Avoid some spa_has_pending_synctask() calls.

Since 8c4fb36a24 (PR #7795) spa_has_pending_synctask() started to
take two more locks per write inside txg_all_lists_empty().  I am
surprised those pool-wide locks are not contended, but still their
operations are visible in CPU profiles under contended vdev lock.

This commit slightly changes vdev_queue_max_async_writes() flow to
not call the function if we are going to return max_active any way
due to high amount of dirty data.  It allows to save some CPU time
exactly when the pool is busy.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-By: Tom Caputi <caputit1@tcnj.edu>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes #11280

diff --git a/module/zfs/vdev_queue.c b/module/zfs/vdev_queue.c
index 6f3c4208aca2cbc42ae047485ec95df93883681b..02040c3ee198416ce6748e38dfd3c219a709cfec 100644 (file)
--- a/module/zfs/vdev_queue.c
+++ b/module/zfs/vdev_queue.c
@@ -338,14 +338,12 @@ vdev_queue_max_async_writes(spa_t *spa)
         * Sync tasks correspond to interactive user actions. To reduce the
         * execution time of those actions we push data out as fast as possible.
         */
-       if (spa_has_pending_synctask(spa))
+       dirty = dp->dp_dirty_total;
+       if (dirty > max_bytes || spa_has_pending_synctask(spa))
                return (zfs_vdev_async_write_max_active);
 
-       dirty = dp->dp_dirty_total;
        if (dirty < min_bytes)
                return (zfs_vdev_async_write_min_active);
-       if (dirty > max_bytes)
-               return (zfs_vdev_async_write_max_active);
 
        /*
         * linear interpolation: