Implement memory and CPU hotplug

ZFS currently doesn't react to hotplugging cpu or memory into the 
system in any way. This patch changes that by adding logic to the ARC 
that allows the system to take advantage of new memory that is added 
for caching purposes. It also adds logic to the taskq infrastructure 
to support dynamically expanding the number of threads allocated to a 
taskq.

Reviewed-by: Brian Behlendorf <[email protected]>
Co-authored-by: Matthew Ahrens <[email protected]>
Co-authored-by: Brian Behlendorf <[email protected]>
Signed-off-by: Paul Dagnelie <[email protected]>
Closes #11212 

14 files changed, 290 insertions, 36 deletions

diff --git a/config/kernel-hotplug.m4 b/config/kernel-hotplug.m4
new file mode 100644
index 000000000..e796a6d2e
--- /dev/null
+++ b/config/kernel-hotplug.m4
@@ -0,0 +1,26 @@
+dnl #
+dnl # 4.6 API change
+dnl # Added CPU hotplug APIs
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_CPU_HOTPLUG], [
+	ZFS_LINUX_TEST_SRC([cpu_hotplug], [
+		#include <linux/cpuhotplug.h>
+	],[
+		enum cpuhp_state state = CPUHP_ONLINE;
+		int (*fp)(unsigned int, struct hlist_node *) = NULL;
+		cpuhp_state_add_instance_nocalls(0, (struct hlist_node *)NULL);
+		cpuhp_state_remove_instance_nocalls(0, (struct hlist_node *)NULL);
+		cpuhp_setup_state_multi(state, "", fp, fp);
+		cpuhp_remove_multi_state(0);
+	])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_CPU_HOTPLUG], [
+	AC_MSG_CHECKING([whether CPU hotplug APIs exist])
+	ZFS_LINUX_TEST_RESULT([cpu_hotplug], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_CPU_HOTPLUG, 1, [yes])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])
diff --git a/config/kernel.m4 b/config/kernel.m4
index 3e01daa5e..14a8d4c58 100644
--- a/config/kernel.m4
+++ b/config/kernel.m4
@@ -124,6 +124,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_TOTALHIGH_PAGES
 	ZFS_AC_KERNEL_SRC_KSTRTOUL
 	ZFS_AC_KERNEL_SRC_PERCPU
+	ZFS_AC_KERNEL_SRC_CPU_HOTPLUG
 
 	AC_MSG_CHECKING([for available kernel interfaces])
 	ZFS_LINUX_TEST_COMPILE_ALL([kabi])
@@ -221,6 +222,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_TOTALHIGH_PAGES
 	ZFS_AC_KERNEL_KSTRTOUL
 	ZFS_AC_KERNEL_PERCPU
+	ZFS_AC_KERNEL_CPU_HOTPLUG
 ])
 
 dnl #
diff --git a/include/os/linux/spl/sys/taskq.h b/include/os/linux/spl/sys/taskq.h
index 16f4349e7..b50175a10 100644
--- a/include/os/linux/spl/sys/taskq.h
+++ b/include/os/linux/spl/sys/taskq.h
@@ -84,6 +84,8 @@ typedef struct taskq {
 	int			tq_nthreads;	/* # of existing threads */
 	int			tq_nspawn;	/* # of threads being spawned */
 	int			tq_maxthreads;	/* # of threads maximum */
+	/* If PERCPU flag is set, percent of NCPUs to have as threads */
+	int			tq_cpu_pct;
 	int			tq_pri;		/* priority */
 	int			tq_minalloc;	/* min taskq_ent_t pool size */
 	int			tq_maxalloc;	/* max taskq_ent_t pool size */
@@ -99,6 +101,9 @@ typedef struct taskq {
 	spl_wait_queue_head_t	tq_work_waitq;	/* new work waitq */
 	spl_wait_queue_head_t	tq_wait_waitq;	/* wait waitq */
 	tq_lock_role_t		tq_lock_class;	/* class when taking tq_lock */
+	/* list node for the cpu hotplug callback */
+	struct hlist_node	tq_hp_cb_node;
+	boolean_t		tq_hp_support;
 } taskq_t;
 
