----- 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. Thoughts ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
