On Fri, Aug 09, 2019 at 05:25:12PM -0400, Joel Fernandes wrote:
> On Fri, Aug 09, 2019 at 04:26:45PM -0400, Joel Fernandes wrote:
> > 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!
>
> And, the number of grace periods did dramatically drop (by 5X) with the
> batching!! I have modified the rcuperf test to show the number of grace
> periods that elapsed during the test.
Very good! Batching for the win! ;-)
Thanx, Paul
