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>

1 files changed, 9 insertions, 3 deletions

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 1a80e3146a59..4ba2e81e35ac 100644
--- a/fs/jbd2/journal.c
+++ b/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,
-                        s->journal->j_max_transaction_buffers);
+        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",