OpenZFS 9284 - arc_reclaim_thread has 2 jobs

Following the fix for 9018 (Replace kmem_cache_reap_now() with
kmem_cache_reap_soon), the arc_reclaim_thread() no longer blocks
while reaping.  However, the code is still confusing and error-prone,
because this thread has two responsibilities.  We should instead
separate this into two threads each with their own responsibility:

 1. keep `arc_size` under `arc_c`, by calling `arc_adjust()`, which
    improves `arc_is_overflowing()`

 2. keep enough free memory in the system, by calling
    `arc_kmem_reap_now()` plus `arc_shrink()`, which improves
    `arc_available_memory()`.

Furthermore, we can use the zthr infrastructure to separate the
"should we do something" from "do it" parts of the logic, and
normalize the start up / shut down of the threads.

Authored by: Brad Lewis <[email protected]>
Reviewed by: Matt Ahrens <[email protected]>
Reviewed by: Serapheim Dimitropoulos <[email protected]>
Reviewed by: Pavel Zakharov <[email protected]>
Reviewed by: Dan Kimmel <[email protected]>
Reviewed by: Paul Dagnelie <[email protected]>
Reviewed by: Dan McDonald <[email protected]>
Reviewed by: Tim Kordas <[email protected]>
Reviewed by: Tim Chase <[email protected]>
Reviewed by: Brian Behlendorf <[email protected]>
Ported-by:  Brad Lewis <[email protected]>
Signed-off-by: Brad Lewis <[email protected]>

OpenZFS-issue: https://www.illumos.org/issues/9284
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/de753e34f9
Closes #8165

7 files changed, 285 insertions, 172 deletions

diff --git a/include/spl/sys/kmem.h b/include/spl/sys/kmem.h
index d6b428551..72d3a7765 100644
--- a/include/spl/sys/kmem.h
+++ b/include/spl/sys/kmem.h
@@ -163,6 +163,7 @@ extern unsigned int spl_kmem_alloc_max;
 #define	kmem_alloc(sz, fl)	spl_kmem_alloc((sz), (fl), __func__, __LINE__)
 #define	kmem_zalloc(sz, fl)	spl_kmem_zalloc((sz), (fl), __func__, __LINE__)
 #define	kmem_free(ptr, sz)	spl_kmem_free((ptr), (sz))
+#define	kmem_cache_reap_active	spl_kmem_cache_reap_active
 
 extern void *spl_kmem_alloc(size_t sz, int fl, const char *func, int line);
 extern void *spl_kmem_zalloc(size_t sz, int fl, const char *func, int line);
@@ -181,5 +182,6 @@ extern void spl_kmem_free_track(const void *buf, size_t size);
 
 extern int spl_kmem_init(void);
 extern void spl_kmem_fini(void);
+extern int spl_kmem_cache_reap_active(void);
 
 #endif	/* _SPL_KMEM_H */
diff --git a/include/sys/zfs_context.h b/include/sys/zfs_context.h
index 3637cc617..11a32bb31 100644
--- a/include/sys/zfs_context.h
+++ b/include/sys/zfs_context.h
@@ -773,6 +773,7 @@ typedef int fstrans_cookie_t;
 extern fstrans_cookie_t spl_fstrans_mark(void);
 extern void spl_fstrans_unmark(fstrans_cookie_t);
 extern int __spl_pf_fstrans_check(void);
+extern int kmem_cache_reap_active(void);
 
 #define	____cacheline_aligned
 
diff --git a/include/sys/zthr.h b/include/sys/zthr.h
index 62da2eea8..ce6033ecb 100644
--- a/include/sys/zthr.h
+++ b/include/sys/zthr.h
@@ -29,6 +29,7 @@ struct zthr {
 	kmutex_t	zthr_lock;
 	kcondvar_t	zthr_cv;
 	boolean_t	zthr_cancel;
+	hrtime_t	zthr_wait_time;
 
 	zthr_checkfunc_t	*zthr_checkfunc;
 	zthr_func_t	*zthr_func;
@@ -38,6 +39,9 @@ struct zthr {
 
 extern zthr_t *zthr_create(zthr_checkfunc_t checkfunc,
     zthr_func_t *func, void *arg);
+extern zthr_t *zthr_create_timer(zthr_checkfunc_t *checkfunc,
+    zthr_func_t *func, void *arg, hrtime_t nano_wait);
+
 extern void zthr_exit(zthr_t *t, int rc);
 extern void zthr_destroy(zthr_t *t);
 
diff --git a/lib/libzpool/kernel.c b/lib/libzpool/kernel.c
index f5eafb917..926f4f4f4 100644
--- a/lib/libzpool/kernel.c
+++ b/lib/libzpool/kernel.c
@@ -1276,6 +1276,12 @@ __spl_pf_fstrans_check(void)
 	return (0);
 }
 
