Re: [PATCH] tty: Only wakeup the line discipline idle queue when queue is active

Jiri Slaby
2013-01-02T09:35:12Z

On 12/18/2012 03:48 PM, Ivo Sieben wrote:
> Before waking up the tty line discipline idle queue first check if the queue is
> active (non empty). This prevents unnecessary entering the critical section in
> the wake_up() function and therefore avoid needless scheduling overhead on a
> PREEMPT_RT system caused by two processes being in the same critical section.
>
> Signed-off-by: Ivo Sieben <meltedpianoman@gmail.com>
> ---
>
> Remark:
> This patch has kind of a long history... I first tried to prevent the critical
> section in the wakeup() function itself by a change in the scheduler. But after
> review remarks from Oleg Nesterov it turned out that using the
> waitqueue_active() was a much nicer way to prevent it. See also
> https://lkml.org/lkml/2012/10/25/159
>
> drivers/tty/tty_ldisc.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
> index c578229..e96d187 100644
> --- a/drivers/tty/tty_ldisc.c
> +++ b/drivers/tty/tty_ldisc.c
> @@ -64,7 +64,9 @@ static void put_ldisc(struct tty_ldisc *ld)
> return;
> }
> raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
> - wake_up(&ld->wq_idle);
> +
> + if (waitqueue_active(&ld->wq_idle))
> + wake_up(&ld->wq_idle);

Looks good, but I would prefer the layer to provide us with
wake_up_if_active...

--
js
suse labs