From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 517A239A057; Wed, 9 Sep 2026 18:16:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788977820; cv=none; b=rKC3vj+aT/AhpFz7cDJ80JR3BikmVoo270lzLu841rCEc+uPcBZ+rUx1FUibbZnmW0ePqtwWiJGp0Tye45gAwxZRo83cBcPTtiZJKvCArILEd4WkYhvMF+4WJDU45N1wF5ZjqOI5LkIZNiJaOzVGIPBT3AE9aTaOESIuut/tqqw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788977820; c=relaxed/simple; bh=kXOctfXUltwhqjTzoFtLL3gVUi//FVz1XuWVTcVuNN8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ewZXILoY6r8KJmLlmqVafpOlv1VoQ7ZkpESMAO0ZRL6l1WfOBigkd3vCvYJIshsWySu1g1qPslAy8r1hAw8vw8dRI0J3vzfvVm2Smuhq9qXvbuNnLV15DEbMx4gLC1722Mq0jNGHnhak8Bh9/NzqS6X/pg1xYn/oqBV4WE+YhUs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=F84p0yyr; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="F84p0yyr" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 962401F00893; Wed, 9 Sep 2026 18:16:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788977815; bh=rmS7BNxOqmi+BeNsZ4oZ5oXHisCR+XOxA/iRVryaknE=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=F84p0yyrHbvT3bpXj+VWGL3evahoR6zG0wTE9+bL32gjOuuHYOcp5jRBCTbBmoesc nUKlyiNo9zgpaA1uQE+puCh0ulCe9TAqCoBisChJDtgib1rgRlKWc7+knS+n4fuz4S 7nSvSwwxqSBsjpa10PXbYDQrFUAV4uGXkFkb2cegO3tPzaqvJNFR9GsDup3MBJdb/o fei0wlIm+cnIeuSWoNfnkZ1S+pAZI84YMz6dsergpXxLlscYpxsi9ZysTp12TVyTtQ b3Y46Ko+oZeGMBiX61KqHlpy4oI7w2IMj6zZPs5hTSlM2IzjYjJGpK1FxVMWDoLj8y b9Pscs7SP3o2A== Date: Wed, 9 Sep 2026 08:16:54 -1000 From: Tejun Heo To: Josef Bacik Cc: Andrew Morton , "Paul E. McKenney" , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jan Kara , Roman Gushchin , Dennis Zhou , "Matthew Wilcox (Oracle)" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] writeback: report a Tasks-RCU quiescent state per cgwb drain pass Message-ID: References: <20260909-cgwb-tasks-rcu-qs-v1-1-967a7754771f@toxicpanda.com> Precedence: bulk X-Mailing-List: bpf@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: <20260909-cgwb-tasks-rcu-qs-v1-1-967a7754771f@toxicpanda.com> (cc'ing Paul) On Wed, Sep 09, 2026 at 06:01:07PM +0000, Josef Bacik wrote: > cleanup_offline_cgwbs_workfn() drains a dying cgwb by calling > cleanup_offline_cgwb() until it returns false, with a cond_resched() > between passes. On a CONFIG_PREEMPTION kernel that cond_resched() does > nothing: _cond_resched() is a plain "return 0", and under > PREEMPT_DYNAMIC the full and lazy modes disable it. Since commit > 7dadeaa6e851 ("sched: Further restrict the preemption modes") those are > the only two models on the architectures with PREEMPT_LAZY support, > arm64 and x86 among them, so the drain loop never reports a Tasks-RCU > quiescent state. > > A worker draining a cgwb with millions of attached inodes runs for > minutes. On a 6.18 arm64 host in lazy mode the cgwb worker drained one > dying cgroup's writeback domain for over 11 minutes. A BPF program > unlink (bpf_trampoline_unlink_prog -> bpf_trampoline_update -> > unregister_ftrace_direct -> ftrace_shutdown -> synchronize_rcu_tasks()) > waited on that grace period while holding the trampoline mutex, 42 > tasks queued behind it in D state, and the hung task detector fired at > 614 s and panicked the host. Any BPF or ftrace detach during a long > drain inherits the drain's length. > > Fix this by calling cond_resched_tasks_rcu_qs() so we do not stall out > anybody who calls sycnrhonize_rcu_tasks(). We put this in a do { } while > loop because if we have many small cgroups cleanup_offline_cgwb() will > return false and we will never call cond_resched_tasks_rcu_qs(), creating > the same problem. > > Fixes: c22d70a162d3 ("writeback, cgroup: release dying cgwbs by switching attached inodes") > Cc: stable@vger.kernel.org > Link: https://lore.kernel.org/bpf/9d444098-7c03-4163-af12-bd0a79a51443@paulmck-laptop/ > Assisted-by: LLM > Signed-off-by: Josef Bacik Acked-by: Tejun Heo > --- > mm/backing-dev.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/mm/backing-dev.c b/mm/backing-dev.c > index cecbcf9060a6..18e999053bae 100644 > --- a/mm/backing-dev.c > +++ b/mm/backing-dev.c > @@ -910,8 +910,9 @@ static void cleanup_offline_cgwbs_workfn(struct work_struct *work) > continue; > > spin_unlock_irq(&cgwb_lock); > - while (cleanup_offline_cgwb(wb)) > - cond_resched(); > + do { > + cond_resched_tasks_rcu_qs(); > + } while (cleanup_offline_cgwb(wb)); The patch looks fine but this overall seems fragile. cond_resched() was already marking "stuff that can take too long" but we need to use cond_resched_tasks_rcu_qs() if it can take *really* long. There gotta be a way to make this more maintainable. If always doing tasks_rcu_qs from cond_resched() is too expensive, can it be be gated behind something cheaper e.g. some tick based test? Thanks. -- tejun