+int
+kmem_cache_reap_active(void)
+{
+	return (0);
+}
+
 void *zvol_tag = "zvol_tag";
 
 void
diff --git a/module/spl/spl-kmem-cache.c b/module/spl/spl-kmem-cache.c
index 5492c6a46..620f03ddf 100644
--- a/module/spl/spl-kmem-cache.c
+++ b/module/spl/spl-kmem-cache.c
@@ -1733,6 +1733,18 @@ out:
 EXPORT_SYMBOL(spl_kmem_cache_reap_now);
 
 /*
+ * This is stubbed out for code consistency with other platforms.  There
+ * is existing logic to prevent concurrent reaping so while this is ugly
+ * it should do no harm.
+ */
+int
+spl_kmem_cache_reap_active()
+{
+	return (0);
+}
+EXPORT_SYMBOL(spl_kmem_cache_reap_active);
+
+/*
  * Reap all free slabs from all registered caches.
  */
 void
diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index 96557054c..9e0ffd06d 100644
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -20,10 +20,10 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2012, Joyent, Inc. All rights reserved.
+ * Copyright (c) 2018, Joyent, Inc.
  * Copyright (c) 2011, 2018 by Delphix. All rights reserved.
  * Copyright (c) 2014 by Saso Kiselkov. All rights reserved.
- * Copyright 2015 Nexenta Systems, Inc.  All rights reserved.
+ * Copyright 2017 Nexenta Systems, Inc.  All rights reserved.
  */
 
 /*
@@ -299,7 +299,7 @@
 #endif
 #include <sys/callb.h>
 #include <sys/kstat.h>
-#include <sys/dmu_tx.h>
+#include <sys/zthr.h>
 #include <zfs_fletcher.h>
 #include <sys/arc_impl.h>
 #include <sys/trace_arc.h>
@@ -311,10 +311,22 @@
 boolean_t arc_watch = B_FALSE;
 #endif
 
-static kmutex_t		arc_reclaim_lock;
-static kcondvar_t	arc_reclaim_thread_cv;
-static boolean_t	arc_reclaim_thread_exit;
-static kcondvar_t	arc_reclaim_waiters_cv;
+/*
+ * This thread's job is to keep enough free memory in the system, by
+ * calling arc_kmem_reap_soon() plus arc_reduce_target_size(), which improves
+ * arc_available_memory().
+ */
+static zthr_t		*arc_reap_zthr;
+
+/*
+ * This thread's job is to keep arc_size under arc_c, by calling
+ * arc_adjust(), which improves arc_is_overflowing().
+ */
+static zthr_t		*arc_adjust_zthr;
+
+static kmutex_t		arc_adjust_lock;
+static kcondvar_t	arc_adjust_waiters_cv;
+static boolean_t	arc_adjust_needed = B_FALSE;
 
 /*
  * The number of headers to evict in arc_evict_state_impl() before
@@ -326,20 +338,25 @@ static kcondvar_t	arc_reclaim_waiters_cv;
 int zfs_arc_evict_batch_limit = 10;
 
 /* number of seconds before growing cache again */
-static int		arc_grow_retry = 5;
+static int arc_grow_retry = 5;
+
+/*
+ * Minimum time between calls to arc_kmem_reap_soon().
+ */
+int arc_kmem_cache_reap_retry_ms = 1000;
 
 /* shift of arc_c for calculating overflow limit in arc_get_data_impl */
-int		zfs_arc_overflow_shift = 8;
+int zfs_arc_overflow_shift = 8;
 
 /* shift of arc_c for calculating both min and max arc_p */
-static int		arc_p_min_shift = 4;
+int arc_p_min_shift = 4;
 
 /* log2(fraction of arc to reclaim) */
-static int		arc_shrink_shift = 7;
+static int arc_shrink_shift = 7;
 
 /* percent of pagecache to reclaim arc to */
 #ifdef _KERNEL
-static uint_t		zfs_arc_pc_percent = 0;
+static uint_t zfs_arc_pc_percent = 0;
 #endif
 
