From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 6888E3A9DA1 for ; Wed, 29 Apr 2026 20:58:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777496307; cv=none; b=I4TiE/fNVEk4yRQ086z1krqT+qwTxypOPzYMYcL2s5MRxmrjsdyTBnYwtpD06BoIy/I8Hu487OmxrBzcRszpek2t3eK3hYHWlbb9336AKVPH0VLQqV9EIaUjrqNkG8bfHSvGYtrcabWlRA1AAlFAWOgj9Nd7tQtUDt9JlycbE8k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777496307; c=relaxed/simple; bh=H4PfGyMnPiSQn1my4AwccDfLneDKKhcFDfiOgWxT+qg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=bPAUB22vGXc53TSkDCMomTADf8eHWY3hEJWUs0JuLv1BwlZGmOSXRosssnwCH56w27zVHI4CzSHb45NaOxwM+C14NaO7593n/USBmJ7+mADqr1+GvE8/N2kaOyWqnlPE2V/E6F5GFBB7zSGdr8wkCoGCNBEUVvFsxfGb9nUiOS0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=f1WZ+nS1; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="f1WZ+nS1" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1777496304; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=y1NyF1QURy3vdICAxiB0jl9JGp6DuWbQaRj+rdabH/M=; b=f1WZ+nS1z5YnCCsIWpsNmYe29Rs3aO49rPRLDjpXoAM9xlo51tZO7qxE1UwL3uZvYg7P+d y2E6mvw2O8WMcOCtx3TvvnZOn+xCOwxk3Zz4BN6TyTakXdmf0LltKji1kJM5W42QKE6OgC cG39kZtWI1Jjduz5qzkbCs6AsYh3WJ8= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-686-rBBwcJqwPxeaG3FBplukoA-1; Wed, 29 Apr 2026 16:58:18 -0400 X-MC-Unique: rBBwcJqwPxeaG3FBplukoA-1 X-Mimecast-MFC-AGG-ID: rBBwcJqwPxeaG3FBplukoA_1777496295 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id D94C118002C0; Wed, 29 Apr 2026 20:58:13 +0000 (UTC) Received: from [10.22.65.94] (unknown [10.22.65.94]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id B3F75300019F; Wed, 29 Apr 2026 20:58:10 +0000 (UTC) Message-ID: <68ba05da-7052-406a-9ddd-b324a349ca80@redhat.com> Date: Wed, 29 Apr 2026 16:58:09 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/memcontrol: Avoid stuck FLUSHING_CACHED_CHARGE on isolated CPU To: Hui Zhu , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , Frederic Weisbecker , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev Cc: Hui Zhu References: <20260429022723.133833-1-hui.zhu@linux.dev> Content-Language: en-US From: Waiman Long In-Reply-To: <20260429022723.133833-1-hui.zhu@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 On 4/28/26 10:27 PM, Hui Zhu wrote: > From: Hui Zhu > > drain_all_stock() sets FLUSHING_CACHED_CHARGE before calling > schedule_drain_work() to queue per-CPU drain work. When the target > CPU is isolated (cpu_is_isolated() == true), the work is silently > not queued, but FLUSHING_CACHED_CHARGE stays set. Every subsequent > drain_all_stock() then sees the bit and skips this stock entirely, > so the entry is effectively pinned until something else on that CPU > runs drain_local_*_stock() and clears the bit -- which on a long- > isolated CPU may never happen. > > The original idea was to actually perform the drain from the calling > CPU on behalf of the isolated one, by adding a lock around the > per-CPU stock so that a remote drainer could safely touch it. In > practice this turned out to be intrusive: the stock data structures > and their fast paths (consume_stock(), refill_stock(), the obj_stock > helpers) are deliberately designed around current-CPU-only access, > and retrofitting cross-CPU serialisation onto them adds non-trivial > locking and PREEMPT_RT concerns for very little gain. > > Looking at the actual amount of charge that can accumulate in a > single per-CPU stock, it is bounded and small, so leaving an > isolated CPU's stock undrained for a while is not a real problem. > The only real bug is that the stuck FLUSHING_CACHED_CHARGE bit > prevents future drain_all_stock() callers from re-attempting once > the CPU is no longer isolated. > > Fix this minimally by clearing FLUSHING_CACHED_CHARGE when the work > could not be queued because the target CPU is isolated. The cached > charge itself is left in place; it will be released the next time > the CPU runs drain_local_*_stock() (e.g. after leaving isolation, > or if the isolated CPU itself calls drain_all_stock() -- in that > case cpu == curcpu causes drain_local_memcg_stock() to be invoked > directly), and the next drain_all_stock() call is free to retry > instead of skipping the stock forever. > > Fixes: 2d05068610a3 ("memcg: Prepare to protect against concurrent isolated cpuset change") I don't think this is the right commit to blame as it didn't really change the logic other than adding RCU locking. I think commit 6a792697a53a ("memcg: do not drain charge pcp caches on remote isolated cpus") is the right one as this is the commit that adds the cpu_is_isolated() check first. Other than that, the patch looks good to me as the list of isolated CPUs is runtime changeable. Cheers, Longman > Signed-off-by: Hui Zhu > --- > mm/memcontrol.c | 28 ++++++++++++++++++++++------ > 1 file changed, 22 insertions(+), 6 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index c3d98ab41f1f..cee77b0a95f5 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2219,7 +2219,8 @@ static bool is_memcg_drain_needed(struct memcg_stock_pcp *stock, > return flush; > } > > -static void schedule_drain_work(int cpu, struct work_struct *work) > +static void > +schedule_drain_work(int cpu, struct work_struct *work, unsigned long *flags) > { > /* > * Protect housekeeping cpumask read and work enqueue together > @@ -2227,9 +2228,22 @@ static void schedule_drain_work(int cpu, struct work_struct *work) > * partition update only need to wait for an RCU GP and flush the > * pending work on newly isolated CPUs. > */ > - guard(rcu)(); > - if (!cpu_is_isolated(cpu)) > - queue_work_on(cpu, memcg_wq, work); > + scoped_guard(rcu) { > + if (!cpu_is_isolated(cpu)) { > + queue_work_on(cpu, memcg_wq, work); > + return; > + } > + } > + > + /* > + * The target CPU is isolated: the drain work was not queued. > + * Clear FLUSHING_CACHED_CHARGE so that future drain_all_stock() > + * callers can re-attempt instead of skipping this stock forever. > + * The cached charge is left in place; it will be released the > + * next time the CPU itself runs drain_local_*_stock() (e.g. > + * after leaving isolation), or by a follow-up mechanism. > + */ > + clear_bit(FLUSHING_CACHED_CHARGE, flags); > } > > /* > @@ -2262,7 +2276,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg) > if (cpu == curcpu) > drain_local_memcg_stock(&memcg_st->work); > else > - schedule_drain_work(cpu, &memcg_st->work); > + schedule_drain_work(cpu, &memcg_st->work, > + &memcg_st->flags); > } > > if (!test_bit(FLUSHING_CACHED_CHARGE, &obj_st->flags) && > @@ -2272,7 +2287,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg) > if (cpu == curcpu) > drain_local_obj_stock(&obj_st->work); > else > - schedule_drain_work(cpu, &obj_st->work); > + schedule_drain_work(cpu, &obj_st->work, > + &obj_st->flags); > } > } > migrate_enable();