From nobody Sun May 24 20:33:03 2026 Received: from out-178.mta1.migadu.com (out-178.mta1.migadu.com [95.215.58.178]) (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 0599F1A683E for ; Fri, 22 May 2026 07:16:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.178 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779434195; cv=none; b=AwLogqCH58kusIiPDpYlNOlePiXo6HBXS3EkQ6WS/e+WKYikd8rG2w+56R6c1tqmpT8j9k9wp5M8oTdZwW3EEx93YA2sJ0FYZTF0XEVCxBF68O3W7YzStsRQ/+enAGzxddo8TNgmQ7d6/GZ9SERdYwQZycviuCXb+xaRvTL7kk4= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779434195; c=relaxed/simple; bh=vI78eeLRQpnyrlePbWhlMsnYV2wcV2LPk0+fsnT9zD8=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=pyrMACl15t0pMaqtQP20JDnpSldkbzaFySJEt1LBoM981C4uhzQBDj3CPQHMBO8xhpZVGW0szT7JbtdASF1IJ3jse0o7LPoTbhcIoRfqxBhgE1+1QQYmsIoPkh0d7HWz8lpSUXgdOqDZDR0JyDwl5OJkB1Kq7TEK/CAKXIdFP2I= 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=nr+B+/VH; arc=none smtp.client-ip=95.215.58.178 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="nr+B+/VH" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1779434190; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=ofTJ2Iw3jGWVDU8AK9RMEoAeb3tQr+vx1xTO7a1aBkY=; b=nr+B+/VHJIU0zdKqf0sEkBhtrCFbOMsL+3ghUpLjMAc4reaEr9gYILTsfaJgBgY+dS9inO ID3FV4DX8x5jioVDqwgFO0mA7jv2ifY+gx0WSfag0iaCJdMBoZoA5xbMsPLJTG+8BXv37E 0GpSX3vtTQ6/pgb+N66nTKvqCwG4NHQ= From: Hui Zhu To: Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , Waiman Long , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev Cc: Hui Zhu Subject: [PATCH v2] mm/memcontrol: Avoid stuck FLUSHING_CACHED_CHARGE on isolated CPU Date: Fri, 22 May 2026 15:16:01 +0800 Message-ID: <20260522071601.21065-1-hui.zhu@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset="utf-8" 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() =3D=3D 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 =3D=3D 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: 6a792697a53a ("memcg: do not drain charge pcp caches on remote isola= ted cpus") Signed-off-by: Hui Zhu --- Changelog: v2: According to the comments of Waiman Long, updated fixes. mm/memcontrol.c | 28 ++++++++++++++++++++++------ 1 file changed, 22 insertions(+), 6 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index c03d4787d466..8985334565a8 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -2228,7 +2228,8 @@ static bool is_memcg_drain_needed(struct memcg_stock_= pcp *stock, return flush; } =20 -static void schedule_drain_work(int cpu, struct work_struct *work) +static void +schedule_drain_work(int cpu, struct work_struct *work, unsigned long *flag= s) { /* * Protect housekeeping cpumask read and work enqueue together @@ -2236,9 +2237,22 @@ 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); } =20 /* @@ -2271,7 +2285,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg) if (cpu =3D=3D 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); } =20 if (!test_bit(FLUSHING_CACHED_CHARGE, &obj_st->flags) && @@ -2281,7 +2296,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg) if (cpu =3D=3D 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(); --=20 2.43.0