On Fri, Aug 09, 2019 at 04:22:26PM -0400, Joel Fernandes wrote: > On Fri, Aug 09, 2019 at 09:33:46AM -0700, Paul E. McKenney wrote: > > On Fri, Aug 09, 2019 at 11:39:24AM -0400, Joel Fernandes wrote: > > > On Fri, Aug 09, 2019 at 08:16:19AM -0700, Paul E. McKenney wrote: > > > > On Thu, Aug 08, 2019 at 07:30:14PM -0400, Joel Fernandes wrote: > > > [snip] > > > > > > But I could make it something like: > > > > > > 1. Letting ->head grow if ->head_free busy > > > > > > 2. If head_free is busy, then just queue/requeue the monitor to try > > > > > > again. > > > > > > > > > > > > This would even improve performance, but will still risk going out > > > > > > of memory. > > > > > > > > > > It seems I can indeed hit an out of memory condition once I changed > > > > > it to > > > > > "letting list grow" (diff is below which applies on top of this > > > > > patch) while > > > > > at the same time removing the schedule_timeout(2) and replacing it > > > > > with > > > > > cond_resched() in the rcuperf test. I think the reason is the > > > > > rcuperf test > > > > > starves the worker threads that are executing in workqueue context > > > > > after a > > > > > grace period and those are unable to get enough CPU time to kfree > > > > > things fast > > > > > enough. But I am not fully sure about it and need to test/trace more > > > > > to > > > > > figure out why this is happening. > > > > > > > > > > If I add back the schedule_uninterruptibe_timeout(2) call, the out of > > > > > memory > > > > > situation goes away. > > > > > > > > > > Clearly we need to do more work on this patch. > > > > > > > > > > In the regular kfree_rcu_no_batch() case, I don't hit this issue. I > > > > > believe > > > > > that since the kfree is happening in softirq context in the > > > > > _no_batch() case, > > > > > it fares better. The question then I guess is how do we run the > > > > > rcu_work in a > > > > > higher priority context so it is not starved and runs often enough. > > > > > I'll > > > > > trace more. > > > > > > > > > > Perhaps I can also lower the priority of the rcuperf threads to give > > > > > the > > > > > worker thread some more room to run and see if anything changes. But > > > > > I am not > > > > > sure then if we're preparing the code for the real world with such > > > > > modifications. > > > > > > > > > > Any thoughts? > > > > > > > > Several! With luck, perhaps some are useful. ;-) > > > > > > > > o Increase the memory via kvm.sh "--memory 1G" or more. The > > > > default is "--memory 500M". > > > > > > Thanks, this definitely helped. > > Also, I can go back to 500M if I just keep KFREE_DRAIN_JIFFIES at HZ/50. So I > am quite happy about that. I think I can declare that the "let list grow > indefinitely" design works quite well even with an insanely heavily loaded > case of every CPU in a 16CPU system with 500M memory, indefinitely doing > kfree_rcu()in a tight loop with appropriate cond_resched(). And I am like > thinking - wow how does this stuff even work at such insane scales :-D
Oh, and I should probably also count whether there are any 'total number of grace periods' reduction, due to the batching! thanks, - Joel
