linux-kernel - Re: Why is kslowd accumulating so much CPU time?

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Message-ID: <20100614184244.GA11480@elliptictech.com>
Date:	Mon, 14 Jun 2010 14:42:44 -0400
From:	Nick Bowler <nbowler@...iptictech.com>
To:	Dave Airlie <airlied@...il.com>
Cc:	tytso@....edu, Dave Airlie <airlied@...hat.com>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Why is kslowd accumulating so much CPU time?

On 06:00 Mon 14 Jun     , Dave Airlie wrote:
> On Mon, Jun 14, 2010 at 5:49 AM,  <tytso@....edu> wrote:
> > It's a Lenovo T400, with an Intel GPU:
> >
[...]
> > Why does KMS need to poll so frequently?  40 minutes of CPU time
> > accumulated in 4 hours of uptime translates to 16% of the CPU being
> > consumed by kslowd daemons, which seems... excessive.
> >
[...]
> > I have the following patch which I'm going to install later tonight to
> > see if I can figure out if it really is drm_crtc_helper.c which is
> > really responsible for all of the kslowd time being burned, but an
> > examination of the source doesn't seem to show any other that I'm
> > using that would likely be using the slow workqueue.
> 
> It most likely is, but polling shouldn't really be taking huge amounts
> of CPU, unless there are some u/mdelays in there which would be bad.
> 
> In theory on Intel with hotplug irqs we shouldn't be poilling at all,
> I must check why, the other thing is you could be suffering from the
> hotplug irq problem that others have reported, this would cause slow
> work triggers which aren't part of the normal poll cycle.

This sounds exactly like the issue I've been seeing on a T500 laptop, as
well (GM45 board).  The slowdowns render the system essentially
unusable, as it can spend a loooong time just moving the mouse cursor a
few pixels on the screen.  During this time, nothing else on the display
is updating (glxgears drops to 0fps).  Things generally seem to be
working fine if I am not moving the mouse, or if I'm not running X.

I do not have this issue on a desktop machine with a G45.

Unfortunately, bisection is proving difficult because the exact set of
conditions to trigger the problems seem to be eluding me: sometimes the
kernel will work perfectly fine for quite some time, and then go
downhill from there.  However, this is definitely a regression
introduced after 2.6.35-rc1.

-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)
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