Call gethrtime() only once per new txg creation

When transitioning current open TXG into QUIESCE state and opening
a new one txg_quiesce() calls gethrtime():
  - to mark the birth time of the new TXG
  - to record the SPA txg history kstat
  - implicitely inside spa_txg_history_add()

These timestamps are practically the same, so that the first one
can be used instead of the other two.  The only visible difference
is that inside spa_txg_history_add() the time spent in kmem_zalloc()
will be counted towards the opened TXG.

Since at this point the new TXG already exists (tx->tx_open_txg
has been already incremented) it is actually a correct accounting.

In any case this extra work is only happening when spa_txg_history
kstat is activated (i.e. zfs_txg_history > 0) and doesn't affect
the normal processing in any way.

Signed-off-by: Cyril Plisko <cyril.plisko@mountall.com>
Issue #2075

include/sys/spa.h

@@ -566,7 +566,7 @@ 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_t *zb,
     uint32_t aflags);
-extern void spa_txg_history_add(spa_t *spa, uint64_t txg);
+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,

module/zfs/spa_stats.c

@@ -422,7 +422,7 @@ spa_txg_history_destroy(spa_t *spa)
  * Add a new txg to historical record.
  */
 void
-spa_txg_history_add(spa_t *spa, uint64_t txg)
+spa_txg_history_add(spa_t *spa, uint64_t txg, hrtime_t birth_time)
 {
 	spa_stats_history_t *ssh = &spa->spa_stats.txg_history;
 	spa_txg_history_t *sth, *rm;
@@ -433,7 +433,7 @@ spa_txg_history_add(spa_t *spa, uint64_t txg)
 	sth = kmem_zalloc(sizeof (spa_txg_history_t), KM_PUSHPAGE);
 	sth->txg = txg;
 	sth->state = TXG_STATE_OPEN;
-	sth->times[TXG_STATE_BIRTH] = gethrtime();
+	sth->times[TXG_STATE_BIRTH] = birth_time;
 
 	mutex_enter(&ssh->lock);
 

module/zfs/txg.c

@@ -377,8 +377,8 @@ txg_quiesce(dsl_pool_t *dp, uint64_t txg)
 	tx->tx_open_txg++;
 	tx->tx_open_time = gethrtime();
 
-	spa_txg_history_set(dp->dp_spa, txg, TXG_STATE_OPEN, gethrtime());
-	spa_txg_history_add(dp->dp_spa, tx->tx_open_txg);
+	spa_txg_history_set(dp->dp_spa, txg, TXG_STATE_OPEN, tx->tx_open_time);
+	spa_txg_history_add(dp->dp_spa, tx->tx_open_txg, tx->tx_open_time);
 
 	DTRACE_PROBE2(txg__quiescing, dsl_pool_t *, dp, uint64_t, txg);
 	DTRACE_PROBE2(txg__opened, dsl_pool_t *, dp, uint64_t, tx->tx_open_txg);