 /*
@@ -366,7 +383,10 @@ static int		arc_min_prescient_prefetch_ms;
  */
 int arc_lotsfree_percent = 10;
 
-static int arc_dead;
+/*
+ * hdr_recl() uses this to determine if the arc is up and running.
+ */
+static boolean_t arc_initialized;
 
 /*
  * The arc has filled available memory and has now warmed up.
@@ -906,6 +926,7 @@ aggsum_t astat_bonus_size;
 aggsum_t astat_hdr_size;
 aggsum_t astat_l2_hdr_size;
 
+static hrtime_t arc_growtime;
 static list_t arc_prune_list;
 static kmutex_t arc_prune_mtx;
 static taskq_t *arc_prune_taskq;
@@ -1380,8 +1401,8 @@ hdr_recl(void *unused)
 	 * umem calls the reclaim func when we destroy the buf cache,
 	 * which is after we do arc_fini().
 	 */
-	if (!arc_dead)
-		cv_signal(&arc_reclaim_thread_cv);
+	if (arc_initialized)
+		zthr_wakeup(arc_reap_zthr);
 }
 
 static void
@@ -4097,13 +4118,14 @@ arc_evict_state_impl(multilist_t *ml, int idx, arc_buf_hdr_t *marker,
 			 * function should proceed in this case).
 			 *
 			 * If threads are left sleeping, due to not
-			 * using cv_broadcast, they will be woken up
-			 * just before arc_reclaim_thread() sleeps.
+			 * using cv_broadcast here, they will be woken
+			 * up via cv_broadcast in arc_adjust_cb() just
+			 * before arc_adjust_zthr sleeps.
 			 */
-			mutex_enter(&arc_reclaim_lock);
+			mutex_enter(&arc_adjust_lock);
 			if (!arc_is_overflowing())
-				cv_signal(&arc_reclaim_waiters_cv);
-			mutex_exit(&arc_reclaim_lock);
+				cv_signal(&arc_adjust_waiters_cv);
+			mutex_exit(&arc_adjust_lock);
 		} else {
 			ARCSTAT_BUMP(arcstat_mutex_miss);
 		}
@@ -4763,8 +4785,8 @@ arc_flush(spa_t *spa, boolean_t retry)
 	(void) arc_flush_state(arc_mfu_ghost, guid, ARC_BUFC_METADATA, retry);
 }
 
-void
-arc_shrink(int64_t to_free)
+static void
+arc_reduce_target_size(int64_t to_free)
 {
 	uint64_t asize = aggsum_value(&arc_size);
 	uint64_t c = arc_c;
@@ -4782,10 +4804,14 @@ arc_shrink(int64_t to_free)
 		arc_c = arc_c_min;
 	}
 
-	if (asize > arc_c)
-		(void) arc_adjust();
+	if (asize > arc_c) {
+		/* See comment in arc_adjust_cb_check() on why lock+flag */
+		mutex_enter(&arc_adjust_lock);
+		arc_adjust_needed = B_TRUE;
+		mutex_exit(&arc_adjust_lock);
+		zthr_wakeup(arc_adjust_zthr);
+	}
 }
-
 /*
  * Return maximum amount of memory that we could possibly use.  Reduced
  * to half of all memory in user space which is primarily used for testing.
@@ -4989,7 +5015,7 @@ arc_reclaim_needed(void)
 }
 
 static void
-arc_kmem_reap_now(void)
+arc_kmem_reap_soon(void)
 {
 	size_t			i;
 	kmem_cache_t		*prev_cache = NULL;
@@ -5044,135 +5070,169 @@ arc_kmem_reap_now(void)
 	}
 }
 
