From patchwork Tue Mar 28 06:16:34 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Yosry Ahmed X-Patchwork-Id: 13190548 X-Patchwork-Delegate: kuba@kernel.org Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 59E7BC76195 for ; Tue, 28 Mar 2023 06:17:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232521AbjC1GR2 (ORCPT ); Tue, 28 Mar 2023 02:17:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34744 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232526AbjC1GRI (ORCPT ); Tue, 28 Mar 2023 02:17:08 -0400 Received: from mail-pg1-x54a.google.com (mail-pg1-x54a.google.com [IPv6:2607:f8b0:4864:20::54a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4A1C33C2B for ; Mon, 27 Mar 2023 23:16:51 -0700 (PDT) Received: by mail-pg1-x54a.google.com with SMTP id t12-20020a65608c000000b005091ec4f2d4so3007672pgu.20 for ; Mon, 27 Mar 2023 23:16:51 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; t=1679984211; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=IdlOdGF0YCCYf+h2+Wx+Pr2285UVOLm52gCW9Jb/OEo=; b=dxPzci0vIh1xyJxlLBjWkEhAjm9YbPSdsUTO5u3B5j0mjvT8KDBeosrn5UiPm3Ksna ECFLKks3+i/JLwnVARkeiq9XCne6PDhOU+plEpcmqroZ0FIo5b3stpDp4azRz7RDEUXr fg0RX+xHWkbr/97SE9yMggMcSWIL9SDqbA2byVm27sTtHVM/RcaUTUFVwNm3hT6OyTW8 KGm1B3oJJOu2mLGqIrSENlsxO5AJatCTwMfnNnO9IVILzvLEURQL9lNGe4Pge9n4mWvC 01ODxHoNCu+DMqEmep2UjWrEts9KjVkOV7sWpi8aLYcEgH/6hlGK54jM+qhbvTAI+Y6D 5qIg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; t=1679984211; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=IdlOdGF0YCCYf+h2+Wx+Pr2285UVOLm52gCW9Jb/OEo=; b=uDQ9XRsMUTGgMDvFVTpWR9V3ccDqXYLU4jvP7/5+itjevGZeoKk1YvP0NDxFNki7oJ u9wjoW0Ui+m/Fi5m5n5jrzttpYoL0bRaKavOnf+bkUbH+iXNF6m1Pt7NhG0O65jPSKpj ya8i1wlRppvmrHo7l75GQuS49gvSnzm3TlfwXXCL4ljH7aDSKlJON2doCCc6qIx5Chuv RvFAI1hizRRxVpvkHONK7VgO2POqX4qcCKjklGqL0eqRdFD3gb98gq9iUROKm7OeGQvm u2NtNDGQT4P7YEPoTgL49UlaVHCoXETlZ4pyOFAJOva9Ax5Egd+MXauoIBraRjrCdMLw uXBA== X-Gm-Message-State: AAQBX9cE4AO25Nu1paoVJ82VbJYvCbdvyU7vtPJecWX7l+i775nLerKX ACtZL7Pi0nfK1EN2QRPwywxk2uGj8s4N/3/t X-Google-Smtp-Source: AKy350ZiMUeqTdYquDazwFQP3GVswROLJyitcbpH2br9NmUFXZZud9qfmMVPueO4yaGFykMkMD2PV8f94RgXrI2o X-Received: from yosry.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:2327]) (user=yosryahmed job=sendgmr) by 2002:a17:90a:74c9:b0:23f:1105:4295 with SMTP id p9-20020a17090a74c900b0023f11054295mr4298168pjl.7.1679984210825; Mon, 27 Mar 2023 23:16:50 -0700 (PDT) Date: Tue, 28 Mar 2023 06:16:34 +0000 In-Reply-To: <20230328061638.203420-1-yosryahmed@google.com> Mime-Version: 1.0 References: <20230328061638.203420-1-yosryahmed@google.com> X-Mailer: git-send-email 2.40.0.348.gf938b09366-goog Message-ID: <20230328061638.203420-6-yosryahmed@google.com> Subject: [PATCH v1 5/9] memcg: replace stats_flush_lock with an atomic From: Yosry Ahmed To: Tejun Heo , Josef Bacik , Jens Axboe , Zefan Li , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , " =?utf-8?q?Michal_Koutn=C3=BD?