 typedef struct taskq_ent {
diff --git a/include/sys/arc.h b/include/sys/arc.h
index 9ade1a432..f58fa53b6 100644
--- a/include/sys/arc.h
+++ b/include/sys/arc.h
@@ -310,6 +310,7 @@ int arc_tempreserve_space(spa_t *spa, uint64_t reserve, uint64_t txg);
 uint64_t arc_all_memory(void);
 uint64_t arc_default_max(uint64_t min, uint64_t allmem);
 uint64_t arc_target_bytes(void);
+void arc_set_limits(uint64_t);
 void arc_init(void);
 void arc_fini(void);
 
diff --git a/include/sys/arc_impl.h b/include/sys/arc_impl.h
index b08a42c75..6adcd4b54 100644
--- a/include/sys/arc_impl.h
+++ b/include/sys/arc_impl.h
@@ -926,6 +926,8 @@ extern int arc_memory_throttle(spa_t *spa, uint64_t reserve, uint64_t txg);
 extern uint64_t arc_free_memory(void);
 extern int64_t arc_available_memory(void);
 extern void arc_tuning_update(boolean_t);
+extern void arc_register_hotplug(void);
+extern void arc_unregister_hotplug(void);
 
 extern int param_set_arc_long(ZFS_MODULE_PARAM_ARGS);
 extern int param_set_arc_int(ZFS_MODULE_PARAM_ARGS);
diff --git a/module/os/freebsd/zfs/arc_os.c b/module/os/freebsd/zfs/arc_os.c
index 94df75003..4fc7468bf 100644
--- a/module/os/freebsd/zfs/arc_os.c
+++ b/module/os/freebsd/zfs/arc_os.c
@@ -243,3 +243,13 @@ arc_lowmem_fini(void)
 	if (arc_event_lowmem != NULL)
 		EVENTHANDLER_DEREGISTER(vm_lowmem, arc_event_lowmem);
 }
+
+void
+arc_register_hotplug(void)
+{
+}
+
+void
+arc_unregister_hotplug(void)
+{
+}
diff --git a/module/os/linux/spl/spl-taskq.c b/module/os/linux/spl/spl-taskq.c
index fafadffc7..e8d89bfea 100644
--- a/module/os/linux/spl/spl-taskq.c
+++ b/module/os/linux/spl/spl-taskq.c
@@ -28,6 +28,9 @@
 #include <sys/kmem.h>
 #include <sys/tsd.h>
 #include <sys/trace_spl.h>
+#ifdef HAVE_CPU_HOTPLUG
+#include <linux/cpuhotplug.h>
+#endif
 
 int spl_taskq_thread_bind = 0;
 module_param(spl_taskq_thread_bind, int, 0644);
@@ -35,7 +38,7 @@ MODULE_PARM_DESC(spl_taskq_thread_bind, "Bind taskq thread to CPU by default");
 
 
 int spl_taskq_thread_dynamic = 1;
-module_param(spl_taskq_thread_dynamic, int, 0644);
+module_param(spl_taskq_thread_dynamic, int, 0444);
 MODULE_PARM_DESC(spl_taskq_thread_dynamic, "Allow dynamic taskq threads");
 
 int spl_taskq_thread_priority = 1;
@@ -59,6 +62,11 @@ EXPORT_SYMBOL(system_delay_taskq);
 static taskq_t *dynamic_taskq;
 static taskq_thread_t *taskq_thread_create(taskq_t *);
 
+#ifdef HAVE_CPU_HOTPLUG
+/* Multi-callback id for cpu hotplugging. */
+static int spl_taskq_cpuhp_state;
+#endif
+
 /* List of all taskqs */
 LIST_HEAD(tq_list);
 struct rw_semaphore tq_list_sem;
@@ -1024,13 +1032,14 @@ taskq_thread_create(taskq_t *tq)
 }
 
 taskq_t *
-taskq_create(const char *name, int nthreads, pri_t pri,
+taskq_create(const char *name, int threads_arg, pri_t pri,
     int minalloc, int maxalloc, uint_t flags)
 {
 	taskq_t *tq;
 	taskq_thread_t *tqt;
 	int count = 0, rc = 0, i;
 	unsigned long irqflags;
+	int nthreads = threads_arg;
 
