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Date:   Fri, 1 Sep 2017 14:32:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ethan Zhao <ethan.zhao@...cle.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        james.puthukattukaran@...cle.com, ethan.kernel@...il.com
Subject: Re: [PATCH] sched: reset sysctl_sched_time_avg to default when

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@...cle.com>
> Signed-off-by: Ethan Zhao <ethan.zhao@...cle.com>
> ---
>  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.

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