From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-186.mta1.migadu.com (out-186.mta1.migadu.com [95.215.58.186]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2378641C0A3; Mon, 2 Mar 2026 16:14:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.186 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772468057; cv=none; b=EdFeiU2WbXQ14X+NwmKito/MBK+K7QIE0JQwmzKfjZmNe+p1j8sP0x6tFAOgX9kk2lH72+1gghoAVdQwe/PJomdSRaPkcczEMPIVxkKDaQzDUc+p9RKNZd1J3tVVeFVzIdzS+BTiyyTsO//Jy3M/lJ5/vfs/RC9S0FBb6ksEcbk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772468057; c=relaxed/simple; bh=YI1rAxGkTWrOMsOCcvJkmb0xR/cuc9o+Z4PTFycl9PA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=MD4CYRNHLgWL2T7I+H31JYTZJ1jD7WgwSzcd6SzotipID56W7b3mSIsBoFVpF+4ItmbHUVBcafTYOQQXi22Lb11tH1uIv6WSR86mAGITFkvv1pVQhSS5jMDwRHrEnyPriiKY+b+9mR0K5Hvx3dA6OHgrfPh1RcPUB7X6yFtld7o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=YG01ILjL; arc=none smtp.client-ip=95.215.58.186 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="YG01ILjL" 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> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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?