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)" <joel@xxxxxxxxxxxxxxxxx> > Date: Thu, 8 Aug 2019 16:29:58 -0400 > Subject: [PATCH] Let list grow > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > --- > 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 >