From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Jeffrey Hugo <jhugo@codeaurora.org>
Cc: linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
pprakash@codeaurora.org, Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Jens Axboe <axboe@kernel.dk>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Thomas Gleixner <tglx@linutronix.de>,
Richard Cochran <rcochran@linutronix.de>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
Richard Weinberger <richard@nod.at>
Subject: Re: [BUG] Deadlock due due to interactions of block, RCU, and cpu offline
Date: Wed, 21 Jun 2017 09:18:53 -0700 [thread overview]
Message-ID: <20170621161853.GB3721@linux.vnet.ibm.com> (raw)
In-Reply-To: <df080eec-62b8-7d19-c201-e1a44febb96d@codeaurora.org>
On Wed, Jun 21, 2017 at 08:39:45AM -0600, Jeffrey Hugo wrote:
> On 6/20/2017 5:46 PM, Paul E. McKenney wrote:
> >On Mon, Mar 27, 2017 at 11:17:11AM -0700, Paul E. McKenney wrote:
> >>On Mon, Mar 27, 2017 at 12:02:27PM -0600, Jeffrey Hugo wrote:
> >>>Hi Paul.
> >>>
> >>>Thanks for the quick reply.
> >>>
> >>>On 3/26/2017 5:28 PM, Paul E. McKenney wrote:
> >>>>On Sun, Mar 26, 2017 at 05:10:40PM -0600, Jeffrey Hugo wrote:
> >>>
> >>>>>It is a race between this work running, and the cpu offline processing.
> >>>>
> >>>>One quick way to test this assumption is to build a kernel with Kconfig
> >>>>options CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y. This will
> >>>>cause call_rcu_sched() to queue the work to a kthread, which can migrate
> >>>>to some other CPU. If your analysis is correct, this should avoid
> >>>>the deadlock. (Note that the deadlock should be fixed in any case,
> >>>>just a diagnostic assumption-check procedure.)
> >>>
> >>>I enabled CONFIG_RCU_EXPERT=y, CONFIG_RCU_NOCB_CPU=y,
> >>>CONFIG_RCU_NOCB_CPU_ALL=y in my build. I've only had time so far to
> >>>do one test run however the issue reproduced, but it took a fair bit
> >>>longer to do so. An initial look at the data indicates that the
> >>>work is still not running. An odd observation, the two threads are
> >>>no longer blocked on the same queue, but different ones.
> >>
> >>I was afraid of that...
> >>
> >>>Let me look at this more and see what is going on now.
> >>
> >>Another thing to try would be to affinity the "rcuo" kthreads to
> >>some CPU that is never taken offline, just in case that kthread is
> >>sometimes somehow getting stuck during the CPU-hotplug operation.
> >>
> >>>>>What is the opinion of the domain experts?
> >>>>
> >>>>I do hope that we can come up with a better fix. No offense intended,
> >>>>as coming up with -any- fix in the CPU-hotplug domain is not to be
> >>>>denigrated, but this looks to be at vest quite fragile.
> >>>>
> >>>> Thanx, Paul
> >>>>
> >>>
> >>>None taken. I'm not particularly attached to the current fix. I
> >>>agree, it does appear to be quite fragile.
> >>>
> >>>I'm still not sure what a better solution would be though. Maybe
> >>>the RCU framework flushes the work somehow during cpu offline? It
> >>>would need to ensure further work is not queued after that point,
> >>>which seems like it might be tricky to synchronize. I don't know
> >>>enough about the working of RCU to even attempt to implement that.
> >>
> >>There are some ways that RCU might be able to shrink the window during
> >>which the outgoing CPU's callbacks are in limbo, but they are not free
> >>of risk, so we really need to compleetly understand what is going on
> >>before making any possibly ill-conceived changes. ;-)
> >>
> >>>In any case, it seem like some more analysis is needed based on the
> >>>latest data.
> >>
> >>Looking forward to hearing about you find!
> >
> >Hearing nothing, I eventually took unilateral action (I am a citizen of
> >USA, after all!) and produced the lightly tested patch shown below.
> >
> >Does it help?
> >
> > Thanx, Paul
> >
>
> Wow, has it been 3 months already? I am extremely sorry, I've been
> preempted multiple times, and this has sat on my todo list where I
> keep thinking I need to find time to come back to it but apparently
> not doing enough to make that happen.
>
> Thank you for not forgetting about this. I promise I will somehow
> clear my schedule to test this next week.
No worries, and I am very much looking forward to seeing the results of
your testing.
Thanx, Paul
next prev parent reply other threads:[~2017-06-21 20:42 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-26 23:10 [BUG] Deadlock due due to interactions of block, RCU, and cpu offline Jeffrey Hugo
2017-03-26 23:28 ` Paul E. McKenney
2017-03-27 18:02 ` Jeffrey Hugo
2017-03-27 18:17 ` Paul E. McKenney
2017-06-20 23:46 ` Paul E. McKenney
2017-06-21 14:39 ` Jeffrey Hugo
2017-06-21 16:18 ` Paul E. McKenney [this message]
2017-06-23 3:34 ` Paul E. McKenney
2017-06-27 22:32 ` Jeffrey Hugo
2017-06-28 0:11 ` Paul E. McKenney
2017-06-29 16:29 ` Jeffrey Hugo
2017-06-30 0:18 ` Paul E. McKenney
2017-08-20 19:31 ` Jeffrey Hugo
2017-08-20 20:56 ` Paul E. McKenney
2017-08-22 16:12 ` Paolo Bonzini
2017-08-22 20:53 ` Jeffrey Hugo
2017-08-15 8:46 ` [tip:core/rcu] rcu: Migrate callbacks earlier in the CPU-offline timeline tip-bot for Paul E. McKenney
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