 	ASSERT(name != NULL);
 	ASSERT(minalloc >= 0);
@@ -1041,15 +1050,27 @@ taskq_create(const char *name, int nthreads, pri_t pri,
 	if (flags & TASKQ_THREADS_CPU_PCT) {
 		ASSERT(nthreads <= 100);
 		ASSERT(nthreads >= 0);
-		nthreads = MIN(nthreads, 100);
+		nthreads = MIN(threads_arg, 100);
 		nthreads = MAX(nthreads, 0);
-		nthreads = MAX((num_online_cpus() * nthreads) / 100, 1);
+		nthreads = MAX((num_online_cpus() * nthreads) /100, 1);
 	}
 
 	tq = kmem_alloc(sizeof (*tq), KM_PUSHPAGE);
 	if (tq == NULL)
 		return (NULL);
 
+	tq->tq_hp_support = B_FALSE;
+#ifdef HAVE_CPU_HOTPLUG
+	if (flags & TASKQ_THREADS_CPU_PCT) {
+		tq->tq_hp_support = B_TRUE;
+		if (cpuhp_state_add_instance_nocalls(spl_taskq_cpuhp_state,
+		    &tq->tq_hp_cb_node) != 0) {
+			kmem_free(tq, sizeof (*tq));
+			return (NULL);
+		}
+	}
+#endif
+
 	spin_lock_init(&tq->tq_lock);
 	INIT_LIST_HEAD(&tq->tq_thread_list);
 	INIT_LIST_HEAD(&tq->tq_active_list);
@@ -1058,6 +1079,7 @@ taskq_create(const char *name, int nthreads, pri_t pri,
 	tq->tq_nthreads = 0;
 	tq->tq_nspawn = 0;
 	tq->tq_maxthreads = nthreads;
+	tq->tq_cpu_pct = threads_arg;
 	tq->tq_pri = pri;
 	tq->tq_minalloc = minalloc;
 	tq->tq_maxalloc = maxalloc;
@@ -1131,6 +1153,12 @@ taskq_destroy(taskq_t *tq)
 	tq->tq_flags &= ~TASKQ_ACTIVE;
 	spin_unlock_irqrestore(&tq->tq_lock, flags);
 
+#ifdef HAVE_CPU_HOTPLUG
+	if (tq->tq_hp_support) {
+		VERIFY0(cpuhp_state_remove_instance_nocalls(
+		    spl_taskq_cpuhp_state, &tq->tq_hp_cb_node));
+	}
+#endif
 	/*
 	 * When TASKQ_ACTIVE is clear new tasks may not be added nor may
 	 * new worker threads be spawned for dynamic taskq.
@@ -1198,7 +1226,6 @@ taskq_destroy(taskq_t *tq)
 }
 EXPORT_SYMBOL(taskq_destroy);
 
-
 static unsigned int spl_taskq_kick = 0;
 
 /*
@@ -1255,12 +1282,96 @@ module_param_call(spl_taskq_kick, param_set_taskq_kick, param_get_uint,
 MODULE_PARM_DESC(spl_taskq_kick,
 	"Write nonzero to kick stuck taskqs to spawn more threads");
 
