Merge patch series "writeback: Avoid lockups when switching inodes"

Jan Kara <jack@suse.cz> says:

This patch series addresses lockups reported by users when systemd unit reading
lots of files from a filesystem mounted with lazytime mount option exits. See
patch 3 for more details about the reproducer.

There are two main issues why switching many inodes between wbs:

1) Multiple workers will be spawned to do the switching but they all contend
on the same wb->list_lock making all the parallelism pointless and just
wasting time.

2) Sorting of wb->b_dirty list by dirtied_time_when is inherently slow.

Patches 1-3 address these problems, patch 4 adds a tracepoint for better
observability of inode writeback switching.

* patches from https://lore.kernel.org/20250912103522.2935-1-jack@suse.cz:
  writeback: Add tracepoint to track pending inode switches
  writeback: Avoid excessively long inode switching times
  writeback: Avoid softlockup when switching many inodes
  writeback: Avoid contention on wb->list_lock when switching inodes

Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index a2848d731a46..15a4bc4ab819 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -265,6 +265,8 @@ static inline void wbc_init_bio(struct writeback_control *wbc, struct bio *bio)
 		bio_associate_blkg_from_css(bio, wbc->wb->blkcg_css);
 }
 
+void inode_switch_wbs_work_fn(struct work_struct *work);
+
 #else	/* CONFIG_CGROUP_WRITEBACK */
 
 static inline void inode_attach_wb(struct inode *inode, struct folio *folio)