Re: [PATCH] sched/isolation: Make NO_HZ_FULL select CPU_ISOLATION

[Frederic Weisbecker]

2017-11-30 21:20 UTC+01:00, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>:
> Commit 5c4991e24c69 ("sched/isolation: Split out new
> CONFIG_CPU_ISOLATION=y config from CONFIG_NO_HZ_FULL") can result in RCU
> CPU stall warnings when running rcutorture with CONFIG_NO_HZ_FULL_ALL=y
> and CONFIG_CPU_ISOLATION=n.  These warnings are caused by RCU's
> grace-period kthreads sleeping for a few jiffies, but never being
> awakened:
>
> [  116.353432] rcu_preempt kthread starved for 9974 jiffies! g4294967208
> +c4294967207 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x402 ->cpu=0
> [  116.355517] rcu_preempt     I 7464     8      2 0x80000000
> [  116.356543] Call Trace:
> [  116.357008]  __schedule+0x493/0x620
> [  116.357682]  schedule+0x24/0x40
> [  116.358291]  schedule_timeout+0x330/0x3b0
> [  116.359024]  ? preempt_count_sub+0xea/0x140
> [  116.359806]  ? collect_expired_timers+0xb0/0xb0
> [  116.360660]  rcu_gp_kthread+0x6bf/0xef0
>
> This commit therefore makes NO_HZ_FULL select CPU_ISOLATION, which
> prevents this behavior and seems like it was the original intention in
> any case.

Although CONFIG_NO_HZ should indeed select CONFIG_CPU_ISOLATION, I'm
surprised about this stall. I'm even  more surprised that setting
CONFIG_CPU_ISOLATION=y is enough to fix the issue because
CONFIG_NO_HZ_FULL_ALL shortcuts CONFIG_CPU_ISOLATION entirely (which
is not good, but work in progress...).

Did you have any nohz_full= or isolcpus= boot options?

Thanks.