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

[Peter Hurley]

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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