+#ifdef HAVE_CPU_HOTPLUG
+/*
+ * This callback will be called exactly once for each core that comes online,
+ * for each dynamic taskq. We attempt to expand taskqs that have
+ * TASKQ_THREADS_CPU_PCT set. We need to redo the percentage calculation every
+ * time, to correctly determine whether or not to add a thread.
+ */
+static int
+spl_taskq_expand(unsigned int cpu, struct hlist_node *node)
+{
+	taskq_t *tq = list_entry(node, taskq_t, tq_hp_cb_node);
+	unsigned long flags;
+	int err = 0;
+
+	ASSERT(tq);
+	spin_lock_irqsave_nested(&tq->tq_lock, flags, tq->tq_lock_class);
+
+	if (!(tq->tq_flags & TASKQ_ACTIVE))
+		goto out;
+
+	ASSERT(tq->tq_flags & TASKQ_THREADS_CPU_PCT);
+	int nthreads = MIN(tq->tq_cpu_pct, 100);
+	nthreads = MAX(((num_online_cpus() + 1) * nthreads) / 100, 1);
+	tq->tq_maxthreads = nthreads;
+
+	if (!((tq->tq_flags & TASKQ_DYNAMIC) && spl_taskq_thread_dynamic) &&
+	    tq->tq_maxthreads > tq->tq_nthreads) {
+		ASSERT3U(tq->tq_maxthreads, ==, tq->tq_nthreads + 1);
+		taskq_thread_t *tqt = taskq_thread_create(tq);
+		if (tqt == NULL)
+			err = -1;
+	}
+
+out:
+	spin_unlock_irqrestore(&tq->tq_lock, flags);
+	return (err);
+}
+
+/*
+ * While we don't support offlining CPUs, it is possible that CPUs will fail
+ * to online successfully. We do need to be able to handle this case
+ * gracefully.
+ */
+static int
+spl_taskq_prepare_down(unsigned int cpu, struct hlist_node *node)
+{
+	taskq_t *tq = list_entry(node, taskq_t, tq_hp_cb_node);
+	unsigned long flags;
+
+	ASSERT(tq);
+	spin_lock_irqsave_nested(&tq->tq_lock, flags, tq->tq_lock_class);
+
+	if (!(tq->tq_flags & TASKQ_ACTIVE))
+		goto out;
+
+	ASSERT(tq->tq_flags & TASKQ_THREADS_CPU_PCT);
+	int nthreads = MIN(tq->tq_cpu_pct, 100);
+	nthreads = MAX(((num_online_cpus()) * nthreads) / 100, 1);
+	tq->tq_maxthreads = nthreads;
+
+	if (!((tq->tq_flags & TASKQ_DYNAMIC) && spl_taskq_thread_dynamic) &&
+	    tq->tq_maxthreads < tq->tq_nthreads) {
+		ASSERT3U(tq->tq_maxthreads, ==, tq->tq_nthreads - 1);
+		taskq_thread_t *tqt = list_entry(tq->tq_thread_list.next,
+		    taskq_thread_t, tqt_thread_list);
+		struct task_struct *thread = tqt->tqt_thread;
+		spin_unlock_irqrestore(&tq->tq_lock, flags);
+
+		kthread_stop(thread);
+
+		return (0);
+	}
+
+out:
+	spin_unlock_irqrestore(&tq->tq_lock, flags);
+	return (0);
+}
+#endif
+
 int
 spl_taskq_init(void)
 {
 	init_rwsem(&tq_list_sem);
 	tsd_create(&taskq_tsd, NULL);
 
+#ifdef HAVE_CPU_HOTPLUG
+	spl_taskq_cpuhp_state = cpuhp_setup_state_multi(CPUHP_AP_ONLINE_DYN,
+	    "fs/spl_taskq:online", spl_taskq_expand, spl_taskq_prepare_down);
+#endif
+
 	system_taskq = taskq_create("spl_system_taskq", MAX(boot_ncpus, 64),
 	    maxclsyspri, boot_ncpus, INT_MAX, TASKQ_PREPOPULATE|TASKQ_DYNAMIC);
 	if (system_taskq == NULL)
@@ -1269,6 +1380,9 @@ spl_taskq_init(void)
 	system_delay_taskq = taskq_create("spl_delay_taskq", MAX(boot_ncpus, 4),
 	    maxclsyspri, boot_ncpus, INT_MAX, TASKQ_PREPOPULATE|TASKQ_DYNAMIC);
 	if (system_delay_taskq == NULL) {
+#ifdef HAVE_CPU_HOTPLUG
+		cpuhp_remove_multi_state(spl_taskq_cpuhp_state);
+#endif
 		taskq_destroy(system_taskq);
 		return (1);
 	}
@@ -1276,6 +1390,9 @@ spl_taskq_init(void)
 	dynamic_taskq = taskq_create("spl_dynamic_taskq", 1,
 	    maxclsyspri, boot_ncpus, INT_MAX, TASKQ_PREPOPULATE);
 	if (dynamic_taskq == NULL) {
+#ifdef HAVE_CPU_HOTPLUG
+		cpuhp_remove_multi_state(spl_taskq_cpuhp_state);
+#endif
 		taskq_destroy(system_taskq);
 		taskq_destroy(system_delay_taskq);
 		return (1);
@@ -1304,4 +1421,9 @@ spl_taskq_fini(void)
 	system_taskq = NULL;
 