+/* ARGSUSED */
+static boolean_t
+arc_adjust_cb_check(void *arg, zthr_t *zthr)
+{
+	/*
+	 * This is necessary in order to keep the kstat information
+	 * up to date for tools that display kstat data such as the
+	 * mdb ::arc dcmd and the Linux crash utility.  These tools
+	 * typically do not call kstat's update function, but simply
+	 * dump out stats from the most recent update.  Without
+	 * this call, these commands may show stale stats for the
+	 * anon, mru, mru_ghost, mfu, and mfu_ghost lists. Even
+	 * with this change, the data might be up to 1 second
+	 * out of date(the arc_adjust_zthr has a maximum sleep
+	 * time of 1 second); but that should suffice.  The
+	 * arc_state_t structures can be queried directly if more
+	 * accurate information is needed.
+	 */
+	if (arc_ksp != NULL)
+		arc_ksp->ks_update(arc_ksp, KSTAT_READ);
+
+	/*
+	 * We have to rely on arc_get_data_impl() to tell us when to adjust,
+	 * rather than checking if we are overflowing here, so that we are
+	 * sure to not leave arc_get_data_impl() waiting on
+	 * arc_adjust_waiters_cv.  If we have become "not overflowing" since
+	 * arc_get_data_impl() checked, we need to wake it up.  We could
+	 * broadcast the CV here, but arc_get_data_impl() may have not yet
+	 * gone to sleep.  We would need to use a mutex to ensure that this
+	 * function doesn't broadcast until arc_get_data_impl() has gone to
+	 * sleep (e.g. the arc_adjust_lock).  However, the lock ordering of
+	 * such a lock would necessarily be incorrect with respect to the
+	 * zthr_lock, which is held before this function is called, and is
+	 * held by arc_get_data_impl() when it calls zthr_wakeup().
+	 */
+	return (arc_adjust_needed);
+}
+
 /*
- * Threads can block in arc_get_data_impl() waiting for this thread to evict
- * enough data and signal them to proceed. When this happens, the threads in
- * arc_get_data_impl() are sleeping while holding the hash lock for their
- * particular arc header. Thus, we must be careful to never sleep on a
- * hash lock in this thread. This is to prevent the following deadlock:
- *
- *  - Thread A sleeps on CV in arc_get_data_impl() holding hash lock "L",
- *    waiting for the reclaim thread to signal it.
- *
- *  - arc_reclaim_thread() tries to acquire hash lock "L" using mutex_enter,
- *    fails, and goes to sleep forever.
- *
- * This possible deadlock is avoided by always acquiring a hash lock
- * using mutex_tryenter() from arc_reclaim_thread().
+ * Keep arc_size under arc_c by running arc_adjust which evicts data
+ * from the ARC.
  */
 /* ARGSUSED */
-static void
-arc_reclaim_thread(void *unused)
+static int
+arc_adjust_cb(void *arg, zthr_t *zthr)
 {
-	fstrans_cookie_t	cookie = spl_fstrans_mark();
-	hrtime_t		growtime = 0;
-	callb_cpr_t		cpr;
-
-	CALLB_CPR_INIT(&cpr, &arc_reclaim_lock, callb_generic_cpr, FTAG);
+	uint64_t evicted = 0;
+	fstrans_cookie_t cookie = spl_fstrans_mark();
 
-	mutex_enter(&arc_reclaim_lock);
-	while (!arc_reclaim_thread_exit) {
-		uint64_t evicted = 0;
-		uint64_t need_free = arc_need_free;
-		arc_tuning_update();
+	/* Evict from cache */
+	evicted = arc_adjust();
 
+	/*
+	 * If evicted is zero, we couldn't evict anything
+	 * via arc_adjust(). This could be due to hash lock
+	 * collisions, but more likely due to the majority of
+	 * arc buffers being unevictable. Therefore, even if
+	 * arc_size is above arc_c, another pass is unlikely to
+	 * be helpful and could potentially cause us to enter an
+	 * infinite loop.  Additionally, zthr_iscancelled() is
+	 * checked here so that if the arc is shutting down, the
+	 * broadcast will wake any remaining arc adjust waiters.
+	 */
+	mutex_enter(&arc_adjust_lock);
+	arc_adjust_needed = !zthr_iscancelled(arc_adjust_zthr) &&
+	    evicted > 0 && aggsum_compare(&arc_size, arc_c) > 0;
+	if (!arc_adjust_needed) {
 		/*
-		 * This is necessary in order for the mdb ::arc dcmd to
-		 * show up to date information. Since the ::arc command
-		 * does not call the kstat's update function, without
-		 * this call, the command may show stale stats for the
-		 * anon, mru, mru_ghost, mfu, and mfu_ghost lists. Even
-		 * with this change, the data might be up to 1 second
-		 * out of date; but that should suffice. The arc_state_t
-		 * structures can be queried directly if more accurate
-		 * information is needed.
+		 * We're either no longer overflowing, or we
+		 * can't evict anything more, so we should wake
+		 * arc_get_data_impl() sooner.
 		 */
-#ifndef __linux__
-		if (arc_ksp != NULL)
-			arc_ksp->ks_update(arc_ksp, KSTAT_READ);
-#endif
-		mutex_exit(&arc_reclaim_lock);
+		cv_broadcast(&arc_adjust_waiters_cv);
+		arc_need_free = 0;
+	}
+	mutex_exit(&arc_adjust_lock);
+	spl_fstrans_unmark(cookie);
+
+	return (0);
+}
 
