Refactor txg history kstat

It was observed that even when the txg history is disabled by
setting `zfs_txg_history=0` the txg_sync thread still fetches
the vdev stats unnecessarily.

This patch refactors the code such that vdev_get_stats() is no
longer called when `zfs_txg_history=0`.  And it further reduces
the  differences between upstream and the ZoL txg_sync_thread()
function.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5412

include/sys/spa.h

@@ -725,6 +725,13 @@ typedef enum txg_state {
 	TXG_STATE_COMMITTED	= 5,
 } txg_state_t;
 
+typedef struct txg_stat {
+	vdev_stat_t		vs1;
+	vdev_stat_t		vs2;
+	uint64_t		txg;
+	uint64_t		ndirty;
+} txg_stat_t;
+
 extern void spa_stats_init(spa_t *spa);
 extern void spa_stats_destroy(spa_t *spa);
 extern void spa_read_history_add(spa_t *spa, const zbookmark_phys_t *zb,
@@ -732,8 +739,9 @@ extern void spa_read_history_add(spa_t *spa, const zbookmark_phys_t *zb,
 extern void spa_txg_history_add(spa_t *spa, uint64_t txg, hrtime_t birth_time);
 extern int spa_txg_history_set(spa_t *spa,  uint64_t txg,
     txg_state_t completed_state, hrtime_t completed_time);
-extern int spa_txg_history_set_io(spa_t *spa,  uint64_t txg, uint64_t nread,
+extern txg_stat_t *spa_txg_history_init_io(spa_t *, uint64_t,
-    uint64_t nwritten, uint64_t reads, uint64_t writes, uint64_t ndirty);
+    struct dsl_pool *);
+extern void spa_txg_history_fini_io(spa_t *, txg_stat_t *);
 extern void spa_tx_assign_add_nsecs(spa_t *spa, uint64_t nsecs);
 
 /* Pool configuration locks */

module/zfs/spa_stats.c

@@ -474,7 +474,7 @@ spa_txg_history_set(spa_t *spa, uint64_t txg, txg_state_t completed_state,
 /*
  * Set txg IO stats.
  */
-int
+static int
 spa_txg_history_set_io(spa_t *spa, uint64_t txg, uint64_t nread,
     uint64_t nwritten, uint64_t reads, uint64_t writes, uint64_t ndirty)
 {
@@ -503,6 +503,54 @@ spa_txg_history_set_io(spa_t *spa, uint64_t txg, uint64_t nread,
 	return (error);
 }
 
+txg_stat_t *
+spa_txg_history_init_io(spa_t *spa, uint64_t txg, dsl_pool_t *dp)
+{
+	txg_stat_t *ts;
+
+	if (zfs_txg_history == 0)
+		return (NULL);
+
+	ts = kmem_alloc(sizeof (txg_stat_t), KM_SLEEP);
+
+	spa_config_enter(spa, SCL_ALL, FTAG, RW_READER);
+	vdev_get_stats(spa->spa_root_vdev, &ts->vs1);
+	spa_config_exit(spa, SCL_ALL, FTAG);
+
+	ts->txg = txg;
+	ts->ndirty = dp->dp_dirty_pertxg[txg & TXG_MASK];
+
+	spa_txg_history_set(spa, txg, TXG_STATE_WAIT_FOR_SYNC, gethrtime());
+
+	return (ts);
+}
+
+void
+spa_txg_history_fini_io(spa_t *spa, txg_stat_t *ts)
+{
+	if (ts == NULL)
+		return;
+
+	if (zfs_txg_history == 0) {
+		kmem_free(ts, sizeof (txg_stat_t));
+		return;
+	}
+
+	spa_config_enter(spa, SCL_ALL, FTAG, RW_READER);
+	vdev_get_stats(spa->spa_root_vdev, &ts->vs2);
+	spa_config_exit(spa, SCL_ALL, FTAG);
+
+	spa_txg_history_set(spa, ts->txg, TXG_STATE_SYNCED, gethrtime());
+	spa_txg_history_set_io(spa, ts->txg,
+	    ts->vs2.vs_bytes[ZIO_TYPE_READ] - ts->vs1.vs_bytes[ZIO_TYPE_READ],
+	    ts->vs2.vs_bytes[ZIO_TYPE_WRITE] - ts->vs1.vs_bytes[ZIO_TYPE_WRITE],
+	    ts->vs2.vs_ops[ZIO_TYPE_READ] - ts->vs1.vs_ops[ZIO_TYPE_READ],
+	    ts->vs2.vs_ops[ZIO_TYPE_WRITE] - ts->vs1.vs_ops[ZIO_TYPE_WRITE],
+	    ts->ndirty);
+
+	kmem_free(ts, sizeof (txg_stat_t));
+}
+
 /*
  * ==========================================================================
  * SPA TX Assign Histogram Routines

module/zfs/txg.c

@@ -481,22 +481,17 @@ txg_sync_thread(dsl_pool_t *dp)
 	spa_t *spa = dp->dp_spa;
 	tx_state_t *tx = &dp->dp_tx;
 	callb_cpr_t cpr;
-	vdev_stat_t *vs1, *vs2;
 	clock_t start, delta;
 
