Received-SPF: pass (google.com: domain of linux-kernel+bounces-123316-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Date: Thu, 28 Mar 2024 17:58:29 +0100
From: Frederic Weisbecker <frederic@kernel.org>
To: Valentin Schneider <vschneid@redhat.com>
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)
Message-ID: <ZgWhteHzLb8Jutp3@localhost.localdomain>
References: <ZfsLtMijRrNZfqh6@localhost.localdomain>
 <6a95b6ac-6681-4492-b155-e30c19bb3341@paulmck-laptop>
 <ZfwdEROGFFmIbkCM@lothringen>
 <bf8689c2-0749-47cb-9535-53cf66e34f5e@paulmck-laptop>
 <Zf1sSv/6mQHJuJ8G@lothringen>
 <Zf2GDjekyemQKn7I@lothringen>
 <xhsmhwmppaqls.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
 <ZgQULtxy0UnoXfGi@localhost.localdomain>
 <xhsmhttkrbvfb.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
 <xhsmhr0fubgaf.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <xhsmhr0fubgaf.mognet@vschneid-thinkpadt14sgen2i.remote.csb>

Le Thu, Mar 28, 2024 at 03:08:08PM +0100, Valentin Schneider a ?crit :
> 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 ?crit :
> >>> > 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=200" --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?
> >>> >
> >>> 
> >>> I would very much assume an offline CPU shouldn't show up in a
> >>> sched_domain_span().
> >>> 
> >>> Now, on top of the above, there's one more thing worth noting:
> >>>   cpu_up_down_serialize_trainwrecks()
> >>> 
> >>> This just flushes the cpuset work, so after that the sched_domain topology
> >>> 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...
> >>
> >>> 
> >>> Maybe as a "quick" test to see if this is the right culprit, you could try
> >>> that with CONFIG_CPUSET=n? Because in that case the sched_domain update 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=n, however it does trigger reliably with
> CONFIG_CPUSETS=y, so I'm back to thinking the cpuset work is a likely
> culprit...

Funny, I just checked again and I can still reliably reproduce with:

/tools/testing/selftests/rcutorture/bin/kvm.sh --kconfig "CONFIG_CPUSETS=n CONFIG_PROC_PID_CPUSET=n" --configs "10*TREE07" --allcpus --bootargs "rcutorture.onoff_interval=200" --duration 2

I'm thinking there might be several culprits... ;-)