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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 15BDBEA4E3A for ; Mon, 2 Mar 2026 16:14:19 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 3891E6B0005; Mon, 2 Mar 2026 11:14:18 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 336876B0088; Mon, 2 Mar 2026 11:14:18 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 218EC6B0089; Mon, 2 Mar 2026 11:14:18 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 1371C6B0005 for ; Mon, 2 Mar 2026 11:14:18 -0500 (EST) Received: from smtpin27.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id B589A89FB6 for ; Mon, 2 Mar 2026 16:14:17 +0000 (UTC) X-FDA: 84501620154.27.4DD33CC Received: from out-178.mta1.migadu.com (out-178.mta1.migadu.com [95.215.58.178]) by imf21.hostedemail.com (Postfix) with ESMTP id 8DE721C000A for ; Mon, 2 Mar 2026 16:14:15 +0000 (UTC) Authentication-Results: imf21.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=YG01ILjL; spf=pass (imf21.hostedemail.com: domain of shakeel.butt@linux.dev designates 95.215.58.178 as permitted sender) smtp.mailfrom=shakeel.butt@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1772468056; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=MLL8CQedq0WsIHC423sVIgR4mj4t68qiQx4HMXd9uDE=; b=3sSvfUPpNVx3KNFxhtJO8+pjbaOIXyvUrikd++eucSk4Fg/1wIkHo/yJWYztx44GP2ry1g w8OEHtTBcDChGbJENXMp9raDEM7PSGeR0mYsjwo623MZpJZpczbXtgm3EJJTyUDS8o1xjn Qvi0g1jyHnGA0n4x9oam5OhfhRA799k= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1772468056; a=rsa-sha256; cv=none; b=p8LE4YsvNZYbvMjnW27llD5hKfuwzio8m4SThhKfOc/3kDle9Ej5AC8HDpgE6TyxyxZ0Aq BvMrBVXBwVHnKE4nz9RObbHSSisO7eWt16bjZ85EMk4cvcVa4pKEwWCyUBEDz25dKQHxQ0 zn66rSrkMU+dq/T8JLXrfxomof0SXto= ARC-Authentication-Results: i=1; imf21.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=YG01ILjL; spf=pass (imf21.hostedemail.com: domain of shakeel.butt@linux.dev designates 95.215.58.178 as permitted sender) smtp.mailfrom=shakeel.butt@linux.dev; dmarc=pass (policy=none) header.from=linux.dev Date: Mon, 2 Mar 2026 08:14:05 -0800 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1772468053; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=MLL8CQedq0WsIHC423sVIgR4mj4t68qiQx4HMXd9uDE=; b=YG01ILjLYI0dk46V+QncPZZT1q3hBjoc84PeXOZJVUm0yLSx284+Q4kliCpzftFLybp+5L kpTVPFxfUITqsKaMqQAgDHi5QNKoE5mvAK0ecsIL/CeH8tWyYKJTYft+DRokoxgqFSwbBG 1ptqeewLBnfoxD5xDpCIAv4liTj8Y2c= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Shakeel Butt To: Chen Ridong Cc: Tejun Heo , Johannes Weiner , Michal =?utf-8?Q?Koutn=C3=BD?= , Roman Gushchin , Kuniyuki Iwashima , Daniel Sedlak , Meta kernel team , linux-mm@kvack.org, netdev@vger.kernel.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Jakub Kicinski Subject: Re: [PATCH 2/3] cgroup: add lockless fast-path checks to cgroup_file_notify() Message-ID: References: <20260228142018.3178529-1-shakeel.butt@linux.dev> <20260228142018.3178529-3-shakeel.butt@linux.dev> <40c77bba-0862-4422-b23e-2a10cd01c728@huaweicloud.