= " Cc: Vasily Averin , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, bpf@vger.kernel.org, Yosry Ahmed Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org As Johannes notes in [1], stats_flush_lock is currently used to: (a) Protect updated to stats_flush_threshold. (b) Protect updates to flush_next_time. (c) Serializes calls to cgroup_rstat_flush() based on those ratelimits. However: 1. stats_flush_threshold is already an atomic 2. flush_next_time is not atomic. The writer is locked, but the reader is lockless. If the reader races with a flush, you could see this: if (time_after(jiffies, flush_next_time)) spin_trylock() flush_next_time = now + delay flush() spin_unlock() spin_trylock() flush_next_time = now + delay flush() spin_unlock() which means we already can get flushes at a higher frequency than FLUSH_TIME during races. But it isn't really a problem. The reader could also see garbled partial updates, so it needs at least READ_ONCE and WRITE_ONCE protection. 3. Serializing cgroup_rstat_flush() calls against the ratelimit factors is currently broken because of the race in 2. But the race is actually harmless, all we might get is the occasional earlier flush. If there is no delta, the flush won't do much. And if there is, the flush is justified. So the lock can be removed all together. However, the lock also served the purpose of preventing a thundering herd problem for concurrent flushers, see [2]. Use an atomic instead to serve the purpose of unifying concurrent flushers. [1]https://lore.kernel.org/lkml/20230323172732.GE739026@cmpxchg.org/ [2]https://lore.kernel.org/lkml/20210716212137.1391164-2-shakeelb@google.com/ Signed-off-by: Yosry Ahmed Acked-by: Johannes Weiner --- mm/memcontrol.c | 17 ++++++++++------- 1 file changed, 10 insertions(+), 7 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index ff39f78f962e..64ff33e02c96 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -585,8 +585,8 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_node *mctz) */ static void flush_memcg_stats_dwork(struct work_struct *w); static DECLARE_DEFERRABLE_WORK(stats_flush_dwork, flush_memcg_stats_dwork); -static DEFINE_SPINLOCK(stats_flush_lock); static DEFINE_PER_CPU(unsigned int, stats_updates); +static atomic_t stats_flush_ongoing = ATOMIC_INIT(0); static atomic_t stats_flush_threshold = ATOMIC_INIT(0); static u64 flush_next_time; @@ -636,15 +636,18 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val) static void __mem_cgroup_flush_stats(void) { - unsigned long flag; - - if (!spin_trylock_irqsave(&stats_flush_lock, flag)) + /* + * We always flush the entire tree, so concurrent flushers can just + * skip. This avoids a thundering herd problem on the rstat global lock + * from memcg flushers (e.g. reclaim, refault, etc). + */ + if (atomic_xchg(&stats_flush_ongoing, 1)) return; - flush_next_time = jiffies_64 + 2*FLUSH_TIME; + WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME); cgroup_rstat_flush_atomic(root_mem_cgroup->css.cgroup); atomic_set(&stats_flush_threshold, 0); - spin_unlock_irqrestore(&stats_flush_lock, flag); + atomic_set(&stats_flush_ongoing, 0); } void mem_cgroup_flush_stats(void) @@ -655,7 +658,7 @@ void mem_cgroup_flush_stats(void) void mem_cgroup_flush_stats_ratelimited(void) { - if (time_after64(jiffies_64, flush_next_time)) + if (time_after64(jiffies_64, READ_ONCE(flush_next_time))) mem_cgroup_flush_stats(); }