Reduce number of arc_prune threads

On FreeBSD vnode reclamation is single-threaded, protected by single
global lock.  Linux seems to be able to use a thread per mount point,
but at this time it creates more harm than good.

Reduce number of threads to 1, adding tunable in case somebody wants
to try more.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes openzfs#12896
Issue openzfs#9966

man/man4/zfs.4

@@ -688,6 +688,13 @@ Minimum time "prescient prefetched" blocks are locked in the ARC.
 These blocks are meant to be prefetched fairly aggressively ahead of
 the code that may use them.
 .
+.It Sy zfs_arc_prune_task_threads Ns = Ns Sy 1 Pq int
+Number of arc_prune threads.
+.Fx
+does not need more than one.
+Linux may theoretically use one per mount point up to number of CPUs,
+but that was not proven to be useful.
+.
 .It Sy zfs_max_missing_tvds Ns = Ns Sy 0 Pq int
 Number of missing top-level vdevs which will be allowed during
 pool import (only in read-only mode).

module/zfs/arc.c

@@ -465,6 +465,11 @@ int zfs_arc_meta_strategy = ARC_STRATEGY_META_BALANCED;
 int zfs_arc_meta_adjust_restarts = 4096;
 int zfs_arc_lotsfree_percent = 10;
 
+/*
+ * Number of arc_prune threads
+ */
+static int zfs_arc_prune_task_threads = 1;
+
 /* The 6 states: */
 arc_state_t ARC_anon;
 arc_state_t ARC_mru;
@@ -8004,9 +8009,8 @@ arc_init(void)
 	    offsetof(arc_prune_t, p_node));
 	mutex_init(&arc_prune_mtx, NULL, MUTEX_DEFAULT, NULL);
 
-	arc_prune_taskq = taskq_create("arc_prune", 100, defclsyspri,
-	    boot_ncpus, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC |
-	    TASKQ_THREADS_CPU_PCT);
+	arc_prune_taskq = taskq_create("arc_prune", zfs_arc_prune_task_threads,
+	    defclsyspri, 100, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
 
 	arc_ksp = kstat_create("zfs", 0, "arcstats", "misc", KSTAT_TYPE_NAMED,
 	    sizeof (arc_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
@@ -11164,4 +11168,7 @@ ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, eviction_pct, INT, ZMOD_RW,
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, evict_batch_limit, INT, ZMOD_RW,
 	"The number of headers to evict per sublist before moving to the next");
+
+ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, prune_task_threads, INT, ZMOD_RW,
+	"Number of arc_prune threads");
 /* END CSTYLED */