linux-kernel - Re: Slow pty's (was Re: libdivecomputer interfaces?)

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Message-Id: <1276701411.30669.6.camel@marge.simson.net>
Date:	Wed, 16 Jun 2010 17:16:51 +0200
From:	Mike Galbraith <efault@....de>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Brian Bloniarz <bmb@...enacr.com>, Chris Wedgwood <cw@...f.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	Jef Driesen <jefdriesen@...enet.be>,
	linux-kernel@...r.kernel.org
Subject: Re: Slow pty's (was Re: libdivecomputer interfaces?)

On Wed, 2010-06-16 at 17:03 +0200, Jiri Kosina wrote:
> On Thu, 10 Jun 2010, Brian Bloniarz wrote:
> 
> > On 06/10/2010 02:10 PM, Chris Wedgwood wrote:
> > > (sorry if this reponse isn't on target, i was just pointed to this
> > > thread a few minutes ago)
> > > 
> > > 
> > > On Thu, Jun 10, 2010 at 10:25:36AM -0700, Linus Torvalds wrote:
> > > 
> > >>  I thought we long since (ie back last fall) fixed the latency
> > >> problems with pty's, but there does seem to be something very fishy
> > >> going on there still.
> > > 
> > > this might not be related, but i have slow serial ports with NOHZ that
> > > goes away when i revert 39c0cbe2150cbd848a25ba6cdb271d1ad46818ad.
> > 
> > Unrelated or not, I think Chris is right about this. Somewhere before
> > -rc1, the emulated serial console on my KVM instance became slow
> > to echo input. I just tested with the commit reverted and it's
> > back to normal.
> 
> So let's CC Mike then.

Chris already gave me a heads up, it's on my todo.  The old P4 box I use
for a serial console box is exploding on boot, or I would have already
had a look.
  
> > > and looking at the only two interesting hunks it's not clear why:

Complete mystery to me.

> > > +int nohz_ratelimit(int cpu)
> > > +{
> > > +       struct rq *rq = cpu_rq(cpu);
> > > +       u64 diff = rq->clock - rq->nohz_stamp;
> > > +
> > > +       rq->nohz_stamp = rq->clock;
> > > +
> > > +       return diff < (NSEC_PER_SEC / HZ) >> 1;
> > > +}
> > > 
> > > +       if (nohz_ratelimit(cpu))
> > > +               goto end;
> > > +
> > > 
> > > network latnecy is fine, and if i create lots of wakeups (network IO
> > > is fine) then the serial port latency is noticable

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