> On Wed, Apr 07, 2010 at 04:21:18PM -0700, john stultz wrote:
> > Further using lockstat I was able to isolate it the contention down to
> > the journal j_state_lock, and then adding some lock owner tracking, I
> > was able to see that the lock owners were almost always in
> > start_this_handle, and jbd2_journal_stop when we saw contention (with
> > the freq breakdown being about 55% in jbd2_journal_stop and 45% in
> > start_this_handle).
>
> Hmm.... I've taken a very close look at jbd2_journal_stop(), and I
> don't think we need to take j_state_lock() at all except if we need to
> call jbd2_log_start_commit(). t_outstanding_credits,
> h_buffer_credits, and t_updates are all documented (and verified by
> me) to be protected by the t_handle_lock spinlock.
>
> So I ***think*** the following might be safe. WARNING! WARNING!! No
> real testing done on this patch, other than "it compiles! ship it!!".
>
> I'll let other people review it, and maybe you could give this a run
> and see what happens with this patch?
I had a look and it really seems that t_handle_lock is enough for
all we do in jbd2_journal_stop (except for jbd2_log_start_commit). So
I think your patch is fine.
I also had a look at jbd2_journal_start. What probably makes things bad there
is that lots of threads accumulate waiting for transaction to get out of
T_LOCKED state. When that happens, all the threads are woken up and start
pondering at j_state_lock which creates contention. This is just a theory and I
might be completely wrong... Some lockstat data would be useful to confirm /
refute this.
Honza
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index bfc70f5..e214d68 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -1311,7 +1311,6 @@ int jbd2_journal_stop(handle_t *handle)
> if (handle->h_sync)
> transaction->t_synchronous_commit = 1;
> current->journal_info = NULL;
> - spin_lock(&journal->j_state_lock);
> spin_lock(&transaction->t_handle_lock);
> transaction->t_outstanding_credits -= handle->h_buffer_credits;
> transaction->t_updates--;
> @@ -1340,8 +1339,7 @@ int jbd2_journal_stop(handle_t *handle)
> jbd_debug(2, "transaction too old, requesting commit for "
> "handle %p\n", handle);
> /* This is non-blocking */
> - __jbd2_log_start_commit(journal, transaction->t_tid);
> - spin_unlock(&journal->j_state_lock);
> + jbd2_log_start_commit(journal, transaction->t_tid);
>
> /*
> * Special case: JBD2_SYNC synchronous updates require us
> @@ -1351,7 +1349,6 @@ int jbd2_journal_stop(handle_t *handle)
> err = jbd2_log_wait_commit(journal, tid);
> } else {
> spin_unlock(&transaction->t_handle_lock);
> - spin_unlock(&journal->j_state_lock);
> }
>
> lock_map_release(&handle->h_lockdep_map);
> --
> To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
--
Jan Kara <jack@xxxxxxx>
SuSE CR Labs
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
