linux-kernel - Re: Powertop shows events/0 waking at high rate due to ptys

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Date:	Mon, 27 Sep 2010 17:27:43 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Ian Jackson <Ian.Jackson@...citrix.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: Powertop shows events/0 waking at high rate due to ptys

 On 09/27/2010 11:15 AM, Alan Cox wrote:
> Really the line discipline should wake the work queue when it sets
> tty->receive_room non-zero, but while only n_tty currently uses that
> facility the existing code doesn't do it in any kind of race-free manner
> and sometimes is only saved by the polling picking it up.
>
> It's all really just a symptom of the fact that input and output buffers
> shouldn't be attached to the tty in the first place but to a struct
> representing the physical port. Fix that and the race conditions in
> serial output go away, as do the potential crashes and this wakeup stuff
> as well as a ton of locking in the irq/tx/rx paths. In several cases it
> also saves you an entire copy.
>
> Unfortunately while I got the tty port structures into lots of places
> needed the job never gone done.

OK, so it sounds like there's a basic design problem here which will
need some non-trivial work to fix.  In the meantime we'll need to look
at doing something to work around the issue, since it ends up consuming
a non-trivial amount of CPU in events/0.  I guess reducing HZ would be
the first, simplest thing to do, but changing xenconsoled to avoid
writing to readerless ptys might not be too hard.

Thanks,
    J
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