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

[Byungchul Park <byungchul.park@lge.com>]

On Sun, Aug 11, 2019 at 05:36:26PM +0900, Byungchul Park wrote:
> On Thu, Aug 08, 2019 at 11:09:16AM -0700, Paul E. McKenney wrote:
> > On Thu, Aug 08, 2019 at 11:23:17PM +0900, Byungchul Park wrote:
> > > On Thu, Aug 8, 2019 at 9:56 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > > >
> > > > On Thu, Aug 08, 2019 at 06:52:32PM +0900, Byungchul Park wrote:
> > > > > On Wed, Aug 07, 2019 at 10:52:15AM -0700, Paul E. McKenney wrote:
> > > > > > > > On Tue, Aug 06, 2019 at 05:20:40PM -0400, Joel Fernandes (Google) wrote:
> > > > > > [ . . . ]
> > > > > > > > > +     for (; head; head = next) {
> > > > > > > > > +             next = head->next;
> > > > > > > > > +             head->next = NULL;
> > > > > > > > > +             __call_rcu(head, head->func, -1, 1);
> > > > > > > >
> > > > > > > > We need at least a cond_resched() here.  200,000 times through this loop
> > > > > > > > in a PREEMPT=n kernel might not always be pretty.  Except that this is
> > > > > > > > invoked directly from kfree_rcu() which might be invoked with interrupts
> > > > > > > > disabled, which precludes calls to cond_resched().  So the realtime guys
> > > > > > > > are not going to be at all happy with this loop.
> > > > > > >
> > > > > > > Ok, will add this here.
> > > > > > >
> > > > > > > > And this loop could be avoided entirely by having a third rcu_head list
> > > > > > > > in the kfree_rcu_cpu structure.  Yes, some of the batches would exceed
> > > > > > > > KFREE_MAX_BATCH, but given that they are invoked from a workqueue, that
> > > > > > > > should be OK, or at least more OK than queuing 200,000 callbacks with
> > > > > > > > interrupts disabled.  (If it turns out not to be OK, an array of rcu_head
> > > > > > > > pointers can be used to reduce the probability of oversized batches.)
> > > > > > > > This would also mean that the equality comparisons with KFREE_MAX_BATCH
> > > > > > > > need to become greater-or-equal comparisons or some such.
> > > > > > >
> > > > > > > Yes, certainly we can do these kinds of improvements after this patch, and
> > > > > > > then add more tests to validate the improvements.
> > > > > >
> > > > > > Out of pity for people bisecting, we need this fixed up front.
> > > > > >
> > > > > > My suggestion is to just allow ->head to grow until ->head_free becomes
> > > > > > available.  That way you are looping with interrupts and preemption
> > > > > > enabled in workqueue context, which is much less damaging than doing so
> > > > > > with interrupts disabled, and possibly even from hard-irq context.
> > > > >
> > > > > Agree.
> > > > >
> > > > > Or after introducing another limit like KFREE_MAX_BATCH_FORCE(>=
> > > > > KFREE_MAX_BATCH):
> > > > >
> > > > > 1. Try to drain it on hitting KFREE_MAX_BATCH as it does.
> > > > >
> > > > >    On success: Same as now.
> > > > >    On fail: let ->head grow and drain if possible, until reaching to
> > > > >             KFREE_MAX_BATCH_FORCE.
> > > 
> > > I should've explain this in more detail. This actually mean:
> > > 
> > > On fail: Let ->head grow and queue rcu_work when ->head_free == NULL,
> > >          until reaching to _FORCE.
> > > 
> > > > > 3. On hitting KFREE_MAX_BATCH_FORCE, give up batching but handle one by
> > > > >    one from now on to prevent too many pending requests from being
> > > > >    queued for batching work.
> > > 
> > > This mean:
> > > 
> > > 3. On hitting KFREE_MAX_BATCH_FORCE, give up batching requests to be added
> > >    from now on but instead handle one by one to prevent too many
> > > pending requests
> 
> Oh! I'm sorry for the weird formatted mail that I wrote with another
> mail client than the one I usually use, outside of office.
> 
> > >    from being queued. Of course, the requests already having been
> > > queued in ->head
> > >    so far should be handled by rcu_work when it's possible which can
> > > be checked by
> > >    the monitor or kfree_rcu() inside every call.
> > 
> > But does this really help?  After all, the reason we have piled up a
> > large number of additional callbacks is likely because the grace period
> > is taking a long time, or because a huge number of callbacks has been
> > queued up.  Sure, these callbacks might get a head start on the following
> > grace period, but at the expense of still retaining the kfree_rcu()
> > special cases in rcu_do_batch().
> 
> Now, I just can see what you want to get with this work. Then we'd
> better avoid that kind of exception as much as possible.
> 
> > Another potential issue is interaction with rcu_barrier().  Currently,
> > rcu_barrier() waits for memory passed to prior kfree_rcu() calls to be
> > freed.  This is useful to allow a large amount of memory be be completely
> > freed before allocating large amounts more memory.  With the earlier
> > version of the patch, an rcu_barrier() followed by a flush_workqueue().
> > But #3 above would reorder the objects so that this approach might not
> > wait for everything.
> 
> It doesn't matter by making the queue operated in FIFO manner though,
> so as to guarantee the order.

I only explained about the re-order problem but yes, we need to come up
with how to deal with the synchronization with rcu_barrier() as you said.

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
> > >