 	tsd_destroy(&taskq_tsd);
+
+#ifdef HAVE_CPU_HOTPLUG
+	cpuhp_remove_multi_state(spl_taskq_cpuhp_state);
+	spl_taskq_cpuhp_state = 0;
+#endif
 }
diff --git a/module/os/linux/zfs/arc_os.c b/module/os/linux/zfs/arc_os.c
index 792c75d46..83d4a3d84 100644
--- a/module/os/linux/zfs/arc_os.c
+++ b/module/os/linux/zfs/arc_os.c
@@ -48,6 +48,8 @@
 #include <sys/vmsystm.h>
 #include <sys/zpl.h>
 #include <linux/page_compat.h>
+#include <linux/notifier.h>
+#include <linux/memory.h>
 #endif
 #include <sys/callb.h>
 #include <sys/kstat.h>
@@ -73,6 +75,9 @@
  */
 int zfs_arc_shrinker_limit = 10000;
 
+#ifdef CONFIG_MEMORY_HOTPLUG
+static struct notifier_block arc_hotplug_callback_mem_nb;
+#endif
 
 /*
  * Return a default max arc size based on the amount of physical memory.
@@ -278,18 +283,9 @@ arc_memory_throttle(spa_t *spa, uint64_t reserve, uint64_t txg)
 	return (0);
 }
 
-void
-arc_lowmem_init(void)
+static void
+arc_set_sys_free(uint64_t allmem)
 {
-	uint64_t allmem = arc_all_memory();
-
-	/*
-	 * Register a shrinker to support synchronous (direct) memory
-	 * reclaim from the arc.  This is done to prevent kswapd from
-	 * swapping out pages when it is preferable to shrink the arc.
-	 */
-	spl_register_shrinker(&arc_shrinker);
-
 	/*
 	 * The ARC tries to keep at least this much memory available for the
 	 * system.  This gives the ARC time to shrink in response to memory
@@ -343,6 +339,20 @@ arc_lowmem_init(void)
 }
 
 void
+arc_lowmem_init(void)
+{
+	uint64_t allmem = arc_all_memory();
+
+	/*
+	 * Register a shrinker to support synchronous (direct) memory
+	 * reclaim from the arc.  This is done to prevent kswapd from
+	 * swapping out pages when it is preferable to shrink the arc.
+	 */
+	spl_register_shrinker(&arc_shrinker);
+	arc_set_sys_free(allmem);
+}
+
+void
 arc_lowmem_fini(void)
 {
 	spl_unregister_shrinker(&arc_shrinker);
@@ -375,6 +385,52 @@ param_set_arc_int(const char *buf, zfs_kernel_param_t *kp)
 
