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Date:   Sat, 9 Dec 2017 14:09:07 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     paulmck@...ux.vnet.ibm.com
Cc:     linux-kernel@...r.kernel.org, xiaolong.ye@...el.com,
        tglx@...utronix.de, cmetcalf@...lanox.com, cl@...ux.com,
        torvalds@...ux-foundation.org, lcapitulino@...hat.com,
        efault@....de, peterz@...radead.org, riel@...hat.com,
        kernellwp@...il.com, mingo@...nel.org, john.stultz@...aro.org
Subject: Re: [PATCH] sched/isolation: Make NO_HZ_FULL select CPU_ISOLATION

2017-12-07 18:29 UTC+01:00, Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> On Thu, Dec 07, 2017 at 05:14:54PM +0100, Frederic Weisbecker wrote:
>> 2017-12-04 18:16 UTC+01:00, Paul E. McKenney
>> <paulmck@...ux.vnet.ibm.com>:
>> > On Mon, Dec 04, 2017 at 04:53:15PM +0100, Frederic Weisbecker wrote:
>> >> 2017-12-02 20:24 UTC+01:00, Paul E. McKenney
>> >> 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.
>> >
>> > How about instead allowing something like "nohz_full=1-" specify that
>> > all CPUs other than CPU 0 should be nohz_full CPUs?  That would shrink
>> > the code by eliminating CONFIG_NO_HZ_FULL_ALL while still allowing
>> > easy automation of that particular scenario.
>> >
>> > (Right now, the boot code complains about "nohz_full=1-", which means
>> > that whatever is generating the boot parameters needs to know how many
>> > CPUs there really are, which as you say can be a pain.)
>>
>> Yes but automated boot testing is rather based on configs than boot
>> options. It's much easier. I think that's how Wu Fengguang lab works,
>> and -tip automated tests as well.
>
> So you have gotten bug reports from them?  Because I see splats from
> rcutorture testing rather frequently.  This thing is in no way a subtle
> low-probability bug.  ;-)

Nope I haven't got anything from them. So far you're the only
reproducer I know :)

>> >> >> 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
>> >
>> > It might well be the same thing, as this schedule_timeout() does look
>> > familiar.  I have some diagnostic patches in -rcu, please see below
>> > for the overall effect.
>>
>> I fear I can hit that even without any nohz_full CPU as well.
>
> Indeed, I do hit that with my TREE01 scenario, which does not set
> CONFIG_NO_HZ_FULL.  But it is much less frequent.  The good news is that
> I have finally figured out a way to extract information from this thing
> without suppressing it.  At the moment it appears to be a rather strange
> deadlock involving CPU hotplug, timers, and RCU.
>
> But that is a completely different bug from the ones for which I have
> the two patches in my tree.
>
> Anyway, I will keep those two patches because I cannot have the
> corresponding bugs possibly hiding RCU bugs in my testing.  If you
> put some other fix in place, I will drop those two patches in favor of
> your fix.

Ok. I'm a bit troubled by this bug, I hate to push a fix for a bug I
don't understand nor can reproduce. But having CONFIG_NO_HZ_FULL
select CONFIG_CPU_ISOLATION is already a fix for sanity that I need to
push. So I think I'm going to take your patch anyway and rewrite the
changelog to take all that into account.

Thanks Paul!

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