linux-ext4 - Re: ext4 dbench performance with CONFIG_PREEMPT

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Message-ID: <20100408034631.GB23188@thunk.org>
Date:	Wed, 7 Apr 2010 23:46:31 -0400
From:	tytso@....edu
To:	john stultz <johnstul@...ibm.com>
Cc:	linux-ext4@...r.kernel.org, Mingming Cao <cmm@...ibm.com>,
	keith maanthey <kmannth@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Darren Hart <dvhltc@...ibm.com>
Subject: Re: ext4 dbench performance with CONFIG_PREEMPT_RT

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?

					- Ted

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);
--
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