From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta0.migadu.com (out-188.mta0.migadu.com [91.218.175.188]) (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 DBF11363C62 for ; Wed, 1 Jul 2026 02:40:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782873628; cv=none; b=YM/AXktuUqv047ZTeURHzoIgTUFS476RABdOj/eUb8PLxZ7RzVPl74uhI9EOSX6hAXOCLI2CgF+P1JOFlmFgP2qk3LwhMEIqZhyOMZyHljhUVgR+T04Yeqg09EwPq7o1E83u1ZUNOCOGLe+mrV/Tw62qs8JlUcfV1UbTmxmWvGI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782873628; c=relaxed/simple; bh=/uNUCpPIvAgfrLvp/TRz/0Kj8XssEJWttncPwsErGyM=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=KHN/yKIR5qqzwv6V2+hYZHoGT8GN32NQ8SmvdnQ4MXGBKdIN06EzStsKmmgCdS19kWhUe/6ADG1Bdf4B73uZzLIZxI0fyD4l71lzJnXp4z4pt3MNyoE47x3RTaezMiKIexDNaXaxjY8cAS1k8QbQRtJ+HJRJ1yDzHLwjbx4n1Do= 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=jU53toVP; arc=none smtp.client-ip=91.218.175.188 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="jU53toVP" 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=1782873624; 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=zOd8DSH7cMYtJD/+/URsy/IxwgqSz9ZSG2fyMnhSXak=; b=jU53toVPumrS9P5nkr8lgqCFP3On/pZhU6agZrcXkfqKCJkvxOKtlYu/72jObB/OC9jhqZ BjXP02wTgs93xHsY/ptW+JuOjrGzS92mQSLkYBuLjiSIiHoqCZZ4dDkxjAAWaKx2pd52T/ 3ULYc9iXvhPdt7Auvx4hQY9Ul009zSM= From: Hui Zhu To: Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev Cc: Hui Zhu Subject: [PATCH v3] mm/memcontrol: Avoid stuck FLUSHING_CACHED_CHARGE on isolated CPU Date: Wed, 1 Jul 2026 10:40:10 +0800 Message-ID: <20260701024010.564830-1-hui.zhu@linux.dev> Precedence: bulk X-Mailing-List: linux-rt-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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: 6a792697a53a ("memcg: do not drain charge pcp caches on remote isolated cpus") Signed-off-by: Hui Zhu --- Changelog: v3: According to the comments of Shakeel Butt, keep the whole schedule_drain_work() body under a single guard(rcu)() instead of splitting it with scoped_guard(rcu){}. v2: According to the comments of Waiman Long, updated fixes. mm/memcontrol.c | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 6dc4888a90f3..42a0d0d6284a 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -2256,7 +2256,8 @@ 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 @@ -2265,8 +2266,21 @@ static void schedule_drain_work(int cpu, struct work_struct *work) * pending work on newly isolated CPUs. */ guard(rcu)(); - if (!cpu_is_isolated(cpu)) - queue_work_on(cpu, memcg_wq, work); + if (cpu_is_isolated(cpu)) { + /* + * The target CPU is isolated: the drain work was not + * queued. Clear FLUSHING_CACHED_CHARGE so a future + * drain_all_stock() call can retry instead of skipping + * this stock forever. + * + * The cached charge itself is left untouched here; it + * will be drained the next time drain_local_stock() runs + * on this CPU (e.g. after it leaves isolation). + */ + clear_bit(FLUSHING_CACHED_CHARGE, flags); + return; + } + queue_work_on(cpu, memcg_wq, work); } /* @@ -2299,7 +2313,8 @@ 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) && @@ -2309,7 +2324,8 @@ 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(); -- 2.43.0