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

[Byungchul Park]

On Sun, Aug 11, 2019 at 04:49:39PM -0700, Paul E. McKenney wrote:
> Maybe.  Note well that I said "potential issue".  When I checked a few
> years ago, none of the uses of rcu_barrier() cared about kfree_rcu().
> They cared instead about call_rcu() callbacks that accessed code or data
> that was going to disappear soon, for example, due to module unload or
> filesystem unmount.
> 
> So it -might- be that rcu_barrier() can stay as it is, but with changes
> as needed to documentation.
> 
> It also -might- be, maybe now or maybe some time in the future, that
> there will need to be a kfree_rcu_barrier() or some such.  But if so,
> let's not create it until it is needed.  For one thing, it is reasonably
> likely that something other than a kfree_rcu_barrier() would really
> be what was needed.  After all, the main point would be to make sure
> that the old memory really was freed before allocating new memory.

Now I fully understand what you meant thanks to you. Thank you for
explaining it in detail.

> But if the system had ample memory, why wait?  In that case you don't
> really need to wait for all the old memory to be freed, but rather for
> sufficient memory to be available for allocation.

Agree. Totally make sense.

Thanks,
Byungchul

> 
> 							Thanx, Paul
> 
> > Thanks,
> > Byungchul
> > 
> > > But now that we can see letting the list just grow works well, we don't
> > > have to consider this one at the moment. Let's consider this method
> > > again once we face the problem in the future by any chance.
> > > 
> > > > We should therefore just let the second list grow.  If experience shows
> > > > a need for callbacks to be sent up more quickly, it should be possible
> > > > to provide an additional list, so that two lists on a given CPU can both
> > > > be waiting for a grace period at the same time.
> > > 
> > > Or the third and fourth list might be needed in some system. But let's
> > > talk about it later too.
> > > 
> > > > > > I also agree. But this _FORCE thing will still not solve the issue Paul is
> > > > > > raising which is doing this loop possibly in irq disabled / hardirq context.
> > > > > 
> > > > > I added more explanation above. What I suggested is a way to avoid not
> > > > > only heavy
> > > > > work within the irq-disabled region of a single kfree_rcu() but also
> > > > > too many requests
> > > > > to be queued into ->head.
> > > > 
> > > > But let's start simple, please!
> > > 
> > > Yes. The simpler, the better.
> > > 
> > > > > > We can't even cond_resched() here. In fact since _FORCE is larger, it will be
> > > > > > even worse. Consider a real-time system with a lot of memory, in this case
> > > > > > letting ->head grow large is Ok, but looping for long time in IRQ disabled
> > > > > > would not be Ok.
> > > > > 
> > > > > Please check the explanation above.
> > > > > 
> > > > > > 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 is exactly what Paul said. The problem with this is ->head can grow too
> > > > > much. That's why I suggested the above one.
> > > > 
> > > > It can grow quite large, but how do you know that limiting its size will
> > > > really help?  Sure, you have limited the size, but does that really do
> > > 
> > > To decide the size, we might have to refer to how much pressure on
> > > memory and RCU there are at that moment and adjust it on runtime.
> > > 
> > > > anything for the larger problem of extreme kfree_rcu() rates on the one
> > > > hand and a desire for more efficient handling of kfree_rcu() on the other?
> > > 
> > > Assuming current RCU logic handles extremly high rate well which is
> > > anyway true, my answer is *yes*, because batching anyway has pros and
> > > cons. One of major cons is there must be inevitable kfree_rcu() requests
> > > that not even request to RCU. By allowing only the size of batching, the
> > > situation can be mitigated.
> > > 
> > > I just answered to you. But again, let's talk about it later once we
> > > face the problem as you said.
> > > 
> > > Thanks,
> > > Byungchul
> > > 
> > > > 							Thanx, Paul
> > > > 
> > > > > > This would even improve performance, but will still risk going out of memory.
> > > > > >
> > > > > > Thoughts?
> > > > > >
> > > > > > thanks,
> > > > > >
> > > > > >  - Joel
> > > > > >
> > > > > > >
> > > > > > > This way, we can avoid both:
> > > > > > >
> > > > > > > 1. too many requests being queued and
> > > > > > > 2. __call_rcu() bunch of requests within a single kfree_rcu().
> > > > > > >
> > > > > > > Thanks,
> > > > > > > Byungchul
> > > > > > >
> > > > > > > >
> > > > > > > > But please feel free to come up with a better solution!
> > > > > > > >
> > > > > > > > [ . . . ]
> > > > > 
> > > > > 
> > > > > 
> > > > > -- 
> > > > > Thanks,
> > > > > Byungchul
> > > > >