[PATCH] 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. Once the shrink has run for one
second, call cond_resched() so the CPU can be scheduled before the
scan continues.

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..c9c70db5eb71 100644
--- a/fs/ext4/extents_status.c
+++ b/fs/ext4/extents_status.c
@@ -1679,6 +1679,7 @@ static int __es_shrink(struct ext4_sb_info *sbi, int nr_to_scan,
 	int nr_to_walk;
 	int nr_shrunk = 0;
 	int retried = 0, nr_skipped = 0;
+	unsigned long end_jiffies = jiffies + HZ;
 
 	es_stats = &sbi->s_es_stats;
 	start_time = ktime_get();
@@ -1719,6 +1720,15 @@ static int __es_shrink(struct ext4_sb_info *sbi, int nr_to_scan,
 		nr_shrunk += es_reclaim_extents(ei, &nr_to_scan);
 		write_unlock(&ei->i_es_lock);
 
+		/*
+		 * Call cond_resched to avoid soft lockup if a kernel with
+		 * preempt disabled.
+		 */
+		if (time_after(jiffies, end_jiffies)) {
+			cond_resched();
+			end_jiffies = jiffies + HZ;
+		}
+
 		if (nr_to_scan <= 0)
 			goto out;
 		spin_lock(&sbi->s_es_lock);
-- 
2.43.7