Re: [PATCH tty-next 6/7] n_tty: Only perform wakeups for waiters

[Peter Hurley]

On 11/23/2013 07:23 PM, One Thousand Gnomes wrote:
> On Fri, 22 Nov 2013 10:59:24 -0500
> Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
>
> > Only wakeup the _waiting_ reader, polls and/or writer(s).
> >
> > Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
> > ---
> >   drivers/tty/n_tty.c | 18 ++++++++++++------
> >   1 file changed, 12 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> > index 8f2356e..aae28a6 100644
> > --- a/drivers/tty/n_tty.c
> > +++ b/drivers/tty/n_tty.c
> > @@ -275,7 +275,8 @@ static void n_tty_check_unthrottle(struct tty_struct *tty)
> >   			return;
> >   		n_tty_set_room(tty);
> >   		n_tty_write_wakeup(tty->link);
> > -		wake_up_interruptible_poll(&tty->link->write_wait, POLLOUT);
> > +		if (waitqueue_active(&tty->link->write_wait))
> > +			wake_up_interruptible_poll(&tty->link->write_wait, POLLOUT);
>
> Does this actually microbenchmark faster ?

Getting on and off the write_wait queue is actually pretty expensive for
the "other" pty (the writer), and the unnecessary wakeup from the reader
doesn't help.

The other chunks are gratuitous.

Regards,
Peter Hurley

PS - This came up because there is some worst-case behavior that I'm looking
into fixing.  When the userspace reader is very far behind (say because it's
reading char-by-char), it doesn't make sense to keep restarting the input
processing worker.

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