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

[Frederic Weisbecker <frederic@kernel.org>]

2017-12-02 20:24 UTC+01:00, Paul E. McKenney <paulmck@linux.vnet.ibm.com>:
> On Sat, Dec 02, 2017 at 02:59:12PM +0100, Frederic Weisbecker wrote:
>> 2017-11-30 21:20 UTC+01:00, Paul E. McKenney
>> <paulmck@linux.vnet.ibm.com>:
>> > 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...).
>
> Yes, and after applying this patch, I get failures a few commits
> later, which appears to be due to other changes that break
> CONFIG_NO_HZ_FULL_ALL=y.  So I have another patch staged that removes
> CONFIG_NO_HZ_FULL_ALL, on the grounds that no one else has complained,
> so rcutorture is likely to be the only user, and I don't see the point
> of having a Kconfig option for only one user.

I would prefer to keep it. It's useful for automated boot testing
based on configs such as 0-day or -tip test machines. But I'm likely
to migrate it to isolcpus implementation. Maybe something along the
lines of CONFIG_CPU_ISOLATION_ALL.

>
>> Did you have any nohz_full= or isolcpus= boot options?
>
> Replacing CONFIG_NO_HZ_FULL_ALL=y with nohz_full=1-7 works, that
> is CONFIG_NO_HZ_FULL=y, CONFIG_NO_HZ_FULL_ALL=n, and nohz_full=1-7
> on an eight-CPU test.
>
> But it is relatively easy to test.  Running the rcutorture TREE04
> scenario on a four-socket x86 gets me RCU CPU stall warnings within
> a few minutes more than half the time.  ;-)

Indeed I managed to trigger something. If it's the same thing I should
be able to track down the root cause.

[  123.907557] ??? Writer stall state RTWS_STUTTER(8) g160 c160 f0x0
->state 0x1 cpu 2
[  123.915184] rcu_torture_wri S    0   111      2 0x80080000
[  123.920673] Call Trace:
[  123.923096]  ? __schedule+0x2bf/0xbb0
[  123.926715]  ? _raw_spin_unlock_irqrestore+0x59/0x70
[  123.931657]  schedule+0x3c/0x90
[  123.934777]  schedule_timeout+0x1e1/0x560
[  123.938785]  ? __next_timer_interrupt+0xd0/0xd0
[  123.943276]  stutter_wait+0xc5/0xe0
[  123.946738]  rcu_torture_writer+0x1ae/0x730
[  123.950912]  ? rcu_torture_pipe_update+0xf0/0xf0
[  123.955491]  kthread+0x15f/0x1a0
[  123.958702]  ? kthread_unpark+0x60/0x60
[  123.962523]  ret_from_fork+0x24/0x30
[  123.966091] rcu_preempt: wait state: 1 ->state: 0x402
[  123.971112] rcu_sched: wait state: 1 ->state: 0x402
[  123.975953] rcu_bh: wait state: 1 ->state: 0x402

Thanks!