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 DBDD4C79FB6 for ; Wed, 9 Sep 2026 18:18:13 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 9D9F26B0092; Wed, 9 Sep 2026 14:18:12 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 98B126B0093; Wed, 9 Sep 2026 14:18:12 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 87A536B0095; Wed, 9 Sep 2026 14:18:12 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id 276AB6B0092 for ; Wed, 9 Sep 2026 14:18:12 -0400 (EDT) Received: from smtpin17.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay03.hostedemail.com (Postfix) with ESMTP id 6612CA0309 for ; Wed, 9 Sep 2026 18:18:11 +0000 (UTC) X-FDA: 85195033182.17.630C5BF Received: from mta0.migadu.com (out-62.mta0.migadu.com [91.218.175.62]) by imf24.hostedemail.com (Postfix) with ESMTP id 8894F18000C for ; Wed, 9 Sep 2026 18:18:07 +0000 (UTC) Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=hdjrczZf; spf=pass (imf24.hostedemail.com: domain of roman.gushchin@linux.dev designates 91.218.175.62 as permitted sender) smtp.mailfrom=roman.gushchin@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1788977889; 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=/kQrjWDVP8VToFUSa5vLPEZUGkwzjIEup3NeNcSxnh0=; b=wlje+vbyi8pIHHQd9t9kCDl3eA6JhVAqjS9tr8I5k27OXblkx6yK31GYp30H850M86daEZ HAqOJm9PKXc12bXblOQW9VV3JgLhlTY5noKn1NPTutNZs1NNi1ENZPhD+8xpQQww2PfMsw z1wQjON7ENPeRPC1ghAB04+KFyKWgh0= ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1788977889; b=tO0vLz/J992MiJcoFH75AYEWB96t5aVBzRRP5EfZdHztmoLFG3SK/gGU6O77Of9OObQNJJ hKLPTMbhD5y/fe9XQZdOls0h7z0iq+8AV8AS6+S6tjGtlAuVs+qQowjGuJIULyWJ6cIkvt wrZ2KM5GqwiTL47/7hWCyo7jkbl8ioU= ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=hdjrczZf; spf=pass (imf24.hostedemail.com: domain of roman.gushchin@linux.dev designates 91.218.175.62 as permitted sender) smtp.mailfrom=roman.gushchin@linux.dev; dmarc=pass (policy=none) header.from=linux.dev X-Envelope-To: linux-mm@kvack.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=1788977884; v=1; x=1789582684; b=hdjrczZfkw2I5j0slrseLFrL0HLbRNdtQMFQrsNdzs1bYi1PYj8Tjts37DA6V9VJp7HsbDBE gqTGEV5KgS4kurpr3VBG0GruXhEvKyTmyOTlL/SjpO7Lybk0tvVNiNmK9wY329ZL5cjWMmleOqu 7L1Y0TMcv1+gqiBUoDXBAtLc= X-Envelope-To: linux-mm@kvack.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> MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 8894F18000C X-Stat-Signature: dxa5uf9cumr67pkoqrbk9bm1acixa15h X-Rspam-User: X-HE-Tag: 1788977887-405378 X-HE-Meta: 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 m6grww9W pIogpMgqYcmJIKg6n5e1yCTTlHnE7lIlSBAd2IMgOBZPSi00X6tIlTcY6fu8pmSvxRKYzOXhL2EoF7klVC6WY5yJl8a3wnX0pIPKdDnp9a+DUmR2uoeeYg6t0Q+FfEch4tDBPINw4hAL8IbG8hzP1eIoY1kZeNyf01VkzI5iK+WcEPXeIHvEDmqqoPTOm3uNgznKAS6BQT4T5cloMDmFJna1itTaLdwSGW4Dj9K3yu9jXSxd/v7r5qICe8VV1nMR0A5D5vv+V7bRpuJePbgnbksP49A== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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!