 	return (0);
 }
+
+#ifdef CONFIG_MEMORY_HOTPLUG
+/* ARGSUSED */
+static int
+arc_hotplug_callback(struct notifier_block *self, unsigned long action,
+    void *arg)
+{
+	uint64_t allmem = arc_all_memory();
+	if (action != MEM_ONLINE)
+		return (NOTIFY_OK);
+
+	arc_set_limits(allmem);
+
+#ifdef __LP64__
+	if (zfs_dirty_data_max_max == 0)
+		zfs_dirty_data_max_max = MIN(4ULL * 1024 * 1024 * 1024,
+		    allmem * zfs_dirty_data_max_max_percent / 100);
+#else
+	if (zfs_dirty_data_max_max == 0)
+		zfs_dirty_data_max_max = MIN(1ULL * 1024 * 1024 * 1024,
+		    allmem * zfs_dirty_data_max_max_percent / 100);
+#endif
+
+	arc_set_sys_free(allmem);
+	return (NOTIFY_OK);
+}
+#endif
+
+void
+arc_register_hotplug(void)
+{
+#ifdef CONFIG_MEMORY_HOTPLUG
+	arc_hotplug_callback_mem_nb.notifier_call = arc_hotplug_callback;
+	/* There is no significance to the value 100 */
+	arc_hotplug_callback_mem_nb.priority = 100;
+	register_memory_notifier(&arc_hotplug_callback_mem_nb);
+#endif
+}
+
+void
+arc_unregister_hotplug(void)
+{
+#ifdef CONFIG_MEMORY_HOTPLUG
+	unregister_memory_notifier(&arc_hotplug_callback_mem_nb);
+#endif
+}
 #else /* _KERNEL */
 int64_t
 arc_available_memory(void)
@@ -405,6 +461,16 @@ arc_free_memory(void)
 {
 	return (spa_get_random(arc_all_memory() * 20 / 100));
 }
+
+void
+arc_register_hotplug(void)
+{
+}
+
+void
+arc_unregister_hotplug(void)
+{
+}
 #endif /* _KERNEL */
 
 /*
diff --git a/module/zfs/aggsum.c b/module/zfs/aggsum.c
index e38f4a66c..e46da95f6 100644
--- a/module/zfs/aggsum.c
+++ b/module/zfs/aggsum.c
@@ -70,6 +70,11 @@
  * zeroing out the borrowed value (forcing that thread to borrow on its next
  * request, which will also be expensive).  This is what makes aggsums well
  * suited for write-many read-rarely operations.
+ *
+ * Note that the aggsums do not expand if more CPUs are hot-added. In that
+ * case, we will have less fanout than boot_ncpus, but we don't want to always
+ * reserve the RAM necessary to create the extra slots for additional CPUs up
+ * front, and dynamically adding them is a complex task.
  */
 
 /*
diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index efc6bb138..1bc27391c 100644
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -7593,6 +7593,15 @@ arc_target_bytes(void)
 }
 
 void
+arc_set_limits(uint64_t allmem)
+{
+	/* Set min cache to 1/32 of all memory, or 32MB, whichever is more. */
+	arc_c_min = MAX(allmem / 32, 2ULL << SPA_MAXBLOCKSHIFT);
+
+	/* How to set default max varies by platform. */
+	arc_c_max = arc_default_max(arc_c_min, allmem);
+}
+void
 arc_init(void)
 {
 	uint64_t percent, allmem = arc_all_memory();
@@ -7607,11 +7616,7 @@ arc_init(void)
 	arc_lowmem_init();
 #endif
 
-	/* Set min cache to 1/32 of all memory, or 32MB, whichever is more. */
-	arc_c_min = MAX(allmem / 32, 2ULL << SPA_MAXBLOCKSHIFT);
-
-	/* How to set default max varies by platform. */
-	arc_c_max = arc_default_max(arc_c_min, allmem);
+	arc_set_limits(allmem);
 
 #ifndef _KERNEL
 	/*
@@ -7648,6 +7653,8 @@ arc_init(void)
 	if (arc_c < arc_c_min)
 		arc_c = arc_c_min;
 
+	arc_register_hotplug();
+
 	arc_state_init();
 
 	buf_init();
@@ -7656,8 +7663,9 @@ 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", boot_ncpus, defclsyspri,
-	    boot_ncpus, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
+	arc_prune_taskq = taskq_create("arc_prune", 100, defclsyspri,
+	    boot_ncpus, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC |
+	    TASKQ_THREADS_CPU_PCT);
 
 	arc_ksp = kstat_create("zfs", 0, "arcstats", "misc", KSTAT_TYPE_NAMED,
 	    sizeof (arc_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
@@ -7754,6 +7762,8 @@ arc_fini(void)
 	buf_fini();
 	arc_state_fini();
 
