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>
diff --git a/fs/btrfs/async-thread.c b/fs/btrfs/async-thread.c
index 8b9e2cf..8d4cc46 100644
--- a/fs/btrfs/async-thread.c
+++ b/fs/btrfs/async-thread.c
@@ -34,6 +34,9 @@
* One of these is allocated per thread.
*/
struct btrfs_worker_thread {
+ /* pool we belong to */
+ struct btrfs_workers *workers;
+
/* list of struct btrfs_work that are waiting for service */
struct list_head pending;
@@ -51,9 +54,45 @@
/* set to non-zero when this thread is already awake and kicking */
int working;
+
+ /* are we currently idle */
+ int idle;
};
/*
+ * helper function to move a thread onto the idle list after it
+ * has finished some requests.
+ */
+static void check_idle_worker(struct btrfs_worker_thread *worker)
+{
+ if (!worker->idle && atomic_read(&worker->num_pending) <
+ worker->workers->idle_thresh / 2) {
+ unsigned long flags;
+ spin_lock_irqsave(&worker->workers->lock, flags);
+ worker->idle = 1;
+ list_move(&worker->worker_list, &worker->workers->idle_list);
+ spin_unlock_irqrestore(&worker->workers->lock, flags);
+ }
+}
+
+/*
+ * helper function to move a thread off the idle list after new
+ * pending work is added.
+ */
+static void check_busy_worker(struct btrfs_worker_thread *worker)
+{
+ if (worker->idle && atomic_read(&worker->num_pending) >=
+ worker->workers->idle_thresh) {
+ unsigned long flags;
+ spin_lock_irqsave(&worker->workers->lock, flags);
+ worker->idle = 0;
+ list_move_tail(&worker->worker_list,
+ &worker->workers->worker_list);
+ spin_unlock_irqrestore(&worker->workers->lock, flags);
+ }
+}
+
+/*
* main loop for servicing work items
*/
static int worker_loop(void *arg)
@@ -76,6 +115,7 @@
atomic_dec(&worker->num_pending);
spin_lock_irq(&worker->lock);
+ check_idle_worker(worker);
}
worker->working = 0;
if (freezing(current)) {
@@ -98,6 +138,7 @@
struct list_head *cur;
struct btrfs_worker_thread *worker;
+ list_splice_init(&workers->idle_list, &workers->worker_list);
while(!list_empty(&workers->worker_list)) {
cur = workers->worker_list.next;
worker = list_entry(cur, struct btrfs_worker_thread,
@@ -116,9 +157,10 @@
{
workers->num_workers = 0;
INIT_LIST_HEAD(&workers->worker_list);
- workers->last = NULL;
+ INIT_LIST_HEAD(&workers->idle_list);
spin_lock_init(&workers->lock);
workers->max_workers = max;
+ workers->idle_thresh = 64;
}
/*
@@ -143,14 +185,14 @@
spin_lock_init(&worker->lock);
atomic_set(&worker->num_pending, 0);
worker->task = kthread_run(worker_loop, worker, "btrfs");
+ worker->workers = workers;
if (IS_ERR(worker->task)) {
ret = PTR_ERR(worker->task);
goto fail;
}
spin_lock_irq(&workers->lock);
- list_add_tail(&worker->worker_list, &workers->worker_list);
- workers->last = worker;
+ list_add_tail(&worker->worker_list, &workers->idle_list);
workers->num_workers++;
spin_unlock_irq(&workers->lock);
}
@@ -169,42 +211,30 @@
{
struct btrfs_worker_thread *worker;
struct list_head *next;
- struct list_head *start;
int enforce_min = workers->num_workers < workers->max_workers;
- /* start with the last thread if it isn't busy */
- worker = workers->last;
- if (atomic_read(&worker->num_pending) < 64)
- goto done;
-
- next = worker->worker_list.next;
- start = &worker->worker_list;
-
/*
- * check all the workers for someone that is bored. FIXME, do
- * something smart here
+ * if we find an idle thread, don't move it to the end of the
+ * idle list. This improves the chance that the next submission
+ * will reuse the same thread, and maybe catch it while it is still
+ * working
*/
- while(next != start) {
- if (next == &workers->worker_list) {
- next = workers->worker_list.next;
- continue;
- }
+ if (!list_empty(&workers->idle_list)) {
+ next = workers->idle_list.next;
worker = list_entry(next, struct btrfs_worker_thread,
worker_list);
- if (atomic_read(&worker->num_pending) < 64 || !enforce_min)
- goto done;
- next = next->next;
+ return worker;
}
+ if (enforce_min || list_empty(&workers->worker_list))
+ return NULL;
+
/*
- * nobody was bored, if we're already at the max thread count,
- * use the last thread
+ * if we pick a busy task, move the task to the end of the list.
+ * hopefully this will keep things somewhat evenly balanced
*/
- if (!enforce_min || atomic_read(&workers->last->num_pending) < 64) {
- return workers->last;
- }
- return NULL;
-done:
- workers->last = worker;
+ next = workers->worker_list.next;
+ worker = list_entry(next, struct btrfs_worker_thread, worker_list);
+ list_move_tail(next, &workers->worker_list);
return worker;
}
@@ -221,11 +251,17 @@
if (!worker) {
spin_lock_irqsave(&workers->lock, flags);
if (workers->num_workers >= workers->max_workers) {
+ struct list_head *fallback = NULL;
/*
* we have failed to find any workers, just
* return the force one
*/
- worker = list_entry(workers->worker_list.next,
+ if (!list_empty(&workers->worker_list))
+ fallback = workers->worker_list.next;
+ if (!list_empty(&workers->idle_list))
+ fallback = workers->idle_list.next;
+ BUG_ON(!fallback);
+ worker = list_entry(fallback,
struct btrfs_worker_thread, worker_list);
spin_unlock_irqrestore(&workers->lock, flags);
} else {
@@ -254,6 +290,7 @@
spin_lock_irqsave(&worker->lock, flags);
atomic_inc(&worker->num_pending);
list_add_tail(&work->list, &worker->pending);
+ check_busy_worker(worker);
spin_unlock_irqrestore(&worker->lock, flags);
out:
return 0;
@@ -276,6 +313,7 @@
spin_lock_irqsave(&worker->lock, flags);
atomic_inc(&worker->num_pending);
+ check_busy_worker(worker);
list_add_tail(&work->list, &worker->pending);
/*