Re: rcu_preempt self-detected stall on CPU from 4.5-rc3, since 3.17

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Mar 26, 2016, at 2:49 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:

> On Sat, Mar 26, 2016 at 08:28:16AM -0700, Paul E. McKenney wrote:
>> On Sat, Mar 26, 2016 at 12:29:31PM +0000, Mathieu Desnoyers wrote:
>> > ----- On Mar 25, 2016, at 5:46 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com
>> > wrote:
>> > 
>> > > On Fri, Mar 25, 2016 at 09:24:14PM +0000, Chatre, Reinette wrote:
>> > >> Hi  Paul,
>> > >> 
>> > >> On 2016-03-23, Paul E. McKenney wrote:
>> > >> > Please boot with the following parameters:
>> > >> > 
>> > >> > 	rcu_tree.rcu_kick_kthreads ftrace
>> > >> > trace_event=sched_waking,sched_wakeup,sched_wake_idle_without_ipi
>> > >> 
>> > >> With these parameters I expected more details to show up in the kernel logs but
>> > >> cannot find any. Even so, today I left the machine running again and when this
>> > >> happened I think I was able to capture the trace data for the event. Please
>> > >> find attached the trace information for the kernel message below. Since the
>> > >> complete trace file is very big I trimmed it to show the time around this event
>> > >> - hopefully this will contain the information you need. I would also like to
>> > >> provide some additional information. The system on which I see these events had
>> > >> a time that was _very_ wrong. I noticed that this issue occurs when
>> > >> system-timesynd was one of the tasks calling the functions of interest to your
>> > >> tracing and am wondering if a very out of sync time in process of being
>> > >> corrected could be the cause of this issue? As an experiment I ensured the
>> > >> system time was accurate before leaving the system idle overnight and I did not
>> > >> see the issue the next morning.
>> > > 
>> > > Ah!  Yes, a sudden jump in time or a disagreement about the time among
>> > > different components of the system can definitely cause these symptoms.
>> > > We have sometimes seen these problems occur when a pair of CPUs have
>> > > wildly different ideas about what time it is, for example.  Please let
>> > > me know how it goes.
>> > > 
>> > > Also, in your trace, there are no sched_waking events for the rcu_preempt
>> > > process that are not immediately followed by sched_wakeup, so your trace
>> > > isn't showing the problem that I am seeing.
>> > 
>> > This is interesting.
>> > 
>> > Perhaps we could try with those commits reverted ?
>> > 
>> > commit e3baac47f0e82c4be632f4f97215bb93bf16b342
>> > Author: Peter Zijlstra <peterz@infradead.org>
>> > Date:   Wed Jun 4 10:31:18 2014 -0700
>> > 
>> >     sched/idle: Optimize try-to-wake-up IPI
>> > 
>> > commit fd99f91aa007ba255aac44fe6cf21c1db398243a
>> > Author: Peter Zijlstra <peterz@infradead.org>
>> > Date:   Wed Apr 9 15:35:08 2014 +0200
>> > 
>> >     sched/idle: Avoid spurious wakeup IPIs
>> > 
>> > They appeared in 3.16.
>> 
>> At this point, I am up for trying pretty much anything.  ;-)
>> 
>> Will give it a go.
> 
> And those certainly don't revert cleanly!  Would patching the kernel
> to remove the definition of TIF_POLLING_NRFLAG be useful?  Or, more
> to the point, is there some other course of action that would be more
> useful?  At this point, the test times are measured in weeks...

Indeed, patching the kernel to remove the TIF_POLLING_NRFLAG
definition would have an effect similar to reverting those two
commits.

Since testing takes a while, we could take a more aggressive
approach towards reproducing a possible race condition: we
could re-implement the _TIF_POLLING_NRFLAG vs _TIF_NEED_RESCHED
dance, along with the ttwu pending lock-list queue, within
a dummy test module, with custom data structures, and
stress-test the invariants. We could also create a Promela
model of these ipi-skip optimisations trying to validate
progress: whenever a wakeup is requested, there should
always be a scheduling performed, even if no further wakeup
is encountered.

Each of the two approaches proposed above might be a significant
endeavor, and would only validate my specific hunch. So it might
be a good idea to just let a test run for a few weeks with
TIF_POLLING_NRFLAG disabled meanwhile.

Thoughts ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com