linux-kernel - Re: [PATCH v2 20/20] tty: Remove extra wakeup from pty write() path

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Message-ID: <51EE7DC8.3060004@hurleysoftware.com>
Date:	Tue, 23 Jul 2013 08:57:44 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Peter Hurley <peter@...leysoftware.com>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	Jiri Slaby <jslaby@...e.cz>,
	Andre Naujoks <nautsch2@...il.com>,
	Dean Jenkins <Dean_Jenkins@...tor.com>
Subject: Re: [PATCH v2 20/20] tty: Remove extra wakeup from pty write() path

On 07/20/2013 01:00 PM, Peter Hurley wrote:
> On 06/15/2013 10:21 AM, Peter Hurley wrote:
>> Acquiring the write_wait queue spin lock now accounts for the largest
>> slice of cpu time on the tty write path. Two factors contribute to
>> this situation; a overly-pessimistic line discipline write loop which
>> _always_ sets up a wait loop even if i/o will immediately succeed, and
>> on ptys, a wakeup storm from reads and writes.
>>
>> Writer wakeup does not need to be performed by the pty driver.
>> Firstly, since the actual i/o is performed within the write, the
>> line discipline write loop will continue while space remains in
>> the flip buffers. Secondly, when space becomes avail in the
>> line discipline receive buffer (and thus also in the flip buffers),
>> the pty unthrottle re-wakes the writer (non-flow-controlled line
>> disciplines unconditionally unthrottle the driver when data is
>> received). Thus, existing in-kernel i/o is guaranteed to advance.
>> Finally, writer wakeup occurs at the conclusion of the line discipline
>> write (in tty_write_unlock()). This guarantees that any user-space write
>> waiters are woken to continue additional i/o.
>
> Greg,
>
> I thought I should let you know I'm tracking down a bug/regression
> related to this patch.
>
> In certain unusual pty/ldisc configurations, i/o fails to make
> forward progress. I still stand by my commit message above, so I'm
> in the process of instrumenting the i/o path so I can uncover the
> cause of the failure.

Mystery solved.

[PATCH v4 23/24] n_tty: Special case pty flow control
from the lockless n_tty receive path series introduced a regression
in which i/o failed to advance.

This only occurred when one end of a pty pair was set to an ldisc
other than N_TTY. The special case optimization which that patch
introduces failed to address that configuration.

I've sent a v5 of that patch to resolve the regression.

Regards,
Peter Hurley
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