Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching

[Joel Fernandes <joel@joelfernandes.org>]

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.

thanks,

 - Joel