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From: Valentin Schneider <vschneid@redhat.com>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>, Thomas Gleixner
 <tglx@linutronix.de>, LKML <linux-kernel@vger.kernel.org>, Ingo Molnar
 <mingo@kernel.org>, Anna-Maria Behnsen <anna-maria@linutronix.de>, Alex
 Shi <alexs@kernel.org>, Peter Zijlstra <peterz@infradead.org>, Vincent
 Guittot <vincent.guittot@linaro.org>, Barry Song
 <song.bao.hua@hisilicon.com>
Subject: Re: for_each_domain()/sched_domain_span() has offline CPUs (was Re:
 [PATCH 2/2] timers: Fix removed self-IPI on global timer's enqueue in
 nohz_full)
In-Reply-To: <xhsmhttkrbvfb.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
References: <20240318230729.15497-3-frederic@kernel.org>
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Date: Thu, 28 Mar 2024 15:08:08 +0100
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On 27/03/24 15:28, Valentin Schneider wrote:
> On 27/03/24 13:42, Frederic Weisbecker wrote:
>> Le Tue, Mar 26, 2024 at 05:46:07PM +0100, Valentin Schneider a =C3=A9cri=
t :
>>> > Then with that patch I ran TREE07, just some short iterations:
>>> >
>>> > tools/testing/selftests/rcutorture/bin/kvm.sh --configs "10*TREE07" -=
-allcpus --bootargs "rcutorture.onoff_interval=3D200" --duration 2
>>> >
>>> > And the warning triggers very quickly. At least since v6.3 but maybe =
since
>>> > earlier. Is this expected behaviour or am I right to assume that
>>> > for_each_domain()/sched_domain_span() shouldn't return an offline CPU?
>>> >
>>>=20
>>> I would very much assume an offline CPU shouldn't show up in a
>>> sched_domain_span().
>>>=20
>>> Now, on top of the above, there's one more thing worth noting:
>>>   cpu_up_down_serialize_trainwrecks()
>>>=20
>>> This just flushes the cpuset work, so after that the sched_domain topol=
ogy
>>> should be sane. However I see it's invoked at the tail end of _cpu_down=
(),
>>> IOW /after/ takedown_cpu() has run, which sounds too late. The comments
>>> around this vs. lock ordering aren't very reassuring however, so I need=
 to
>>> look into this more.
>>
>> Ouch...
>>
>>>=20
>>> Maybe as a "quick" test to see if this is the right culprit, you could =
try
>>> that with CONFIG_CPUSET=3Dn? Because in that case the sched_domain upda=
te is
>>> ran within sched_cpu_deactivate().
>>
>> I just tried and I fear that doesn't help. It still triggers even without
>> cpusets :-s
>>
>
> What, you mean I can't always blame cgroups? What has the world come to?
>
> That's interesting, it means the deferred work item isn't the (only)
> issue. I'll grab your test patch and try to reproduce on TREE07.
>

Unfortunately I haven't been able to trigger your warning with ~20 runs of
TREE07 & CONFIG_CPUSETS=3Dn, however it does trigger reliably with
CONFIG_CPUSETS=3Dy, so I'm back to thinking the cpuset work is a likely
culprit...

>> Thanks.