+/* ARGSUSED */
+static boolean_t
+arc_reap_cb_check(void *arg, zthr_t *zthr)
+{
+	int64_t free_memory = arc_available_memory();
+
+	/*
+	 * If a kmem reap is already active, don't schedule more.  We must
+	 * check for this because kmem_cache_reap_soon() won't actually
+	 * block on the cache being reaped (this is to prevent callers from
+	 * becoming implicitly blocked by a system-wide kmem reap -- which,
+	 * on a system with many, many full magazines, can take minutes).
+	 */
+	if (!kmem_cache_reap_active() && free_memory < 0) {
+
+		arc_no_grow = B_TRUE;
+		arc_warm = B_TRUE;
 		/*
-		 * We call arc_adjust() before (possibly) calling
-		 * arc_kmem_reap_now(), so that we can wake up
-		 * arc_get_data_buf() sooner.
+		 * Wait at least zfs_grow_retry (default 5) seconds
+		 * before considering growing.
 		 */
-		evicted = arc_adjust();
+		arc_growtime = gethrtime() + SEC2NSEC(arc_grow_retry);
+		return (B_TRUE);
+	} else if (free_memory < arc_c >> arc_no_grow_shift) {
+		arc_no_grow = B_TRUE;
+	} else if (gethrtime() >= arc_growtime) {
+		arc_no_grow = B_FALSE;
+	}
 
-		int64_t free_memory = arc_available_memory();
-		if (free_memory < 0) {
+	return (B_FALSE);
+}
 
-			arc_no_grow = B_TRUE;
-			arc_warm = B_TRUE;
+/*
+ * Keep enough free memory in the system by reaping the ARC's kmem
+ * caches.  To cause more slabs to be reapable, we may reduce the
+ * target size of the cache (arc_c), causing the arc_adjust_cb()
+ * to free more buffers.
+ */
+/* ARGSUSED */
+static int
+arc_reap_cb(void *arg, zthr_t *zthr)
+{
+	int64_t free_memory;
+	fstrans_cookie_t cookie = spl_fstrans_mark();
 
-			/*
-			 * Wait at least zfs_grow_retry (default 5) seconds
-			 * before considering growing.
-			 */
-			growtime = gethrtime() + SEC2NSEC(arc_grow_retry);
+	/*
+	 * Kick off asynchronous kmem_reap()'s of all our caches.
+	 */
+	arc_kmem_reap_soon();
 
-			arc_kmem_reap_now();
+	/*
+	 * Wait at least arc_kmem_cache_reap_retry_ms between
+	 * arc_kmem_reap_soon() calls. Without this check it is possible to
+	 * end up in a situation where we spend lots of time reaping
+	 * caches, while we're near arc_c_min.  Waiting here also gives the
+	 * subsequent free memory check a chance of finding that the
+	 * asynchronous reap has already freed enough memory, and we don't
+	 * need to call arc_reduce_target_size().
+	 */
+	delay((hz * arc_kmem_cache_reap_retry_ms + 999) / 1000);
 
-			/*
-			 * If we are still low on memory, shrink the ARC
-			 * so that we have arc_shrink_min free space.
-			 */
-			free_memory = arc_available_memory();
+	/*
+	 * Reduce the target size as needed to maintain the amount of free
+	 * memory in the system at a fraction of the arc_size (1/128th by
+	 * default).  If oversubscribed (free_memory < 0) then reduce the
+	 * target arc_size by the deficit amount plus the fractional
+	 * amount.  If free memory is positive but less then the fractional
+	 * amount, reduce by what is needed to hit the fractional amount.
+	 */
+	free_memory = arc_available_memory();
 
