sched_avg_period(void)
>>  {
>> +     if (unlikely(sysctl_sched_time_avg <= 0))
>> +             sysctl_sched_time_avg = MSEC_PER_SEC;
>> +
>
> Sight, no.. you can set limits in the sysctl table.