+	arc_unregister_hotplug();
+
 	/*
 	 * We destroy the zthrs after all the ARC state has been
 	 * torn down to avoid the case of them receiving any
diff --git a/module/zfs/dsl_pool.c b/module/zfs/dsl_pool.c
index 3a2028625..8f6675820 100644
--- a/module/zfs/dsl_pool.c
+++ b/module/zfs/dsl_pool.c
@@ -220,11 +220,12 @@ 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", boot_ncpus, defclsyspri,
-	    boot_ncpus * 8, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
+	dp->dp_zrele_taskq = taskq_create("z_zrele", 100, defclsyspri,
+	    boot_ncpus * 8, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC |
+	    TASKQ_THREADS_CPU_PCT);
 	dp->dp_unlinked_drain_taskq = taskq_create("z_unlinked_drain",
-	    boot_ncpus, defclsyspri, boot_ncpus, INT_MAX,
-	    TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
+	    100, defclsyspri, boot_ncpus, INT_MAX,
+	    TASKQ_PREPOPULATE | TASKQ_DYNAMIC | TASKQ_THREADS_CPU_PCT);
 
 	return (dp);
 }
diff --git a/module/zfs/multilist.c b/module/zfs/multilist.c
index a3adfd317..36c0d33bf 100644
--- a/module/zfs/multilist.c
+++ b/module/zfs/multilist.c
@@ -96,9 +96,12 @@ multilist_create_impl(size_t size, size_t offset,
 }
 
 /*
- * Allocate a new multilist, using the default number of sublists
- * (the number of CPUs, or at least 4, or the tunable
- * zfs_multilist_num_sublists).
+ * Allocate a new multilist, using the default number of sublists (the number
+ * of CPUs, or at least 4, or the tunable zfs_multilist_num_sublists). Note
+ * that the multilists do not expand if more CPUs are hot-added. In that case,
+ * we will have less fanout than boot_ncpus, but we don't want to always
+ * reserve the RAM necessary to create the extra slots for additional CPUs up
+ * front, and dynamically adding them is a complex task.
  */
 multilist_t *
 multilist_create(size_t size, size_t offset,
diff --git a/module/zfs/spa.c b/module/zfs/spa.c
index 65c907d9d..50822cfae 100644
--- a/module/zfs/spa.c
+++ b/module/zfs/spa.c
@@ -1281,15 +1281,15 @@ spa_activate(spa_t *spa, spa_mode_t mode)
 	 * pool traverse code from monopolizing the global (and limited)
 	 * system_taskq by inappropriately scheduling long running tasks on it.
 	 */
-	spa->spa_prefetch_taskq = taskq_create("z_prefetch", boot_ncpus,
-	    defclsyspri, 1, INT_MAX, TASKQ_DYNAMIC);
+	spa->spa_prefetch_taskq = taskq_create("z_prefetch", 100,
+	    defclsyspri, 1, INT_MAX, TASKQ_DYNAMIC | TASKQ_THREADS_CPU_PCT);
 
 	/*
 	 * The taskq to upgrade datasets in this pool. Currently used by
 	 * feature SPA_FEATURE_USEROBJ_ACCOUNTING/SPA_FEATURE_PROJECT_QUOTA.
 	 */
-	spa->spa_upgrade_taskq = taskq_create("z_upgrade", boot_ncpus,
-	    defclsyspri, 1, INT_MAX, TASKQ_DYNAMIC);
+	spa->spa_upgrade_taskq = taskq_create("z_upgrade", 100,
+	    defclsyspri, 1, INT_MAX, TASKQ_DYNAMIC | TASKQ_THREADS_CPU_PCT);
 }
 
 /*
diff --git a/module/zfs/txg.c b/module/zfs/txg.c
index 420244abb..3efd26155 100644
--- a/module/zfs/txg.c
+++ b/module/zfs/txg.c
@@ -446,8 +446,9 @@ 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",
-			    boot_ncpus, defclsyspri, boot_ncpus, boot_ncpus * 2,
-			    TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
+			    100, defclsyspri, boot_ncpus, boot_ncpus * 2,
+			    TASKQ_PREPOPULATE | TASKQ_DYNAMIC |
+			    TASKQ_THREADS_CPU_PCT);
 		}
 
 		cb_list = kmem_alloc(sizeof (list_t), KM_SLEEP);