-			int64_t to_free =
-			    (arc_c >> arc_shrink_shift) - free_memory;
-			if (to_free > 0) {
+	int64_t to_free =
+	    (arc_c >> arc_shrink_shift) - free_memory;
+	if (to_free > 0) {
 #ifdef _KERNEL
-				to_free = MAX(to_free, need_free);
+		to_free = MAX(to_free, arc_need_free);
 #endif
-				arc_shrink(to_free);
-			}
-		} else if (free_memory < arc_c >> arc_no_grow_shift) {
-			arc_no_grow = B_TRUE;
-		} else if (gethrtime() >= growtime) {
-			arc_no_grow = B_FALSE;
-		}
-
-		mutex_enter(&arc_reclaim_lock);
-
-		/*
-		 * If evicted is zero, we couldn't evict anything via
-		 * arc_adjust(). This could be due to hash lock
-		 * collisions, but more likely due to the majority of
-		 * arc buffers being unevictable. Therefore, even if
-		 * arc_size is above arc_c, another pass is unlikely to
-		 * be helpful and could potentially cause us to enter an
-		 * infinite loop.
-		 */
-		if (aggsum_compare(&arc_size, arc_c) <= 0|| evicted == 0) {
-			/*
-			 * We're either no longer overflowing, or we
-			 * can't evict anything more, so we should wake
-			 * up any threads before we go to sleep and remove
-			 * the bytes we were working on from arc_need_free
-			 * since nothing more will be done here.
-			 */
-			cv_broadcast(&arc_reclaim_waiters_cv);
-			ARCSTAT_INCR(arcstat_need_free, -need_free);
-
-			/*
-			 * Block until signaled, or after one second (we
-			 * might need to perform arc_kmem_reap_now()
-			 * even if we aren't being signalled)
-			 */
-			CALLB_CPR_SAFE_BEGIN(&cpr);
-			(void) cv_timedwait_sig_hires(&arc_reclaim_thread_cv,
-			    &arc_reclaim_lock, SEC2NSEC(1), MSEC2NSEC(1), 0);
-			CALLB_CPR_SAFE_END(&cpr, &arc_reclaim_lock);
-		}
+		arc_reduce_target_size(to_free);
 	}
-
-	arc_reclaim_thread_exit = B_FALSE;
-	cv_broadcast(&arc_reclaim_thread_cv);
-	CALLB_CPR_EXIT(&cpr);		/* drops arc_reclaim_lock */
 	spl_fstrans_unmark(cookie);
-	thread_exit();
+
+	return (0);
 }
 
 #ifdef _KERNEL
@@ -5276,21 +5336,21 @@ __arc_shrinker_func(struct shrinker *shrink, struct shrink_control *sc)
 		return (SHRINK_STOP);
 
 	/* Reclaim in progress */
-	if (mutex_tryenter(&arc_reclaim_lock) == 0) {
+	if (mutex_tryenter(&arc_adjust_lock) == 0) {
 		ARCSTAT_INCR(arcstat_need_free, ptob(sc->nr_to_scan));
 		return (0);
 	}
 
-	mutex_exit(&arc_reclaim_lock);
+	mutex_exit(&arc_adjust_lock);
 
 	/*
 	 * Evict the requested number of pages by shrinking arc_c the
 	 * requested amount.
 	 */
 	if (pages > 0) {
-		arc_shrink(ptob(sc->nr_to_scan));
+		arc_reduce_target_size(ptob(sc->nr_to_scan));
 		if (current_is_kswapd())
-			arc_kmem_reap_now();
+			arc_kmem_reap_soon();
 #ifdef HAVE_SPLIT_SHRINKER_CALLBACK
 		pages = MAX((int64_t)pages -
 		    (int64_t)btop(arc_evictable_memory()), 0);
@@ -5300,7 +5360,7 @@ __arc_shrinker_func(struct shrinker *shrink, struct shrink_control *sc)
 		/*
 		 * We've shrunk what we can, wake up threads.
 		 */
-		cv_broadcast(&arc_reclaim_waiters_cv);
+		cv_broadcast(&arc_adjust_waiters_cv);
 	} else
 		pages = SHRINK_STOP;
 
@@ -5315,7 +5375,7 @@ __arc_shrinker_func(struct shrinker *shrink, struct shrink_control *sc)
 		ARCSTAT_BUMP(arcstat_memory_indirect_count);
 	} else {
 		arc_no_grow = B_TRUE;
-		arc_kmem_reap_now();
+		arc_kmem_reap_soon();
 		ARCSTAT_BUMP(arcstat_memory_direct_count);
 	}
 
@@ -5369,8 +5429,11 @@ arc_adapt(int bytes, arc_state_t *state)
 	}
 	ASSERT((int64_t)arc_p >= 0);
 
