From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
oleg@redhat.com
Subject: Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop() enable tick on all online CPUs
Date: Fri, 9 Aug 2019 11:39:57 -0700 [thread overview]
Message-ID: <20190809183957.GH28441@linux.ibm.com> (raw)
In-Reply-To: <20190809180721.GA255533@google.com>
On Fri, Aug 09, 2019 at 02:07:21PM -0400, Joel Fernandes wrote:
> On Fri, Aug 09, 2019 at 09:51:20AM -0700, Paul E. McKenney wrote:
> > > > > And of course I forgot to dump out the online CPUs, so I really had no
> > > > > idea whether or not all the CPUs were accounted for. I added tracing
> > > > > to dump out the online CPUs at the beginning of __stop_cpus() and then
> > > > > reworked it a few times to get the problem to happen in reasonable time.
> > > > > Please see below for the resulting annotated trace.
> > > > >
> > > > > I was primed to expect a lost IPI, perhaps due to yet another qemu bug,
> > > > > but all the migration threads are running within about 2 milliseconds.
> > > > > It is then almost two minutes(!) until the next trace message.
> > > > >
> > > > > Looks like time to (very carefully!) instrument multi_cpu_stop().
> > > > >
> > > > > Of course, if you have any ideas, please do not keep them a secret!
> > > >
> > > > Functionally, multi_cpu_stop() is working fine, according to the trace
> > > > below (search for a line beginning with TAB). But somehow CPU 2 took
> > > > almost three -minutes- to do one iteration of the loop. The prime suspect
> > > > in that loop is cpu_relax() due to the hypervisor having an opportunity
> > > > to do something at that point. The commentary below (again, search for
> > > > a line beginning with TAB) gives my analysis.
> > > >
> > > > Of course, if I am correct, it should be possible to catch cpu_relax()
> > > > in the act. That is the next step, give or take the Heisenbuggy nature
> > > > of this beast.
> > > >
> > > > Another thing for me to try is to run longer with !NO_HZ_FULL, just in
> > > > case the earlier runs just got lucky.
> > > >
> > > > Thoughts?
> > >
> > > And it really can happen:
> > >
> > > [ 1881.467922] migratio-33 4...1 1879530317us : stop_machine_yield: cpu_relax() took 756140 ms
> > >
> > > The previous timestamp was 1123391100us, so the cpu_relax() is almost
> > > exactly the full delay.
> > >
> > > But another instance stalled for many minutes without a ten-second
> > > cpu_relax(). So it is not just cpu_relax() causing trouble. I could
> > > rationalize that vCPU preemption being at fault...
> > >
> > > And my diagnostic patch is below, just in case I am doing something
> > > stupid with that.
> >
> > I did a 12-hour run with the same configuration except for leaving out the
> > "nohz_full=1-7" kernel parameter without problems (aside from the RCU CPU
> > stall warnings due to the ftrace_dump() at the end of the run -- isn't
> > there some way to clear the ftrace buffer without actually printing it?).
>
> I think if tracing_reset_all_online_cpus() is exposed, then calling that with
> the appropriate locks held can reset the ftrace ring buffer and clear the
> trace. Steve, is there a better way?
On the off-chance that it helps, here is my use case:
o I have a race condition that becomes extremely unlikely if
I leave tracing enabled at all times.
o I therefore enable tracing at the beginning of a CPU-hotplug
removal.
o At the end of that CPU-hotplug removal, I disable tracing.
o I can test whether the problem occurred without affecting its
probability. If it occurred, I want to dump only that portion
of the trace buffer since enabling it above. If the problem
did not occur, I want to flush (not dump) the trace buffer.
o I cannot reasonably just make the trace buffer small because
the number of events in a given occurrence of the problem
can vary widely.
