On Thu, Aug 08, 2019 at 07:30:14PM -0400, Joel Fernandes wrote:
> On Thu, Aug 08, 2019 at 08:56:07AM -0400, Joel Fernandes 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.
> > >
> > > 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.
>
> 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".
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.
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!)
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.
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)" <j...@joelfernandes.org>
> Date: Thu, 8 Aug 2019 16:29:58 -0400
> Subject: [PATCH] Let list grow
>
> Signed-off-by: Joel Fernandes (Google) <j...@joelfernandes.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
>