+	/*
+	 * Wake reap thread if we do not have any available memory
+	 */
 	if (arc_reclaim_needed()) {
-		cv_signal(&arc_reclaim_thread_cv);
+		zthr_wakeup(arc_reap_zthr);
 		return;
 	}
 
@@ -5478,7 +5541,7 @@ arc_get_data_impl(arc_buf_hdr_t *hdr, uint64_t size, void *tag)
 	 * overflowing; thus we don't use a while loop here.
 	 */
 	if (arc_is_overflowing()) {
-		mutex_enter(&arc_reclaim_lock);
+		mutex_enter(&arc_adjust_lock);
 
 		/*
 		 * Now that we've acquired the lock, we may no longer be
@@ -5492,11 +5555,12 @@ arc_get_data_impl(arc_buf_hdr_t *hdr, uint64_t size, void *tag)
 		 * shouldn't cause any harm.
 		 */
 		if (arc_is_overflowing()) {
-			cv_signal(&arc_reclaim_thread_cv);
-			cv_wait(&arc_reclaim_waiters_cv, &arc_reclaim_lock);
+			arc_adjust_needed = B_TRUE;
+			zthr_wakeup(arc_adjust_zthr);
+			(void) cv_wait(&arc_adjust_waiters_cv,
+			    &arc_adjust_lock);
 		}
-
-		mutex_exit(&arc_reclaim_lock);
+		mutex_exit(&arc_adjust_lock);
 	}
 
 	VERIFY3U(hdr->b_type, ==, type);
@@ -7687,10 +7751,8 @@ void
 arc_init(void)
 {
 	uint64_t percent, allmem = arc_all_memory();
-
-	mutex_init(&arc_reclaim_lock, NULL, MUTEX_DEFAULT, NULL);
-	cv_init(&arc_reclaim_thread_cv, NULL, CV_DEFAULT, NULL);
-	cv_init(&arc_reclaim_waiters_cv, NULL, CV_DEFAULT, NULL);
+	mutex_init(&arc_adjust_lock, NULL, MUTEX_DEFAULT, NULL);
+	cv_init(&arc_adjust_waiters_cv, NULL, CV_DEFAULT, NULL);
 
 	arc_min_prefetch_ms = 1000;
 	arc_min_prescient_prefetch_ms = 6000;
@@ -7750,6 +7812,13 @@ arc_init(void)
 		arc_c = arc_c_min;
 
 	arc_state_init();
+
+	/*
+	 * The arc must be "uninitialized", so that hdr_recl() (which is
+	 * registered by buf_init()) will not access arc_reap_zthr before
+	 * it is created.
+	 */
+	ASSERT(!arc_initialized);
 	buf_init();
 
 	list_create(&arc_prune_list, sizeof (arc_prune_t),
@@ -7759,8 +7828,6 @@ arc_init(void)
 	arc_prune_taskq = taskq_create("arc_prune", max_ncpus, defclsyspri,
 	    max_ncpus, INT_MAX, TASKQ_PREPOPULATE | TASKQ_DYNAMIC);
 
-	arc_reclaim_thread_exit = B_FALSE;
-
 	arc_ksp = kstat_create("zfs", 0, "arcstats", "misc", KSTAT_TYPE_NAMED,
 	    sizeof (arc_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
 
@@ -7770,10 +7837,12 @@ arc_init(void)
 		kstat_install(arc_ksp);
 	}
 
-	(void) thread_create(NULL, 0, arc_reclaim_thread, NULL, 0, &p0,
-	    TS_RUN, defclsyspri);
+	arc_adjust_zthr = zthr_create(arc_adjust_cb_check,
+	    arc_adjust_cb, NULL);
+	arc_reap_zthr = zthr_create_timer(arc_reap_cb_check,
+	    arc_reap_cb, NULL, SEC2NSEC(1));
 
-	arc_dead = B_FALSE;
+	arc_initialized = B_TRUE;
 	arc_warm = B_FALSE;
 
 	/*
@@ -7805,22 +7874,10 @@ arc_fini(void)
 	spl_unregister_shrinker(&arc_shrinker);
 #endif /* _KERNEL */
 
-	mutex_enter(&arc_reclaim_lock);
-	arc_reclaim_thread_exit = B_TRUE;
-	/*
-	 * The reclaim thread will set arc_reclaim_thread_exit back to
-	 * B_FALSE when it is finished exiting; we're waiting for that.
-	 */
-	while (arc_reclaim_thread_exit) {
-		cv_signal(&arc_reclaim_thread_cv);
-		cv_wait(&arc_reclaim_thread_cv, &arc_reclaim_lock);
-	}
-	mutex_exit(&arc_reclaim_lock);
-
 	/* Use B_TRUE to ensure *all* buffers are evicted */
 	arc_flush(NULL, B_TRUE);
 
