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

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Message-ID: <20100930071907.GA2429@ucw.cz>
Date:	Thu, 30 Sep 2010 09:19:07 +0200
From:	Pavel Machek <pavel@....cz>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	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 Mon 2010-09-27 17:27:43, Jeremy Fitzhardinge wrote:
>  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.

Replace "1" in schedule_delayed_work with (HZ/100) should make it
better, and looks like good thing while better fix is prepared?



-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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