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

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Message-ID: <5291647D.2070703@hurleysoftware.com>
Date:	Sat, 23 Nov 2013 21:29:17 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH tty-next 6/7] n_tty: Only perform wakeups for waiters

On 11/23/2013 07:23 PM, One Thousand Gnomes wrote:
> On Fri, 22 Nov 2013 10:59:24 -0500
> Peter Hurley <peter@...leysoftware.com> wrote:
>
>> Only wakeup the _waiting_ reader, polls and/or writer(s).
>>
>> Signed-off-by: Peter Hurley <peter@...leysoftware.com>
>> ---
>>   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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