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <40c77bba-0862-4422-b23e-2a10cd01c728@huaweicloud.com> X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 8DE721C000A X-Stat-Signature: 7kb6r7g7bwiwwn98aqentm44knh9xfaa X-Rspam-User: X-HE-Tag: 1772468055-41725 X-HE-Meta: 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 2EWJg0eo pPaPhY+8GRQzBjgzpSq9ja+4CW9Djpgz2XPWJ8L7E4bAksSPzNdLAx9/y0MuC9+krOV36xlA5Uu2VWL0nkMhu1TC5PEOedbod6rel7zY9Gca+tmPBIuSjMWcmrqCDcXCSovYqWUVF0OUwZN9QLEVLsuidNY0ZgJfd1P/PZtvN04GSaYpwnfvjQw9N4InCLc9D5RZHXeeAGSBQwlIV2bEfxbuDai9kPgz4BOH5F9beVaN96jKpPbQRkpbHylCZjcwkhy5yRuiMV43ryJ1H/bztDv44WA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Chen, thanks for taking a look. On Mon, Mar 02, 2026 at 09:50:53AM +0800, Chen Ridong wrote: > > Hi Shakeel, > > Good series to move away from the global lock. > > On 2026/2/28 22:20, Shakeel Butt wrote: > > Add two lockless checks before acquiring the lock: > > > > 1. READ_ONCE(cfile->kn) NULL check to skip torn-down files. > > 2. READ_ONCE(cfile->notified_at) check to skip when within the > > rate-limit window (~10ms). > > > > Both checks have safe error directions -- a stale read can only cause > > unnecessary lock acquisition, never a missed notification. Annotate > > all write sites with WRITE_ONCE() to pair with the lockless readers. > > > > The trade-off is that trailing timer_reduce() calls during bursts are > > skipped, so the deferred notification that delivers the final state > > may be lost. This is acceptable for the primary callers like > > __memcg_memory_event() where events keep arriving. > > > > Signed-off-by: Shakeel Butt > > Reported-by: Jakub Kicinski > > --- > > kernel/cgroup/cgroup.c | 21 ++++++++++++++------- > > 1 file changed, 14 insertions(+), 7 deletions(-) > > > > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c > > index 33282c7d71e4..5473ebd0f6c1 100644 > > --- a/kernel/cgroup/cgroup.c > > +++ b/kernel/cgroup/cgroup.c > > @@ -1749,7 +1749,7 @@ static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft) > > struct cgroup_file *cfile = (void *)css + cft->file_offset; > > > > spin_lock_irq(&cgroup_file_kn_lock); > > - cfile->kn = NULL; > > + WRITE_ONCE(cfile->kn, NULL); > > spin_unlock_irq(&cgroup_file_kn_lock); > > > > timer_delete_sync(&cfile->notify_timer); > > @@ -4430,7 +4430,7 @@ static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp, > > timer_setup(&cfile->notify_timer, cgroup_file_notify_timer, 0); > > > > spin_lock_irq(&cgroup_file_kn_lock); > > - cfile->kn = kn; > > + WRITE_ONCE(cfile->kn, kn); > > spin_unlock_irq(&cgroup_file_kn_lock); > > } > > > > @@ -4686,20 +4686,27 @@ int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts) > > */ > > void cgroup_file_notify(struct cgroup_file *cfile) > > { > > - unsigned long flags; > > + unsigned long flags, last, next; > > struct kernfs_node *kn = NULL; > > > > + if (!READ_ONCE(cfile->kn)) > > + return; > > + > > + last = READ_ONCE(cfile->notified_at); > > + if (time_before_eq(jiffies, last + CGROUP_FILE_NOTIFY_MIN_INTV)) > > + return; > > + > > Previously, if a notification arrived within the rate-limit window, we would > still call timer_reduce(&cfile->notify_timer, next) to schedule a deferred > notification. > > With this change, returning early here bypasses that timer scheduling entirely. > Does this risk missing notifications that would have been delivered by the timer? > You are indeed right that this can cause missed notifications. After giving some thought I think the lockless check-and-return can be pretty much simplified to timer_pending() check. If timer is active, just do nothing and the notification will be delivered eventually. I will send the updated version soon. Any comments on the other two patches?