[PATCH v2] ext4: fix soft lockup while shrinking the extent status cache

[Jianping Liu <frankljpliu@gmail.com>]

From: Jianping Liu <frankjpliu@tencent.com>

__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 <frankjpliu@tencent.com>
---
 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