From nobody Tue Feb 10 05:10:47 2026 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 6F9E8C761A6 for ; Thu, 30 Mar 2023 19:18:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232194AbjC3TS2 (ORCPT ); Thu, 30 Mar 2023 15:18:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48526 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231845AbjC3TST (ORCPT ); Thu, 30 Mar 2023 15:18:19 -0400 Received: from mail-pl1-x649.google.com (mail-pl1-x649.google.com [IPv6:2607:f8b0:4864:20::649]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6FD7EFF35 for ; Thu, 30 Mar 2023 12:18:11 -0700 (PDT) Received: by mail-pl1-x649.google.com with SMTP id q9-20020a170902dac900b001a18ceff5ebso11612229plx.4 for ; Thu, 30 Mar 2023 12:18:11 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; t=1680203891; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=4nnUf1Bi8zb8HMhH9DtFsVEWdtAjMz82Sf34ZimV0fg=; b=X2RFp1pZWOYnRsIh7IWjsVsM+KEFc9WJSHxF/6WWA0h+C4aKaj5fmmxIF9JIbSgACd wSFJlPVZHSShfit9sIv2gEMkHiLDYI1aj0ZjNxN+KDFEUuJ5hrS0dAiMoZtgG8tE01Qw Z6IuI1YwDe0K3jJKl09PQ+88WTXCXmLj3mXjZyc3NpqNvz62t2kYidmfKl22OQzA3sQQ EAjXhE1Jh+vHizPfZUZ3LQ+IzSkqjVL6en02vxZp22lS6vBDlsf8gGPBHsHSCXP5Ac15 JluiS/D70cGu14FmTesFiFf/4cN9gJ7flhGTivB535PauxJnWfGkMyHN0wJ36oB1Mtk7 YlVQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; t=1680203891; 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=4nnUf1Bi8zb8HMhH9DtFsVEWdtAjMz82Sf34ZimV0fg=; b=Zm+lShhCPbLalzq59B8ZMMAt7Sk1N2DiS8uEa9o2MEcLdyl7aos1Ou4UviPfuaw3GB zMYozFQw0mMDnjRlOVKbn2OPfhaE/hDmHotuEKQfN4Uunh3WeyINHDanuGhT13VFLHzF /BH0vlZxZrBgOXWB6YlAIkAtujfhoTw9jPscO3c0stN7ZkhCW/6WDJpO9wGieeFGeNDr pMEUD5R5lYLcsABQ5F5LtnlIWpmvPhG5mtP5sAIeJ/EBOqBVhchVfXFySSV1P9umfGZB vWHZpF5o0scs9Z4j0UT5okgZtpAV8SVuplPXVNnpKRKsKFWk4B2xvYMD1mhlJPfjQxyX VKaA== X-Gm-Message-State: AAQBX9dATVZiU+26Z1/mTKaPo1mCCM3UwDn+Kqq3bsYQjLQKVJoJPxva Ud78J5C51DfohbZBum2mVx/RY9nC6m6sxxBY X-Google-Smtp-Source: AKy350ZmAXD9xXrcG5sJYCYyMiAcFBYVD5KvS63XKanxHd2/adSILGt8yBb9ajmjtAKFUTblNAGcd8BbimVIOaMZ X-Received: from yosry.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:2327]) (user=yosryahmed job=sendgmr) by 2002:a05:6a00:2286:b0:627:e6d5:ba2d with SMTP id f6-20020a056a00228600b00627e6d5ba2dmr13290706pfe.6.1680203890966; Thu, 30 Mar 2023 12:18:10 -0700 (PDT) Date: Thu, 30 Mar 2023 19:17:57 +0000 In-Reply-To: <20230330191801.1967435-1-yosryahmed@google.com> Mime-Version: 1.0 References: <20230330191801.1967435-1-yosryahmed@google.com> X-Mailer: git-send-email 2.40.0.348.gf938b09366-goog Message-ID: <20230330191801.1967435-5-yosryahmed@google.com> Subject: [PATCH v3 4/8] 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=20Koutn=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 , Michal Hocko Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 =3D now + delay flush() spin_unlock() spin_trylock() flush_next_time =3D 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 if the compiler decides to split the write, 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.co= m/ Signed-off-by: Yosry Ahmed Acked-by: Johannes Weiner Acked-by: Shakeel Butt Acked-by: Michal Hocko --- mm/memcontrol.c | 18 +++++++++++------- 1 file changed, 11 insertions(+), 7 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index ff39f78f962e..65750f8b8259 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -585,8 +585,8 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tr= ee_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 =3D ATOMIC_INIT(0); static atomic_t stats_flush_threshold =3D ATOMIC_INIT(0); static u64 flush_next_time; =20 @@ -636,15 +636,19 @@ static inline void memcg_rstat_updated(struct mem_cgr= oup *memcg, int val) =20 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_read(&stats_flush_ongoing) || + atomic_xchg(&stats_flush_ongoing, 1)) return; =20 - flush_next_time =3D 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); } =20 void mem_cgroup_flush_stats(void) @@ -655,7 +659,7 @@ void mem_cgroup_flush_stats(void) =20 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(); } =20 --=20 2.40.0.348.gf938b09366-goog