linux-kernel - Re: [PATCH] jbd2: do not start commit when t

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Message-ID: <20190329212523.GC8757@quack2.suse.cz>
Date:   Fri, 29 Mar 2019 22:25:23 +0100
From:   Jan Kara <jack@...e.cz>
To:     Liu Song <fishland@...yun.com>
Cc:     tytso@....edu, jack@...e.com, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, liu.song11@....com.cn
Subject: Re: [PATCH] jbd2: do not start commit when t_updates does not back
 to zero

On Sun 24-03-19 11:38:35, Liu Song wrote:
> When t_updates back to zero, it guaranteed wake up process which
> waiting on j_wait_updates. If we triggered a commit start without
> considered t_updates, the commit thread wakes up and find t_updates
> is not zero, it have to wait on it once again. So, add checking code
> to avoid this happen.
> 
> Signed-off-by: Liu Song <liu.song11@....com.cn>

Do I understand correctly that this is a performance improvement? If yes,
did you measure any benefit of the patch? Because I have some doubts that
t_updates == 0 case is very common.

								Honza

> ---
>  fs/jbd2/transaction.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index 79a028a7a579..e0499fd73b1e 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -144,12 +144,13 @@ static void wait_transaction_locked(journal_t *journal)
>  	__releases(journal->j_state_lock)
>  {
>  	DEFINE_WAIT(wait);
> -	int need_to_start;
> +	int need_to_start = 0;
>  	tid_t tid = journal->j_running_transaction->t_tid;
>  
>  	prepare_to_wait(&journal->j_wait_transaction_locked, &wait,
>  			TASK_UNINTERRUPTIBLE);
> -	need_to_start = !tid_geq(journal->j_commit_request, tid);
> +	if (!atomic_read(&journal->j_running_transaction->t_updates))
> +		need_to_start = !tid_geq(journal->j_commit_request, tid);
>  	read_unlock(&journal->j_state_lock);
>  	if (need_to_start)
>  		jbd2_log_start_commit(journal, tid);
> -- 
> 2.19.1
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR