From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 08B25C79FB6 for ; Wed, 9 Sep 2026 18:17:01 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id F08866B008C; Wed, 9 Sep 2026 14:16:59 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id E92A06B0092; Wed, 9 Sep 2026 14:16:59 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D5A516B0093; Wed, 9 Sep 2026 14:16:59 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id B4A416B008C for ; Wed, 9 Sep 2026 14:16:59 -0400 (EDT) Received: from smtpin16.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay07.hostedemail.com (Postfix) with ESMTP id 2F617160302 for ; Wed, 9 Sep 2026 18:16:58 +0000 (UTC) X-FDA: 85195030116.16.6044AA0 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf26.hostedemail.com (Postfix) with ESMTP id 9395C140010 for ; Wed, 9 Sep 2026 18:16:56 +0000 (UTC) Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=F84p0yyr; spf=pass (imf26.hostedemail.com: domain of tj@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=tj@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1788977816; h=from:from:sender: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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=rmS7BNxOqmi+BeNsZ4oZ5oXHisCR+XOxA/iRVryaknE=; b=7UY0hEjtpu8gf/ukD3ucATz69VoffN36Eqs285/grVQBGvFLruwLka7tE3UNEjMvF6YwdE +AAyFP0AjJHBJAQaHhCbxidmdqzy34/YzHvr91sHb7v/lMjNSJGzZ1Qfwf3PkEguGQbffx 1v3IrLQcPERkVWfL9PPjRDblAoz9o0c= ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1788977816; b=x8Dts8DsMSULnkwPU8sv18rJaY4Z+hr5gz2/TwGQJ2w2F9QHa1l/XQ9QaS6VVznGJn4alv UB67qK4dEiesNh3rJBMlVCJCOWYZPBoJ8lS3rAyogr1fuzIlGBesMMyZB0kn/0UJveGMYO Lp81dOrwEbDp7sQ/I9QpPSS/RnVSlcs= ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=F84p0yyr; spf=pass (imf26.hostedemail.com: domain of tj@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=tj@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 1BE54600CB; Wed, 9 Sep 2026 18:16:56 +0000 (UTC) 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> 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> X-Rspamd-Server: rspam04 X-Rspam-User: X-Stat-Signature: 331tscyym5rwzhc5uyzoxmupj7d6mjgu X-Rspamd-Queue-Id: 9395C140010 X-HE-Tag: 1788977816-103036 X-HE-Meta: U2FsdGVkX1+obqBTYbgX0bhJmd1s4Og1seAZAH37wmHX9UlmPcSwM7MPpcYYPh+HYxWUU6bEfArw7EfTpiqzk19PHjdJgndeHlIwrOlIqSR7Io3DURo5hrHibAHtZXKdNUlqJ1NaSMCQzQZLO/r4HQ8gxHzwFSghWDc32Xh31EfhLDmcEtHO5TdwihxHU0mve1qyZdH20ybdK+D+M42a5wJ3wDjIQD22w11/RoEi7GkS0y7ggGICPtkJlVRHpw4hs9TQ0ZtOHKMevJXk6pL9YsRLG70CSj6C9OHzY8WxN3+wizcC3w2D86lI89fxacUhf8Lui4MpfGYyVLOzZnNBmxWQ1E5bO3kd4QScFs6oyUUN522EwCwWQ5ljXECXTo5AH9ZkKu7D+OVfybWs9iR+j4hjKep0G4AznNk6+L6Rgtf8+dytm6R0IFsfSPNXHhACw+RzkMIGoccr7/EoE1+uLXj1TJTWX2z9/OwbLmqDxSRh8CL8uvfPFDvGPlxT3czUHSKczKaCIGIJVH+GPKsQ/XnEJF0pRjWqwdwwWFifT41q5mi8de4/kWmeAQtigAQG3Wy/jmja3Cq7X5zZAwygjOc6dfb/pFsaS1/3fgkHHbyXSftQo0Q3p6byKE5lL48UO/U8aLcMj17mQ7VVfpTUzdeUM4W+FxIP9iCeICLyK7oPlY2sSAEO63eK6lPkuq0VZk5s2njTISrgtMQPrrvzWBt81X5IL2sFECvdZdQNiqN2E/IG6ogCBeCUsH9tiy4rAqUQae1kkVOLMN3VwgcexUBijQI8CCWl0/Kgb8qwOdDKyAMpnmoaVt/Ko/N95G2NpM4AkRq8ivN3tPehkD8vQAlSkUhZF3fM0SGV1WNpNWCn4zWGWizrV84yQtHdFmIFY/VpBHLVHa1fIk14UoGULmM2gKvkw3tut1cPmYVnoPUQqZ45sMLzTU+wGPRsRlmPHwQ5ynYEONoYtpW+IwD 7fJZabBE zjqpGd+uORDVwQJ2oAbHoDUrOD31WPqC4OQ8HbVkVgZVUAphdz8Nwaq0oeY0YPp/gi3Mo0ZQUf2aS7+kMSZ5fafJQnL3mgM0kAtA3FPTWvkOuE8ASMcZd6G4bmXm4ptCEcPRB3RWgoohqkbcUbWz4Y/kfDOGRT3gmW44yzkUfypqxqQh9mKUEpBUvI/sT8cVex06/d4YbOR0H4ze7alUg2K+rVSkOeuEppLNrciKaV2N8J5XjfkKxzFNIJgt59q0qF7xSOQmbsgMdYRiSXXUpHbAnP+YBo066jE7J3b7ViI2Yypg= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: (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