On Sat, Mar 26, 2016 at 10:22:57PM +0000, Mathieu Desnoyers wrote:
> ----- On Mar 26, 2016, at 2:49 PM, Paul E. McKenney
> paul...@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
> >> > paul...@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 <pet...@infradead.org>
> >> > Date: Wed Jun 4 10:31:18 2014 -0700
> >> >
> >> > sched/idle: Optimize try-to-wake-up IPI
> >> >
> >> > commit fd99f91aa007ba255aac44fe6cf21c1db398243a
> >> > Author: Peter Zijlstra <pet...@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.
This makes a lot of sense. I did some short runs, and nothing broke
too badly. However, I left some diagnostic stuff in that obscured
the outcome. I disabled the diagnostic stuff and am running overnight.
I might need to go further and revert some of my diagnostic patches,
but let's see where it is in the morning.
Thanx, Paul
