[PATCH v2 0/4] writeback: Avoid lockups when switching inodes

[Jan Kara <jack@suse.cz>]

Hello!

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.

Changes since v1:
* Added Acked-by's from Tejun
* Modified patch 1 to directly insert isw items into a list of switches to
  make instead of queueing rcu_work for that. This actually speeded up
  large scale switching about 2x.

								Honza

Previous versions:
Link: http://lore.kernel.org/r/20250909143734.30801-1-jack@suse.cz # v1