From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-58.mta0.migadu.com [91.218.175.58]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E4137399000 for ; Wed, 9 Sep 2026 18:17:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.58 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788977880; cv=none; b=pBJXlvt7rSKyfk6LaX5+BpHdq3RnuKCZrHaZBICSh7u3a60Cf/v9BWakDGIeWJ+HMeTjKwEr8tkZkW4Jm6c/TCKSfmg7lh/zy9L8xVgiT7kRZboIlX5/RlTRrUGRvV8QN5MUVFojCV1Fo1Gg0qUAPjDwY48xgLO1gQ8Etgp4yyw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788977880; c=relaxed/simple; bh=a5KoTsru4QeX3ourrXoG/HiW8Ls4AkpUlsNfQy7SGEw=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=hatnBqtyuSiLhC+PvXOfLx4VE/0Cw+O7309hM6XGf3ypZrdkhM3sYm4/h4ANVt/H+7rqfT8K3is0mOUU7mGIYH1U7fsQOERh0Kd574NZ73qYJl9sN4fU5/Vs50/BrIJklq6h7JqyRvo38AalQ+NGxSbVjeh1A3wM3q1ghX91ALc= 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=hPS3R6FZ; arc=none smtp.client-ip=91.218.175.58 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="hPS3R6FZ" X-Envelope-To: bpf@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=a5KoTsru4QeX3ourrXoG/HiW8Ls4AkpUlsNfQy7SGEw=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1788977874; v=1; x=1789582674; b=hPS3R6FZ/bOOovAHZdFzdDlRydNTo1aeniCMI93ZZ3l3di3ytdQSNkp6CtrVkq6Vokb2Bguj nzoZsN0pnHlW08e60iKem1Z54LqFa/w126jeUlLKhuxw1kcJoju02bcKMkrH5+TpgZqxdC4wrmg +U0bR5KujZanYeuzhuYaVtg8= X-Envelope-To: bpf@vger.kernel.org Received: by mta10.migadu.com with ESMTPS id 7a7c3fb00e682847; Wed, 09 Sep 2026 18:17:54 +0000 X-Mizu-Trace-ID: 7a7c3fb00e682847 X-Migadu-Flow: FLOW_OUT From: Roman Gushchin To: Josef Bacik Cc: Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jan Kara , Tejun Heo , 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 In-Reply-To: <20260909-cgwb-tasks-rcu-qs-v1-1-967a7754771f@toxicpanda.com> (Josef Bacik's message of "Wed, 09 Sep 2026 18:01:07 +0000") References: <20260909-cgwb-tasks-rcu-qs-v1-1-967a7754771f@toxicpanda.com> User-Agent: mu4e 1.12.15; emacs 30.2 Date: Wed, 09 Sep 2026 18:17:45 +0000 Message-ID: <7ia4ik4ep9va.fsf@castle.c.googlers.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Josef Bacik writes: > 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 > --- > 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)); Reviewed-by: Roman Gushchin Thanks!