From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 33176C0650F for ; Thu, 8 Aug 2019 23:30:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E21432086A for ; Thu, 8 Aug 2019 23:30:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="ACPVRgsD" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1733258AbfHHXaR (ORCPT ); Thu, 8 Aug 2019 19:30:17 -0400 Received: from mail-pg1-f194.google.com ([209.85.215.194]:36945 "EHLO mail-pg1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732708AbfHHXaR (ORCPT ); Thu, 8 Aug 2019 19:30:17 -0400 Received: by mail-pg1-f194.google.com with SMTP id d1so12042236pgp.4 for ; Thu, 08 Aug 2019 16:30:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=fbtUbKzMjQggfyJoXEwOvtq7UssFlb6V5+VWKviPdQc=; b=ACPVRgsD+IdMhNn6fmN5OZenUm3BevU8eQ3cPcdHIdgK4aygRe36SXj2b8dA9kPf1I 0QQkNGpnbmH1MC1NdxOd3FpFqHQUTrF7UvzeOKcYkV011GhrWCR//qP/qrzAodPXhwpq Od0TeHoB8ErWKLMuPLT9iScnHp0XtCmRzpS3U= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=fbtUbKzMjQggfyJoXEwOvtq7UssFlb6V5+VWKviPdQc=; b=qw2JhDEOFKhck9e5Y84ig2OO1QHQQRxxVOKrKKzPTfSRby+8yppqnTgPqZKCvUEy9d f3c4+0QtwwgR560Q9UKeqdN6WBgvEHlGjMUJPYtzBp5u6o54U36ViqGn7QXe2v5EBTWy nPwW4CO2KWfg6BGauvyrNzjGrBGL7ukfFXX1Leaqcp/0ypN63SqTxAB3vMJqRw2cqbj7 ly75f1ygwr7SJz7FSJ9tZJ/apYHvcNsQ5BlShe0MEMEsNJrNqCY3++Vny29+I1nLVaZv ki1uimzag19Vc9mK3mjA5sVnfq0vVQYfrWItxi67jzLw1RqaNjGbvuzrKVFWXMIWkvii hr4w== X-Gm-Message-State: APjAAAUniHUbIphpwEUEFVj7b/6SuQhbRiuCyMn+B13UX+sadNq39OX/ UQck4aEDd/6Hl812QQiAqIlS3Q== X-Google-Smtp-Source: APXvYqwlBgxNYBk4zVtuAsxKB/6z770GZoySjs5PO5M1n4vtH9PuJYbRVvwVUODvTX3bKoKW4aZ4bg== X-Received: by 2002:a62:e301:: with SMTP id g1mr17410292pfh.119.1565307016304; Thu, 08 Aug 2019 16:30:16 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id p13sm3734889pjb.30.2019.08.08.16.30.14 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Thu, 08 Aug 2019 16:30:15 -0700 (PDT) Date: Thu, 8 Aug 2019 19:30:14 -0400 From: Joel Fernandes To: Byungchul Park Cc: "Paul E. McKenney" , 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 Message-ID: <20190808233014.GA184373@google.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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190808125607.GB261256@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org 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? 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