Fix txg_wait_open() load average inflation

Callers of txg_wait_open() which set should_quiesce=B_TRUE should be
accounted for as iowait time.  Otherwise, the caller is understood
to be idle and cv_wait_sig() is used to prevent incorrectly inflating
the system load average.

Similarly txg_wait_wait() has been updated to use cv_wait_io() to
be accounted against iowait.

Reviewed-by: Tim Chase <[email protected]>
Reviewed-by: Olaf Faaland <[email protected]>
Reviewed-by: Matt Ahrens <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #8550
Closes #8558 

2 files changed, 25 insertions, 5 deletions

diff --git a/module/zfs/txg.c b/module/zfs/txg.c
index b3f895302..0fcd569e3 100644
--- a/module/zfs/txg.c
+++ b/module/zfs/txg.c
@@ -242,11 +242,17 @@ txg_thread_wait(tx_state_t *tx, callb_cpr_t *cpr, kcondvar_t *cv, clock_t time)
 {
 	CALLB_CPR_SAFE_BEGIN(cpr);
 
-	if (time)
+	/*
+	 * cv_wait_sig() is used instead of cv_wait() in order to prevent
+	 * this process from incorrectly contributing to the system load
+	 * average when idle.
+	 */
+	if (time) {
 		(void) cv_timedwait_sig(cv, &tx->tx_sync_lock,
 		    ddi_get_lbolt() + time);
-	else
+	} else {
 		cv_wait_sig(cv, &tx->tx_sync_lock);
+	}
 
 	CALLB_CPR_SAFE_END(cpr, &tx->tx_sync_lock);
 }
@@ -689,7 +695,7 @@ txg_wait_synced(dsl_pool_t *dp, uint64_t txg)
 		    "tx_synced=%llu waiting=%llu dp=%p\n",
 		    tx->tx_synced_txg, tx->tx_sync_txg_waiting, dp);
 		cv_broadcast(&tx->tx_sync_more_cv);
-		cv_wait(&tx->tx_sync_done_cv, &tx->tx_sync_lock);
+		cv_wait_io(&tx->tx_sync_done_cv, &tx->tx_sync_lock);
 	}
 	mutex_exit(&tx->tx_sync_lock);
 }
@@ -715,7 +721,17 @@ txg_wait_open(dsl_pool_t *dp, uint64_t txg, boolean_t should_quiesce)
 	    txg, tx->tx_quiesce_txg_waiting, tx->tx_sync_txg_waiting);
 	while (tx->tx_open_txg < txg) {
 		cv_broadcast(&tx->tx_quiesce_more_cv);
-		cv_wait(&tx->tx_quiesce_done_cv, &tx->tx_sync_lock);
+		/*
+		 * Callers setting should_quiesce will use cv_wait_io() and
+		 * be accounted for as iowait time.  Otherwise, the caller is
+		 * understood to be idle and cv_wait_sig() is used to prevent
+		 * incorrectly inflating the system load average.
+		 */
+		if (should_quiesce == B_TRUE) {
+			cv_wait_io(&tx->tx_quiesce_done_cv, &tx->tx_sync_lock);
+		} else {
+			cv_wait_sig(&tx->tx_quiesce_done_cv, &tx->tx_sync_lock);
+		}
 	}
 	mutex_exit(&tx->tx_sync_lock);
 }
diff --git a/module/zfs/zthr.c b/module/zfs/zthr.c
index dd8505caa..532e8ce0f 100644
--- a/module/zfs/zthr.c
+++ b/module/zfs/zthr.c
@@ -234,7 +234,11 @@ zthr_procedure(void *arg)
 			t->zthr_func(t->zthr_arg, t);
 			mutex_enter(&t->zthr_state_lock);
 		} else {
-			/* go to sleep */
+			/*
+			 * cv_wait_sig() is used instead of cv_wait() in
+			 * order to prevent this process from incorrectly
+			 * contributing to the system load average when idle.
+			 */
 			if (t->zthr_wait_time == 0) {
 				cv_wait_sig(&t->zthr_cv, &t->zthr_state_lock);
 			} else {