 	(void) spl_fstrans_mark();
 	txg_thread_enter(tx, &cpr);
 
-	vs1 = kmem_alloc(sizeof (vdev_stat_t), KM_SLEEP);
-	vs2 = kmem_alloc(sizeof (vdev_stat_t), KM_SLEEP);
-
 	start = delta = 0;
 	for (;;) {
-		clock_t timer, timeout;
+		clock_t timeout = zfs_txg_timeout * hz;
+		clock_t timer;
 		uint64_t txg;
-		uint64_t ndirty;
+		txg_stat_t *ts;
-
-		timeout = zfs_txg_timeout * hz;
 
 		/*
 		 * We sync when we're scanning, there's someone waiting
@@ -527,15 +522,8 @@ txg_sync_thread(dsl_pool_t *dp)
 			txg_thread_wait(tx, &cpr, &tx->tx_quiesce_done_cv, 0);
 		}
 
-		if (tx->tx_exiting) {
+		if (tx->tx_exiting)
-			kmem_free(vs2, sizeof (vdev_stat_t));
-			kmem_free(vs1, sizeof (vdev_stat_t));
 			txg_thread_exit(tx, &cpr, &tx->tx_sync_thread);
-		}
-
-		spa_config_enter(spa, SCL_ALL, FTAG, RW_READER);
-		vdev_get_stats(spa->spa_root_vdev, vs1);
-		spa_config_exit(spa, SCL_ALL, FTAG);
 
 		/*
 		 * Consume the quiesced txg which has been handed off to
@@ -546,16 +534,13 @@ txg_sync_thread(dsl_pool_t *dp)
 		tx->tx_quiesced_txg = 0;
 		tx->tx_syncing_txg = txg;
 		DTRACE_PROBE2(txg__syncing, dsl_pool_t *, dp, uint64_t, txg);
+		ts = spa_txg_history_init_io(spa, txg, dp);
 		cv_broadcast(&tx->tx_quiesce_more_cv);
 
 		dprintf("txg=%llu quiesce_txg=%llu sync_txg=%llu\n",
 		    txg, tx->tx_quiesce_txg_waiting, tx->tx_sync_txg_waiting);
 		mutex_exit(&tx->tx_sync_lock);
 
-		spa_txg_history_set(spa, txg, TXG_STATE_WAIT_FOR_SYNC,
-		    gethrtime());
-		ndirty = dp->dp_dirty_pertxg[txg & TXG_MASK];
-
 		start = ddi_get_lbolt();
 		spa_sync(spa, txg);
 		delta = ddi_get_lbolt() - start;
@@ -564,23 +549,13 @@ txg_sync_thread(dsl_pool_t *dp)
 		tx->tx_synced_txg = txg;
 		tx->tx_syncing_txg = 0;
 		DTRACE_PROBE2(txg__synced, dsl_pool_t *, dp, uint64_t, txg);
+		spa_txg_history_fini_io(spa, ts);
 		cv_broadcast(&tx->tx_sync_done_cv);
 
 		/*
 		 * Dispatch commit callbacks to worker threads.
 		 */
 		txg_dispatch_callbacks(dp, txg);
-
-		spa_config_enter(spa, SCL_ALL, FTAG, RW_READER);
-		vdev_get_stats(spa->spa_root_vdev, vs2);
-		spa_config_exit(spa, SCL_ALL, FTAG);
-		spa_txg_history_set_io(spa, txg,
-		    vs2->vs_bytes[ZIO_TYPE_READ]-vs1->vs_bytes[ZIO_TYPE_READ],
-		    vs2->vs_bytes[ZIO_TYPE_WRITE]-vs1->vs_bytes[ZIO_TYPE_WRITE],
-		    vs2->vs_ops[ZIO_TYPE_READ]-vs1->vs_ops[ZIO_TYPE_READ],
-		    vs2->vs_ops[ZIO_TYPE_WRITE]-vs1->vs_ops[ZIO_TYPE_WRITE],
-		    ndirty);
-		spa_txg_history_set(spa, txg, TXG_STATE_SYNCED, gethrtime());
 	}
 }