Use boot_ncpus in place of max_ncpus in taskq_create

Due to hotplug support or BIOS bugs sometimes max_ncpus can be
an absurdly high value. I have a system with 32 cores/threads
but reports max_ncpus == 440. This many threads potentially
cripples the system during arc_prune floods for example.

boot_ncpus is the number of working CPUs when called so use
that instead.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: DHE <git@dehacked.net>
Closes #10282

module/zfs/arc.c

@@ -7348,8 +7348,8 @@ 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", max_ncpus, defclsyspri,
-	    max_ncpus, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
+	arc_prune_taskq = taskq_create("arc_prune", boot_ncpus, defclsyspri,
+	    boot_ncpus, 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);

module/zfs/dsl_pool.c

@@ -220,10 +220,10 @@ dsl_pool_open_impl(spa_t *spa, uint64_t txg)
 	mutex_init(&dp->dp_lock, NULL, MUTEX_DEFAULT, NULL);
 	cv_init(&dp->dp_spaceavail_cv, NULL, CV_DEFAULT, NULL);
 
-	dp->dp_zrele_taskq = taskq_create("z_zrele", max_ncpus, defclsyspri,
-	    max_ncpus * 8, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
+	dp->dp_zrele_taskq = taskq_create("z_zrele", boot_ncpus, defclsyspri,
+	    boot_ncpus * 8, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
 	dp->dp_unlinked_drain_taskq = taskq_create("z_unlinked_drain",
-	    max_ncpus, defclsyspri, max_ncpus, INT_MAX,
+	    boot_ncpus, defclsyspri, boot_ncpus, INT_MAX,
 	    TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
 
 	return (dp);

module/zfs/txg.c

@@ -453,7 +453,7 @@ txg_dispatch_callbacks(dsl_pool_t *dp, uint64_t txg)
 			 * Commit callback taskq hasn't been created yet.
 			 */
 			tx->tx_commit_cb_taskq = taskq_create("tx_commit_cb",
-			    max_ncpus, defclsyspri, max_ncpus, max_ncpus * 2,
+			    boot_ncpus, defclsyspri, boot_ncpus, boot_ncpus * 2,
 			    TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
 		}