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 A lot of work by a lot of people over a long period of time. On their behalf, I thank you for the implied compliment. So once this patch gets in, perhaps you will have complimented yourself as well. ;-) But more work is needed, and will continue to be as new workloads, compiler optimizations, and hardware appears. And it would be good to try this on a really big system at some point. > > > > o Leave a CPU free to run things like the RCU grace-period kthread. > > > > You might also need to bind that kthread to that CPU. > > > > > > > > o Alternatively, use the "rcutree.kthread_prio=" boot parameter to > > > > boost the RCU kthreads to real-time priority. This won't do > > > > anything for ksoftirqd, though. > > > > > > I will try these as well. > > kthread_prio=50 definitely reduced the probability of OOM but it still > occurred. OK, interesting. > > > > o Along with the above boot parameter, use "rcutree.use_softirq=0" > > > > to cause RCU to use kthreads instead of softirq. (You might well > > > > find issues in priority setting as well, but might as well find > > > > them now if so!) > > > > > > Doesn't think one actually reduce the priority of the core RCU work? softirq > > > will always have higher priority than any there. So wouldn't that have the > > > effect of not reclaiming things fast enough? (Or, in my case not scheduling > > > the rcu_work which does the reclaim). > > > > For low kfree_rcu() loads, yes, it increases overhead due to the need > > for context switches instead of softirq running at the tail end of an > > interrupt. But for high kfree_rcu() loads, it gets you realtime priority > > (in conjunction with "rcutree.kthread_prio=", that is). > > I meant for high kfree_rcu() loads, a softirq context executing RCU callback > is still better from the point of view of the callback running because the > softirq will run above all else (higher than the highest priority task) so > use_softirq=0 would be a down grade from that perspective if something higher > than rcutree.kthread_prio is running on the CPU. So unless kthread_prio is > set to the highest prio, then softirq running would work better. Did I miss > something? Under heavy load, softirq stops running at the tail end of interrupts and is instead run within the context of a per-CPU ksoftirqd kthread. At normal SCHED_OTHER priority. > > > > o With any of the above, invoke rcu_momentary_dyntick_idle() along > > > > with cond_resched() in your kfree_rcu() loop. This simulates > > > > a trip to userspace for nohz_full CPUs, so if this helps for > > > > non-nohz_full CPUs, adjustments to the kernel might be called for. > > I did not try this yet. But I am thinking why would this help in nohz_idle > case? In nohz_idle we already have the tick active when CPU is idle. I guess > it is because there may be a long time that elapses before > rcu_data.rcu_need_heavy_qs == true ? Under your heavy rcuperf load, none of the CPUs would ever be idle. Nor would they every be in nohz_full userspace context, either. In contrast, a heavy duty userspace-driven workload would transition to and from userspace for each kfree_rcu(), and that would increment the dyntick-idle count on each transition to and from userspace. Adding the rcu_momentary_dyntick_idle() emulates a pair of such transitions. Thanx, Paul > > > Ok, will try it. > > > > > > Save these bullet points for future reference! ;-) thanks, > > > > I guess this is helping me to prepare for Plumbers. ;-) > > :-) > > thanks, Paul! > > - Joel >