Thanx, Paul
next prev parent reply other threads:[~2019-08-09 18:40 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-02 15:14 [PATCH tip/core/rcu 0/14] No-CBs bypass addition for v5.4 Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 01/14] rcu/nocb: Atomic ->len field in rcu_segcblist structure Paul E. McKenney
2019-08-04 14:50 ` Peter Zijlstra
2019-08-04 14:52 ` Peter Zijlstra
2019-08-04 18:45 ` Paul E. McKenney
2019-08-04 18:42 ` Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 02/14] rcu/nocb: Add bypass callback queueing Paul E. McKenney
2019-08-07 0:03 ` Joel Fernandes
2019-08-07 0:16 ` Joel Fernandes
2019-08-07 0:35 ` Paul E. McKenney
2019-08-07 0:40 ` Steven Rostedt
2019-08-07 1:17 ` Paul E. McKenney
2019-08-07 1:24 ` Steven Rostedt
2019-08-07 3:47 ` Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 03/14] rcu/nocb: EXP Check use and usefulness of ->nocb_lock_contended Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 04/14] rcu/nocb: Print no-CBs diagnostics when rcutorture writer unduly delayed Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 05/14] rcu/nocb: Avoid synchronous wakeup in __call_rcu_nocb_wake() Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 06/14] rcu/nocb: Advance CBs after merge in rcutree_migrate_callbacks() Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 07/14] rcu/nocb: Reduce nocb_cb_wait() leaf rcu_node ->lock contention Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 08/14] rcu/nocb: Reduce __call_rcu_nocb_wake() " Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 09/14] rcu/nocb: Don't wake no-CBs GP kthread if timer posted under overload Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 10/14] rcu: Allow rcu_do_batch() to dynamically adjust batch sizes Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 11/14] EXP nohz: Add TICK_DEP_BIT_RCU Paul E. McKenney
2019-08-02 15:14 ` [PATCH RFC tip/core/rcu 12/14] rcu/nohz: Force on tick when invoking lots of callbacks Paul E. McKenney
2019-08-02 15:15 ` [PATCH RFC tip/core/rcu 13/14] rcutorture: Force on tick for readers and callback flooders Paul E. McKenney
2019-08-02 15:15 ` [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop() enable tick on all online CPUs Paul E. McKenney
2019-08-04 14:43 ` Peter Zijlstra
2019-08-04 14:48 ` Peter Zijlstra
2019-08-04 18:41 ` Paul E. McKenney
2019-08-04 20:24 ` Paul E. McKenney
2019-08-05 4:19 ` Paul E. McKenney
2019-08-05 8:07 ` Peter Zijlstra
2019-08-05 14:47 ` Paul E. McKenney
2019-08-05 8:05 ` Peter Zijlstra
2019-08-05 14:54 ` Paul E. McKenney
2019-08-05 15:50 ` Peter Zijlstra
2019-08-05 17:48 ` Paul E. McKenney
2019-08-06 18:08 ` Paul E. McKenney
2019-08-07 21:41 ` Paul E. McKenney
2019-08-08 20:35 ` Paul E. McKenney
2019-08-08 21:30 ` Paul E. McKenney
2019-08-09 16:51 ` Paul E. McKenney
2019-08-09 18:07 ` Joel Fernandes
2019-08-09 18:39 ` Paul E. McKenney [this message]
2019-08-12 21:02 ` Frederic Weisbecker
2019-08-12 23:23 ` Paul E. McKenney
2019-08-13 1:33 ` Joel Fernandes
2019-08-13 12:30 ` Frederic Weisbecker
2019-08-13 14:48 ` Paul E. McKenney
2019-08-14 17:55 ` Joel Fernandes
2019-08-14 22:05 ` Paul E. McKenney
2019-08-15 15:07 ` Joel Fernandes
2019-08-15 17:23 ` Paul E. McKenney
2019-08-15 18:15 ` Joel Fernandes
2019-08-15 18:39 ` Paul E. McKenney
2019-08-15 19:42 ` Joel Fernandes
2019-08-13 21:06 ` Paul E. McKenney
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