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Thu, 05 Nov 2026 19:31:48 -0800 (PST) Received: from localhost.localdomain ([14.22.11.168]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3b0b323cd3csm5899802a91.3.2026.11.05.19.31.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Nov 2026 19:31:47 -0800 (PST) From: Jianping Liu To: tytso@mit.edu, adilger.kernel@dilger.ca, libaokun@linux.alibaba.com, jack@suse.cz, ojaswin@linux.ibm.com, ritesh.list@gmail.com, yi.zhang@huawei.com, frankjpliu@tencent.com, linux-ext4@vger.kernel.org Cc: linux-kernel@vger.kernel.org Subject: [PATCH v2] ext4: fix soft lockup while shrinking the extent status cache Date: Fri, 6 Nov 2026 11:31:41 +0800 Message-ID: <20261106033141.1509656-1-frankljpliu@gmail.com> X-Mailer: git-send-email 2.43.7 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Jianping Liu __es_shrink() walks sbi->s_es_list and reclaims extents from each inode. On a non-preemptible kernel that walk never gives up the CPU. There is a soft lockup report on our company's machine. The machine with millions of active inodes, 256 CPUs, and heavy memory pressure, concurrent shrinkers run too long. i_es_lock is dropped after es_reclaim_extents(), and s_es_lock is not taken again until the next inode. When nr_to_scan remains, call cond_resched_tasks_rcu_qs() before continuing the scan. cond_resched() does not report the RCU quiescent state needed here. It is compiled out when CONFIG_PREEMPTION=y. Otherwise it schedules through preempt_schedule(), and RCU records that switch as a preemption. An RCU-tasks read-side section ends only at a voluntary context switch, cond_resched_tasks_rcu_qs(), idle, or return to userspace. The helper reports that voluntary quiescent state and still lets a non-preemptible kernel reschedule. --- Changes from v1: - Delete time out logic. - Change "cond_resched" to "cond_resched_tasks_rcu_qs". --- ==================== Link: https://patchwork.ozlabs.org/project/linux-ext4/patch/20261028061628.1209508-1-frankljpliu@gmail.com/ Signed-off-by: Jianping Liu --- fs/ext4/extents_status.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c index 018b6146b9d8..62346eaf4350 100644 --- a/fs/ext4/extents_status.c +++ b/fs/ext4/extents_status.c @@ -1721,6 +1721,16 @@ static int __es_shrink(struct ext4_sb_info *sbi, int nr_to_scan, if (nr_to_scan <= 0) goto out; + + /* + * Report a voluntary RCU-tasks quiescent state before continuing. + * cond_resched() is not enough: it is compiled out under + * CONFIG_PREEMPTION, and otherwise switches via preemption, which + * does not end an RCU-tasks read-side section. + * This also lets a non-preemptible kernel reschedule. + */ + cond_resched_tasks_rcu_qs(); + spin_lock(&sbi->s_es_lock); } spin_unlock(&sbi->s_es_lock); -- 2.43.7