OpenZFS 8558, 8602 - lwp_create() returns EAGAIN

8558 lwp_create() returns EAGAIN on system with more than 80K ZFS filesystems

On a system with more than 80K ZFS filesystems, we've seen cases
where lwp_create() will start to fail by returning EAGAIN. The
problem being, for each of those 80K ZFS filesystems, a taskq will
be created for each dataset as part of the ZIL for each dataset.

Porting Notes:
- The new nomem taskq kstat was dropped.
- Added module options and documentation for new tunings
  zfs_zil_clean_taskq_nthr_pct, zfs_zil_clean_taskq_minalloc,
  zfs_zil_clean_taskq_maxalloc, and zfs_sync_taskq_batch_pct.

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Authored by: Prakash Surya <prakash.surya@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/8558
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/216d772

8602 remove unused "dp_early_sync_tasks" field from "dsl_pool" structure

Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Authored by: Prakash Surya <prakash.surya@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/8602
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/2bcb545
Closes #6779

include/sys/dsl_pool.h

@@ -127,6 +127,7 @@ typedef struct dsl_pool {
 	txg_list_t dp_dirty_dirs;
 	txg_list_t dp_sync_tasks;
 	taskq_t *dp_sync_taskq;
+	taskq_t *dp_zil_clean_taskq;
 
 	/*
 	 * Protects administrative changes (properties, namespace)

include/sys/zil_impl.h

@@ -124,7 +124,6 @@ struct zilog {
 	list_t		zl_lwb_list;	/* in-flight log write list */
 	kmutex_t	zl_vdev_lock;	/* protects zl_vdev_tree */
 	avl_tree_t	zl_vdev_tree;	/* vdevs to flush in zil_commit() */
-	taskq_t		*zl_clean_taskq; /* runs lwb and itx clean tasks */
 	avl_tree_t	zl_bp_tree;	/* track bps during log parse */
 	clock_t		zl_replay_time;	/* lbolt of when replay started */
 	uint64_t	zl_replay_blks;	/* number of log blocks replayed */

man/man5/zfs-module-parameters.5

@@ -1745,6 +1745,18 @@ Rewrite new block pointers starting in this pass
 Default value: \fB2\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_sync_taskq_batch_pct\fR (int)
+.ad
+.RS 12n
+This controls the number of threads used by the dp_sync_taskq.  The default
+value of 75% will create a maximum of one thread per cpu.
+.sp
+Default value: \fB75\fR.
+.RE
+
 .sp
 .ne 2
 .na
@@ -1999,6 +2011,42 @@ in the queue can be viewed with the \fBzpool events\fR command.
 Default value: \fB0\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_zil_clean_taskq_maxalloc\fR (int)
+.ad
+.RS 12n
+The maximum number of taskq entries that are allowed to be cached.  When this
+limit is exceeded itx's will be cleaned synchronously.
+.sp
+Default value: \fB1048576\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_zil_clean_taskq_minalloc\fR (int)
+.ad
+.RS 12n
+The number of taskq entries that are pre-populated when the taskq is first
+created and are immediately available for use.
+.sp
+Default value: \fB1024\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_zil_clean_taskq_nthr_pct\fR (int)
+.ad
+.RS 12n
+This controls the number of threads used by the dp_zil_clean_taskq.  The default
+value of 100% will create a maximum of one thread per cpu.
+.sp
+Default value: \fB100\fR.
+.RE
+
 .sp
 .ne 2
 .na

module/zfs/dsl_pool.c

@@ -135,6 +135,36 @@ unsigned long zfs_delay_scale = 1000 * 1000 * 1000 / 2000;
  */
 int zfs_sync_taskq_batch_pct = 75;
 
+/*
+ * These tunables determine the behavior of how zil_itxg_clean() is
+ * called via zil_clean() in the context of spa_sync(). When an itxg
+ * list needs to be cleaned, TQ_NOSLEEP will be used when dispatching.
+ * If the dispatch fails, the call to zil_itxg_clean() will occur
+ * synchronously in the context of spa_sync(), which can negatively
+ * impact the performance of spa_sync() (e.g. in the case of the itxg
+ * list having a large number of itxs that needs to be cleaned).
+ *
+ * Thus, these tunables can be used to manipulate the behavior of the
+ * taskq used by zil_clean(); they determine the number of taskq entries
+ * that are pre-populated when the taskq is first created (via the
+ * "zfs_zil_clean_taskq_minalloc" tunable) and the maximum number of
+ * taskq entries that are cached after an on-demand allocation (via the
+ * "zfs_zil_clean_taskq_maxalloc").
+ *
+ * The idea being, we want to try reasonably hard to ensure there will
+ * already be a taskq entry pre-allocated by the time that it is needed
+ * by zil_clean(). This way, we can avoid the possibility of an
+ * on-demand allocation of a new taskq entry from failing, which would
+ * result in zil_itxg_clean() being called synchronously from zil_clean()
+ * (which can adversely affect performance of spa_sync()).
+ *
+ * Additionally, the number of threads used by the taskq can be
+ * configured via the "zfs_zil_clean_taskq_nthr_pct" tunable.
+ */
+int zfs_zil_clean_taskq_nthr_pct = 100;
+int zfs_zil_clean_taskq_minalloc = 1024;
+int zfs_zil_clean_taskq_maxalloc = 1024 * 1024;
+
 int
 dsl_pool_open_special_dir(dsl_pool_t *dp, const char *name, dsl_dir_t **ddp)
 {
@@ -176,6 +206,12 @@ dsl_pool_open_impl(spa_t *spa, uint64_t txg)
 	    zfs_sync_taskq_batch_pct, minclsyspri, 1, INT_MAX,
 	    TASKQ_THREADS_CPU_PCT);
 
