[PATCH v2] jbd2: speed up jbd2_transaction_committed()

[Zhang Yi]

From: Zhang Yi <yi.zhang@xxxxxxxxxx>

jbd2_transaction_committed() is used to check whether a transaction with
the given tid has already committed, it holds j_state_lock in read mode
and check the tid of current running transaction and committing
transaction, but holding the j_state_lock is expensive.

We have already stored the sequence number of the most recently
committed transaction in journal t->j_commit_sequence, we could do this
check by comparing it with the given tid instead. If the given tid isn't
smaller than j_commit_sequence, we can ensure that the given transaction
has been committed. That way we could drop the expensive lock and
achieve about 10% ~ 20% performance gains in concurrent DIOs on may
virtual machine with 100G ramdisk.

fio -filename=/mnt/foo -direct=1 -iodepth=10 -rw=$rw -ioengine=libaio \
    -bs=4k -size=10G -numjobs=10 -runtime=60 -overwrite=1 -name=test \
    -group_reporting

Before:
  overwrite       IOPS=88.2k, BW=344MiB/s
  read            IOPS=95.7k, BW=374MiB/s
  rand overwrite  IOPS=98.7k, BW=386MiB/s
  randread        IOPS=102k, BW=397MiB/s

After:
  overwrite       IOPS=105k, BW=410MiB/s
  read            IOPS=112k, BW=436MiB/s
  rand overwrite  IOPS=104k, BW=404MiB/s
  randread        IOPS=111k, BW=432MiB/s

CC: Dave Chinner <david@xxxxxxxxxxxxx>
Suggested-by: Dave Chinner <david@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/linux-ext4/ZjILCPNZRHeazSqV@xxxxxxxxxxxxxxxxxxx/
Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx>
---
v1->v2:
 - Add READ_ONCE and WRITE_ONCE to access ->j_commit_sequence
   concurrently.
 - Keep the jbd2_transaction_committed() helper.

 fs/jbd2/commit.c  |  2 +-
 fs/jbd2/journal.c | 12 +-----------
 2 files changed, 2 insertions(+), 12 deletions(-)

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 5e122586e06e..8244cab17688 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -1108,7 +1108,7 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 
 	commit_transaction->t_state = T_COMMIT_CALLBACK;
 	J_ASSERT(commit_transaction == journal->j_committing_transaction);
-	journal->j_commit_sequence = commit_transaction->t_tid;
+	WRITE_ONCE(journal->j_commit_sequence, commit_transaction->t_tid);
 	journal->j_committing_transaction = NULL;
 	commit_time = ktime_to_ns(ktime_sub(ktime_get(), start_time));
 
diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index b6c114c11b97..cc586e3c4ee1 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -789,17 +789,7 @@ EXPORT_SYMBOL(jbd2_fc_end_commit_fallback);
 /* Return 1 when transaction with given tid has already committed. */
 int jbd2_transaction_committed(journal_t *journal, tid_t tid)
 {
-	int ret = 1;
-
-	read_lock(&journal->j_state_lock);
-	if (journal->j_running_transaction &&
-	    journal->j_running_transaction->t_tid == tid)
-		ret = 0;
-	if (journal->j_committing_transaction &&
-	    journal->j_committing_transaction->t_tid == tid)
-		ret = 0;
-	read_unlock(&journal->j_state_lock);
-	return ret;
+	return tid_geq(READ_ONCE(journal->j_commit_sequence), tid);
 }
 EXPORT_SYMBOL(jbd2_transaction_committed);
 
-- 
2.39.2