-	arc_dead = B_TRUE;
+	arc_initialized = B_FALSE;
 
 	if (arc_ksp != NULL) {
 		kstat_delete(arc_ksp);
@@ -7841,9 +7898,14 @@ arc_fini(void)
 
 	list_destroy(&arc_prune_list);
 	mutex_destroy(&arc_prune_mtx);
-	mutex_destroy(&arc_reclaim_lock);
-	cv_destroy(&arc_reclaim_thread_cv);
-	cv_destroy(&arc_reclaim_waiters_cv);
+	(void) zthr_cancel(arc_adjust_zthr);
+	zthr_destroy(arc_adjust_zthr);
+
+	(void) zthr_cancel(arc_reap_zthr);
+	zthr_destroy(arc_reap_zthr);
+
+	mutex_destroy(&arc_adjust_lock);
+	cv_destroy(&arc_adjust_waiters_cv);
 
 	/*
 	 * buf_fini() must proceed arc_state_fini() because buf_fin() may
diff --git a/module/zfs/zthr.c b/module/zfs/zthr.c
index 1c4a8e02c..c5b11dafc 100644
--- a/module/zfs/zthr.c
+++ b/module/zfs/zthr.c
@@ -47,6 +47,10 @@
  * 3] When the zthr is done, it changes the indicator to stopped, allowing
  *    a new cycle to start.
  *
+ * Besides being awakened by other threads, a zthr can be configured
+ * during creation to wakeup on its own after a specified interval
+ * [see zthr_create_timer()].
+ *
  * == ZTHR creation
  *
  * Every zthr needs three inputs to start running:
@@ -74,6 +78,9 @@
  *
  * To start a zthr:
  *     zthr_t *zthr_pointer = zthr_create(checkfunc, func, args);
+ * or
+ *     zthr_t *zthr_pointer = zthr_create_timer(checkfunc, func,
+ *         args, max_sleep);
  *
  * After that you should be able to wakeup, cancel, and resume the
  * zthr from another thread using zthr_pointer.
@@ -189,7 +196,13 @@ zthr_procedure(void *arg)
 			mutex_enter(&t->zthr_lock);
 		} else {
 			/* go to sleep */
-			cv_wait_sig(&t->zthr_cv, &t->zthr_lock);
+			if (t->zthr_wait_time == 0) {
+				cv_wait_sig(&t->zthr_cv, &t->zthr_lock);
+			} else {
+				(void) cv_timedwait_sig_hires(&t->zthr_cv,
+				    &t->zthr_lock, t->zthr_wait_time,
+				    MSEC2NSEC(1), 0);
+			}
 		}
 	}
 	mutex_exit(&t->zthr_lock);
@@ -200,6 +213,18 @@ zthr_procedure(void *arg)
 zthr_t *
 zthr_create(zthr_checkfunc_t *checkfunc, zthr_func_t *func, void *arg)
 {
+	return (zthr_create_timer(checkfunc, func, arg, (hrtime_t)0));
+}
+
+/*
+ * Create a zthr with specified maximum sleep time.  If the time
+ * in sleeping state exceeds max_sleep, a wakeup(do the check and
+ * start working if required) will be triggered.
+ */
+zthr_t *
+zthr_create_timer(zthr_checkfunc_t *checkfunc, zthr_func_t *func,
+    void *arg, hrtime_t max_sleep)
+{
 	zthr_t *t = kmem_zalloc(sizeof (*t), KM_SLEEP);
 	mutex_init(&t->zthr_lock, NULL, MUTEX_DEFAULT, NULL);
 	cv_init(&t->zthr_cv, NULL, CV_DEFAULT, NULL);
@@ -208,6 +233,7 @@ zthr_create(zthr_checkfunc_t *checkfunc, zthr_func_t *func, void *arg)
 	t->zthr_checkfunc = checkfunc;
 	t->zthr_func = func;
 	t->zthr_arg = arg;
+	t->zthr_wait_time = max_sleep;
 
 	t->zthr_thread = thread_create(NULL, 0, zthr_procedure, t,
 	    0, &p0, TS_RUN, minclsyspri);