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Message-Id: <20190809163346.GF28441@linux.ibm.com>
Date: Fri, 9 Aug 2019 09:33:46 -0700
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Byungchul Park <byungchul.park@....com>,
linux-kernel@...r.kernel.org, Rao Shoaib <rao.shoaib@...cle.com>,
max.byungchul.park@...il.com, kernel-team@...roid.com,
kernel-team@....com, Davidlohr Bueso <dave@...olabs.net>,
Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu
batching
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.
>
> > o Leave a CPU free to run things like the RCU grace-period kthread.
> > You might also need to bind that kthread to that CPU.
> >
> > o Alternatively, use the "rcutree.kthread_prio=" boot parameter to
> > boost the RCU kthreads to real-time priority. This won't do
> > anything for ksoftirqd, though.
>
> I will try these as well.
>
> >
> > o Along with the above boot parameter, use "rcutree.use_softirq=0"
> > to cause RCU to use kthreads instead of softirq. (You might well
> > find issues in priority setting as well, but might as well find
> > them now if so!)
>
> Doesn't think one actually reduce the priority of the core RCU work? softirq
> will always have higher priority than any there. So wouldn't that have the
> effect of not reclaiming things fast enough? (Or, in my case not scheduling
> the rcu_work which does the reclaim).
For low kfree_rcu() loads, yes, it increases overhead due to the need
for context switches instead of softirq running at the tail end of an
interrupt. But for high kfree_rcu() loads, it gets you realtime priority
(in conjunction with "rcutree.kthread_prio=", that is).
> > o With any of the above, invoke rcu_momentary_dyntick_idle() along
> > with cond_resched() in your kfree_rcu() loop. This simulates
> > a trip to userspace for nohz_full CPUs, so if this helps for
> > non-nohz_full CPUs, adjustments to the kernel might be called for.
>
> Ok, will try it.
>
> Save these bullet points for future reference! ;-) thanks,
I guess this is helping me to prepare for Plumbers. ;-)
Thanx, Paul
> - Joel
>
>
> >
> > Probably others, but this should do for a start.
> >
> > Thanx, Paul
> >
> > > thanks,
> > >
> > > - Joel
> > >
> > > ---8<-----------------------
> > >
> > > >From 098d62e5a1b84a11139236c9b1f59e7f32289b40 Mon Sep 17 00:00:00 2001
> > > From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> > > Date: Thu, 8 Aug 2019 16:29:58 -0400
> > > Subject: [PATCH] Let list grow
> > >
> > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > > ---
> > > kernel/rcu/rcuperf.c | 2 +-
> > > kernel/rcu/tree.c | 52 +++++++++++++++++++-------------------------
> > > 2 files changed, 23 insertions(+), 31 deletions(-)
> > >
> > > diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
> > > index 34658760da5e..7dc831db89ae 100644
> > > --- a/kernel/rcu/rcuperf.c
> > > +++ b/kernel/rcu/rcuperf.c
> > > @@ -654,7 +654,7 @@ kfree_perf_thread(void *arg)
> > > }
> > > }
> > >
> > > - schedule_timeout_uninterruptible(2);
> > > + cond_resched();
> > > } while (!torture_must_stop() && ++l < kfree_loops);
> > >
> > > kfree(alloc_ptrs);
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index bdbd483606ce..bab77220d8ac 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2595,7 +2595,7 @@ EXPORT_SYMBOL_GPL(call_rcu);
> > >
> > >
> > > /* Maximum number of jiffies to wait before draining batch */
> > > -#define KFREE_DRAIN_JIFFIES 50
> > > +#define KFREE_DRAIN_JIFFIES (HZ / 20)
> > >
> > > /*
> > > * Maximum number of kfree(s) to batch, if limit is hit
> > > @@ -2684,27 +2684,19 @@ static void kfree_rcu_drain_unlock(struct kfree_rcu_cpu *krc,
> > > {
> > > struct rcu_head *head, *next;
> > >
> > > - /* It is time to do bulk reclaim after grace period */
> > > - krc->monitor_todo = false;
> > > + /* It is time to do bulk reclaim after grace period. */
> > > if (queue_kfree_rcu_work(krc)) {
> > > spin_unlock_irqrestore(&krc->lock, flags);
> > > return;
> > > }
> > >
> > > - /*
> > > - * Use non-batch regular call_rcu for kfree_rcu in case things are too
> > > - * busy and batching of kfree_rcu could not be used.
> > > - */
> > > - head = krc->head;
> > > - krc->head = NULL;
> > > - krc->kfree_batch_len = 0;
> > > - spin_unlock_irqrestore(&krc->lock, flags);
> > > -
> > > - for (; head; head = next) {
> > > - next = head->next;
> > > - head->next = NULL;
> > > - __call_rcu(head, head->func, -1, 1);
> > > + /* Previous batch did not get free yet, let us try again soon. */
> > > + if (krc->monitor_todo == false) {
> > > + schedule_delayed_work_on(smp_processor_id(),
> > > + &krc->monitor_work, KFREE_DRAIN_JIFFIES/4);
> > > + krc->monitor_todo = true;
> > > }
> > > + spin_unlock_irqrestore(&krc->lock, flags);
> > > }
> > >
> > > /*
> > > --
> > > 2.23.0.rc1.153.gdeed80330f-goog
> > >
> >
>
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