Steven Rostedt <[email protected]> wrote:
>
> I noticed that the code in commit.c of the jbd system can waste CPU
> cycles.
How did you notice? By code inspection or by runtime observation? If the
latter, please share.
> The offending code is as follows.
>
> static int inverted_lock(journal_t *journal, struct buffer_head *bh)
> {
> if (!jbd_trylock_bh_state(bh)) {
> spin_unlock(&journal->j_list_lock);
> schedule();
> return 0;
> }
> return 1;
> }
"offending" is a good description. That code sucks. But it sits on the
edge between two subsystems which really really want to take those locks in
opposite order.
> This code makes a loop if the jbd_trylock_bh_state fails. This code will
> wait till whoever owns the lock releases it. But it is really in a busy
> loop and will only be interrupted when the kjournald uses up all its
> quota. So it's basically just wasting CPU cycles here.
Yeah. But these _are_ spinlocks, so spinning is what's supposed to happen.
Maybe we should dump that silly schedule() and just do cpu_relax().
Although I do recall once theorising that the time we spend in the
schedule() might be preventing livelocks.
> The following
> patch should fix this.
>
> Signed-off-by: Steven Rostedt [email protected]
Please put "<>" around the email address.
> ---
> --- a/fs/jbd/commit.c 2005-07-11 17:51:37.000000000 -0400
> +++ b/fs/jbd/commit.c 2005-07-11 17:51:58.000000000 -0400
> @@ -87,7 +87,7 @@ static int inverted_lock(journal_t *jour
> {
> if (!jbd_trylock_bh_state(bh)) {
> spin_unlock(&journal->j_list_lock);
> - schedule();
> + yield();
> return 0;
> }
> return 1;
Nope, yield() can cause terribly long delays when other tasks are cpu-bound.
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