+	dp->dp_zil_clean_taskq = taskq_create("dp_zil_clean_taskq",
+	    zfs_zil_clean_taskq_nthr_pct, minclsyspri,
+	    zfs_zil_clean_taskq_minalloc,
+	    zfs_zil_clean_taskq_maxalloc,
+	    TASKQ_PREPOPULATE | TASKQ_THREADS_CPU_PCT);
+
 	mutex_init(&dp->dp_lock, NULL, MUTEX_DEFAULT, NULL);
 	cv_init(&dp->dp_spaceavail_cv, NULL, CV_DEFAULT, NULL);
 
@@ -334,6 +370,7 @@ dsl_pool_close(dsl_pool_t *dp)
 	txg_list_destroy(&dp->dp_sync_tasks);
 	txg_list_destroy(&dp->dp_dirty_dirs);
 
+	taskq_destroy(dp->dp_zil_clean_taskq);
 	taskq_destroy(dp->dp_sync_taskq);
 
 	/*
@@ -1155,5 +1192,18 @@ MODULE_PARM_DESC(zfs_delay_scale, "how quickly delay approaches infinity");
 module_param(zfs_sync_taskq_batch_pct, int, 0644);
 MODULE_PARM_DESC(zfs_sync_taskq_batch_pct,
 	"max percent of CPUs that are used to sync dirty data");
+
+module_param(zfs_zil_clean_taskq_nthr_pct, int, 0644);
+MODULE_PARM_DESC(zfs_zil_clean_taskq_nthr_pct,
+	"max percent of CPUs that are used per dp_sync_taskq");
+
+module_param(zfs_zil_clean_taskq_minalloc, int, 0644);
+MODULE_PARM_DESC(zfs_zil_clean_taskq_minalloc,
+	"number of taskq entries that are pre-populated");
+
+module_param(zfs_zil_clean_taskq_maxalloc, int, 0644);
+MODULE_PARM_DESC(zfs_zil_clean_taskq_maxalloc,
+	"max number of taskq entries that are cached");
+
 /* END CSTYLED */
 #endif

module/zfs/zil.c

@@ -1473,8 +1473,7 @@ zil_clean(zilog_t *zilog, uint64_t synced_txg)
 		return;
 	}
 	ASSERT3U(itxg->itxg_txg, <=, synced_txg);
-	ASSERT(itxg->itxg_txg != 0);
-	ASSERT(zilog->zl_clean_taskq != NULL);
+	ASSERT3U(itxg->itxg_txg, !=, 0);
 	clean_me = itxg->itxg_itxs;
 	itxg->itxg_itxs = NULL;
 	itxg->itxg_txg = 0;
@@ -1485,8 +1484,11 @@ zil_clean(zilog_t *zilog, uint64_t synced_txg)
 	 * free it in-line. This should be rare. Note, using TQ_SLEEP
 	 * created a bad performance problem.
 	 */
-	if (taskq_dispatch(zilog->zl_clean_taskq,
-	    (void (*)(void *))zil_itxg_clean, clean_me, TQ_NOSLEEP) == 0)
+	ASSERT3P(zilog->zl_dmu_pool, !=, NULL);
+	ASSERT3P(zilog->zl_dmu_pool->dp_zil_clean_taskq, !=, NULL);
+	taskqid_t id = taskq_dispatch(zilog->zl_dmu_pool->dp_zil_clean_taskq,
+	    (void (*)(void *))zil_itxg_clean, clean_me, TQ_NOSLEEP);
+	if (id == TASKQID_INVALID)
 		zil_itxg_clean(clean_me);
 }
 
@@ -1959,13 +1961,10 @@ zil_open(objset_t *os, zil_get_data_t *get_data)
 {
 	zilog_t *zilog = dmu_objset_zil(os);
 
-	ASSERT(zilog->zl_clean_taskq == NULL);
 	ASSERT(zilog->zl_get_data == NULL);
 	ASSERT(list_is_empty(&zilog->zl_lwb_list));
 
 	zilog->zl_get_data = get_data;
-	zilog->zl_clean_taskq = taskq_create("zil_clean", 1, defclsyspri,
-	    2, 2, TASKQ_PREPOPULATE);
 
 	return (zilog);
 }
@@ -2000,8 +1999,6 @@ zil_close(zilog_t *zilog)
 	if (txg < spa_freeze_txg(zilog->zl_spa))
 		VERIFY(!zilog_is_dirty(zilog));
 
-	taskq_destroy(zilog->zl_clean_taskq);
-	zilog->zl_clean_taskq = NULL;
 	zilog->zl_get_data = NULL;
 
 	/*