From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-171.mta1.migadu.com (out-171.mta1.migadu.com [95.215.58.171]) (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 871922FD69D for ; Tue, 30 Jun 2026 15:08:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.171 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782832096; cv=none; b=mXy0EB3isctmKs3qxDP8IRQEDLGZvOvBTBczgBG1dyZNEEyeRFVFyVoMlvGdwGESLIJV8k4ec5NBGY2eWYULHCTABMhu61UadspeyBEZjSZzGRlo5m4/QbybQbtk5YR6m4Pvug0lJZbZ7H3kpnzQqxZb09epl9M/h1KWNn7SGi8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782832096; c=relaxed/simple; bh=DTKadkgUJEsm8vau+Kht82J5nMgaemZk4nMq4zha+HE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=R2GMJEtvMd09D3C3aMh5rWrg+o65Ux9HoySskkeSFJ8fzSRLeppy0xZOR4ezK4e83L4QGeCj/AuAazYE+U6PrWmb2sLGoGZ2udRuaR6kBVOgROi5aEaSQpaBmjcEg/NUNqeq3D0eD7N1I+vg08qVhnnsPSk9qO3tgn1owuUQ/s0= 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=DdA85Oso; arc=none smtp.client-ip=95.215.58.171 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="DdA85Oso" Date: Tue, 30 Jun 2026 08:07:47 -0700 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1782832082; 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=gHtO69diIM0k7S0Wj5v0VMDMfaI21iwZjDPJcsxigLA=; b=DdA85OsoBrLMdEteQcGdKeX0AzDe/7nYRg6vYyk+ql7QNoPRZhPyh6W4lrwEt3aIUsY85A JE813+iwrtqbl676jixEw8tFQvESMIQJk7/QjLpMcGpXjiQKc/SGLfsaN5zJ4egZANbrfF VdBlPwM6d0VpkL2r/zLQAQQmH4qak78= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Shakeel Butt To: Hui Zhu Cc: Johannes Weiner , Michal Hocko , Roman Gushchin , 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, Hui Zhu Subject: Re: [RESEND PATCH v2] mm/memcontrol: Avoid stuck FLUSHING_CACHED_CHARGE on isolated CPU Message-ID: References: <20260630070004.470181-1-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-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260630070004.470181-1-hui.zhu@linux.dev> X-Migadu-Flow: FLOW_OUT On Tue, Jun 30, 2026 at 03:00:04PM +0800, 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: 6a792697a53a ("memcg: do not drain charge pcp caches on remote isolated 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 6dc4888a90f3..2e66b4a2c25d 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 > @@ -2264,9 +2265,22 @@ 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); Let's do something like the following. guard(rcu)(); if (cpu_is_isolated(cpu)) { 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 >