Btrfs: Worker thread optimizations

This changes the worker thread pool to maintain a list of idle threads,
avoiding a complex search for a good thread to wake up.

Threads have two states:

idle - we try to reuse the last thread used in hopes of improving the batching
ratios

busy - each time a new work item is added to a busy task, the task is
rotated to the end of the line.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/fs/btrfs/async-thread.h b/fs/btrfs/async-thread.h
index 52fc9da0f9e7..3436ff897597 100644
--- a/fs/btrfs/async-thread.h
+++ b/fs/btrfs/async-thread.h
@@ -60,11 +60,12 @@ struct btrfs_workers {
         /* max number of workers allowed.  changed by btrfs_start_workers */
         int max_workers;
 
+        /* once a worker has this many requests or fewer, it is idle */
+        int idle_thresh;
+
         /* list with all the work threads */
         struct list_head worker_list;
-
-        /* the last worker thread to have something queued */
-        struct btrfs_worker_thread *last;
+        struct list_head idle_list;
 
         /* lock for finding the next worker thread to queue on */
         spinlock_t lock;