jbd2: track request delay statistics

Track the delay between when we first request that the commit begin
and when it actually begins, so we can see how much of a gap exists.
In theory, this should just be the remaining scheduling quantuum of
the thread which requested the commit (assuming it was not a
synchronous operation which triggered the commit request) plus
scheduling overhead; however, it's possible that real time processes
might get in the way of letting the kjournald thread from executing.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/jbd2/commit.c

@@ -435,7 +435,12 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 
 	trace_jbd2_commit_locking(journal, commit_transaction);
 	stats.run.rs_wait = commit_transaction->t_max_wait;
+	stats.run.rs_request_delay = 0;
 	stats.run.rs_locked = jiffies;
+	if (commit_transaction->t_requested)
+		stats.run.rs_request_delay =
+			jbd2_time_diff(commit_transaction->t_requested,
+				       stats.run.rs_locked);
 	stats.run.rs_running = jbd2_time_diff(commit_transaction->t_start,
 					      stats.run.rs_locked);
 
@@ -1116,7 +1121,10 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 	 */
 	spin_lock(&journal->j_history_lock);
 	journal->j_stats.ts_tid++;
+	if (commit_transaction->t_requested)
+		journal->j_stats.ts_requested++;
 	journal->j_stats.run.rs_wait += stats.run.rs_wait;
+	journal->j_stats.run.rs_request_delay += stats.run.rs_request_delay;
 	journal->j_stats.run.rs_running += stats.run.rs_running;
 	journal->j_stats.run.rs_locked += stats.run.rs_locked;
 	journal->j_stats.run.rs_flushing += stats.run.rs_flushing;

fs/jbd2/journal.c

@@ -533,6 +533,7 @@ int __jbd2_log_start_commit(journal_t *journal, tid_t target)
 		jbd_debug(1, "JBD2: requesting commit %d/%d\n",
 			  journal->j_commit_request,
 			  journal->j_commit_sequence);
+		journal->j_running_transaction->t_requested = jiffies;
 		wake_up(&journal->j_wait_commit);
 		return 1;
 	} else if (!tid_geq(journal->j_commit_request, target))
@@ -898,13 +899,18 @@ static int jbd2_seq_info_show(struct seq_file *seq, void *v)
 
 	if (v != SEQ_START_TOKEN)
 		return 0;
-	seq_printf(seq, "%lu transaction, each up to %u blocks\n",
-			s->stats->ts_tid,
+	seq_printf(seq, "%lu transactions (%lu requested), "
+		   "each up to %u blocks\n",
+		   s->stats->ts_tid, s->stats->ts_requested,
 		   s->journal->j_max_transaction_buffers);
 	if (s->stats->ts_tid == 0)
 		return 0;
 	seq_printf(seq, "average: \n  %ums waiting for transaction\n",
 	    jiffies_to_msecs(s->stats->run.rs_wait / s->stats->ts_tid));
+	seq_printf(seq, "  %ums request delay\n",
+	    (s->stats->ts_requested == 0) ? 0 :
+	    jiffies_to_msecs(s->stats->run.rs_request_delay /
+			     s->stats->ts_requested));
 	seq_printf(seq, "  %ums running transaction\n",
 	    jiffies_to_msecs(s->stats->run.rs_running / s->stats->ts_tid));
 	seq_printf(seq, "  %ums transaction was being locked\n",

fs/jbd2/transaction.c

@@ -100,6 +100,7 @@ jbd2_get_transaction(journal_t *journal, transaction_t *transaction)
 	journal->j_running_transaction = transaction;
 	transaction->t_max_wait = 0;
 	transaction->t_start = jiffies;
+	transaction->t_requested = 0;
 
 	return transaction;
 }

include/linux/jbd2.h

@@ -580,6 +580,11 @@ struct transaction_s
 	 */
 	unsigned long		t_start;
 
+	/*
+	 * When commit was requested
+	 */
+	unsigned long		t_requested;
+
 	/*
 	 * Checkpointing stats [j_checkpoint_sem]
 	 */
@@ -637,6 +642,7 @@ struct transaction_s
 
 struct transaction_run_stats_s {
 	unsigned long		rs_wait;
+	unsigned long		rs_request_delay;
 	unsigned long		rs_running;
 	unsigned long		rs_locked;
 	unsigned long		rs_flushing;
@@ -649,6 +655,7 @@ struct transaction_run_stats_s {
 
 struct transaction_stats_s {
 	unsigned long		ts_tid;
+	unsigned long		ts_requested;
 	struct transaction_run_stats_s run;
 };
 

include/trace/events/jbd2.h

@@ -142,6 +142,7 @@ TRACE_EVENT(jbd2_run_stats,
 		__field(		dev_t,	dev		)
 		__field(	unsigned long,	tid		)
 		__field(	unsigned long,	wait		)
+		__field(	unsigned long,	request_delay	)
 		__field(	unsigned long,	running		)
 		__field(	unsigned long,	locked		)
 		__field(	unsigned long,	flushing	)
@@ -155,6 +156,7 @@ TRACE_EVENT(jbd2_run_stats,
 		__entry->dev		= dev;
 		__entry->tid		= tid;
 		__entry->wait		= stats->rs_wait;
+		__entry->request_delay	= stats->rs_request_delay;
 		__entry->running	= stats->rs_running;
 		__entry->locked		= stats->rs_locked;
 		__entry->flushing	= stats->rs_flushing;
@@ -164,10 +166,12 @@ TRACE_EVENT(jbd2_run_stats,
 		__entry->blocks_logged	= stats->rs_blocks_logged;
 	),
 
-	TP_printk("dev %d,%d tid %lu wait %u running %u locked %u flushing %u "
-		  "logging %u handle_count %u blocks %u blocks_logged %u",
+	TP_printk("dev %d,%d tid %lu wait %u request_delay %u running %u "
+		  "locked %u flushing %u logging %u handle_count %u "
+		  "blocks %u blocks_logged %u",
 		  MAJOR(__entry->dev), MINOR(__entry->dev), __entry->tid,
 		  jiffies_to_msecs(__entry->wait),
+		  jiffies_to_msecs(__entry->request_delay),
 		  jiffies_to_msecs(__entry->running),
 		  jiffies_to_msecs(__entry->locked),
 		  jiffies_to_msecs(__entry->flushing),