[PATCH] sched: reset sysctl_sched_time_avg to default when

[Ethan Zhao]

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;
+
 	return (u64)sysctl_sched_time_avg * NSEC_PER_MSEC / 2;
 }
 
-- 
1.8.3.1