From: Byungchul Park <byungchul.park@lge.com>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Byungchul Park <max.byungchul.park@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
LKML <linux-kernel@vger.kernel.org>,
Rao Shoaib <rao.shoaib@oracle.com>,
kernel-team@android.com, kernel-team <kernel-team@lge.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
rcu <rcu@vger.kernel.org>, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching
Date: Mon, 12 Aug 2019 19:10:52 +0900 [thread overview]
Message-ID: <20190812101052.GA10478@X58A-UD3R> (raw)
In-Reply-To: <20190811234939.GC28441@linux.ibm.com>
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
> > > > >
next prev parent reply other threads:[~2019-08-12 10:12 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-06 21:20 [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching Joel Fernandes (Google)
2019-08-06 21:20 ` [PATCH RFC v1 2/2] rcuperf: Add kfree_rcu performance Tests Joel Fernandes (Google)
2019-08-07 0:29 ` Paul E. McKenney
2019-08-07 10:22 ` Joel Fernandes
2019-08-07 17:56 ` Paul E. McKenney
2019-08-09 16:01 ` Joel Fernandes
2019-08-11 2:01 ` Joel Fernandes
2019-08-11 23:42 ` Paul E. McKenney
2019-08-06 23:56 ` [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching Paul E. McKenney
2019-08-07 9:45 ` Joel Fernandes
2019-08-07 17:52 ` Paul E. McKenney
2019-08-08 9:52 ` Byungchul Park
2019-08-08 12:56 ` Joel Fernandes
2019-08-08 14:23 ` Byungchul Park
2019-08-08 18:09 ` Paul E. McKenney
2019-08-11 8:36 ` Byungchul Park
2019-08-11 8:49 ` Byungchul Park
2019-08-11 23:49 ` Paul E. McKenney
2019-08-12 10:10 ` Byungchul Park [this message]
2019-08-12 13:12 ` Joel Fernandes
2019-08-13 5:29 ` Byungchul Park
2019-08-13 15:41 ` Paul E. McKenney
2019-08-14 0:11 ` Byungchul Park
2019-08-14 2:53 ` Paul E. McKenney
2019-08-14 3:43 ` Byungchul Park
2019-08-14 16:59 ` Paul E. McKenney
2019-08-11 10:37 ` Byungchul Park
2019-08-08 23:30 ` Joel Fernandes
2019-08-09 15:16 ` Paul E. McKenney
2019-08-09 15:39 ` Joel Fernandes
2019-08-09 16:33 ` Paul E. McKenney
2019-08-09 20:22 ` Joel Fernandes
2019-08-09 20:26 ` Joel Fernandes
2019-08-09 21:25 ` Joel Fernandes
2019-08-10 3:38 ` Paul E. McKenney
2019-08-09 20:29 ` Joel Fernandes
2019-08-09 20:42 ` Paul E. McKenney
2019-08-09 21:36 ` Joel Fernandes
2019-08-10 3:40 ` Paul E. McKenney
2019-08-10 3:52 ` Joel Fernandes
2019-08-10 2:42 ` Joel Fernandes
2019-08-10 3:38 ` Paul E. McKenney
2019-08-10 4:20 ` Joel Fernandes
2019-08-10 18:24 ` Paul E. McKenney
2019-08-11 2:26 ` Joel Fernandes
2019-08-11 23:35 ` Paul E. McKenney
2019-08-12 13:13 ` Joel Fernandes
2019-08-12 14:44 ` Paul E. McKenney
2019-08-08 10:26 ` Byungchul Park
2019-08-08 18:11 ` Paul E. McKenney
2019-08-08 20:13 ` Joel Fernandes
2019-08-08 20:51 ` Paul E. McKenney
2019-08-08 22:34 ` Joel Fernandes
2019-08-08 22:37 ` Paul E. McKenney
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