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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Fri, 9 Aug 2019 17:33:46 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x79GXjOj24052074 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 9 Aug 2019 16:33:45 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6461DB2067; Fri, 9 Aug 2019 16:33:45 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 2B21EB205F; Fri, 9 Aug 2019 16:33:45 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.154]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Fri, 9 Aug 2019 16:33:45 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id CD4A816C9A29; Fri, 9 Aug 2019 09:33:46 -0700 (PDT) Date: Fri, 9 Aug 2019 09:33:46 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Byungchul Park , linux-kernel@vger.kernel.org, Rao Shoaib , max.byungchul.park@gmail.com, kernel-team@android.com, kernel-team@lge.com, Davidlohr Bueso , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching Reply-To: paulmck@linux.ibm.com References: <20190806212041.118146-1-joel@joelfernandes.org> <20190806235631.GU28441@linux.ibm.com> <20190807094504.GB169551@google.com> <20190807175215.GE28441@linux.ibm.com> <20190808095232.GA30401@X58A-UD3R> <20190808125607.GB261256@google.com> <20190808233014.GA184373@google.com> <20190809151619.GD28441@linux.ibm.com> <20190809153924.GB211412@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190809153924.GB211412@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19080916-0060-0000-0000-0000036B59CE X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011575; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000287; SDB=6.01244392; UDB=6.00656519; IPR=6.01025874; MB=3.00028109; MTD=3.00000008; XFM=3.00000015; UTC=2019-08-09 16:33:50 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19080916-0061-0000-0000-00004A7D3341 Message-Id: <20190809163346.GF28441@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-09_05:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908090162 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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)" > > > Date: Thu, 8 Aug 2019 16:29:58 -0400 > > > Subject: [PATCH] Let list grow > > > > > > Signed-off-by: Joel Fernandes (Google) > > > --- > > > 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 > > > > > >