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
