From: Joel Fernandes <joel@joelfernandes.org>
To: Byungchul Park <byungchul.park@lge.com>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>,
linux-kernel@vger.kernel.org, Rao Shoaib <rao.shoaib@oracle.com>,
max.byungchul.park@gmail.com, kernel-team@android.com,
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@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: Thu, 8 Aug 2019 08:56:07 -0400 [thread overview]
Message-ID: <20190808125607.GB261256@google.com> (raw)
In-Reply-To: <20190808095232.GA30401@X58A-UD3R>
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.
>
> 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.
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.
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.
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.
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!
> >
> > [ . . . ]
next prev parent reply other threads:[~2019-08-08 12:56 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 [this message]
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
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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