Re: 2.5.69 Interrupt Latency

From: Greg KH <greg@kroah.com>
To: Paul Fulghum <paulkf@microgate.com>
Cc: Andrew Morton <akpm@digeo.com>,
	stern@rowland.harvard.edu,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	johannes@erdfelt.com
Subject: Re: 2.5.69 Interrupt Latency
Date: Mon, 12 May 2003 10:30:23 -0700	[thread overview]
Message-ID: <20030512173023.GB28381@kroah.com> (raw)
In-Reply-To: <1052759300.1995.4.camel@diemos>

On Mon, May 12, 2003 at 12:08:21PM -0500, Paul Fulghum wrote:
> On Mon, 2003-05-12 at 11:24, Greg KH wrote: 
> > On Mon, May 12, 2003 at 08:57:42AM -0500, Paul Fulghum wrote:
> > > On Fri, 2003-05-09 at 16:28, Andrew Morton wrote:
> > > 
> > > > This code was added to wakeup_hc().  It is called from uhci_irq():
> > > > 
> > > > +	/* Global resume for 20ms */
> > > > +	outw(USBCMD_FGR | USBCMD_EGSM, io_addr + USBCMD);
> > > > +	wait_ms(20);
> > > > 
> > > > The changelog just says "Minor patch for uhci-hcd.c"
> > > > 
> > > > Can you delete the wait_ms() and see if that is our culprit?
> > > 
> > > This is the culprit.
> > > 
> > > Removing this line corrects the latency problems on
> > > the server. A 20ms delay seems pretty excessive for an
> > > interrupt handler. I'm not sure what it is supposed to
> > > accomplish, but this seems like something that should
> > > be scheduled to run outside of the ISR.
> > 
> > This should only happen when your hardware receives a "RESUME" signal
> > from a USB device AND the host controller is in a global suspend state
> > at that time.
> > 
> > Now I think the wait_ms() call is valid for when this is really
> > happening, but it sounds like you are having this happen all the time
> > during normal operation.
> 
> It does appear to happen on a regular basis.
> 
> Should the 20ms delay be in the ISR though?
> I thought it was standard practice to move such
> lengthy operations outside of the ISR so as not to
> impact interrupt latency for the system.

This should only happen when the hardware is suspended, and we are being
woken up by a device.  So this should be a _very_ rare occurance, and
when it does happen, the latency isn't that big of a deal (we need it to
wake up the hardware properly.)

> > Are you using any USB devices with this
> > server?  Is USB enabled in the BIOS or not?
> 
> There are no USB devices attached to the server.
> There are no actual USB connectors, and the
> server's specs do not list USB. There is no
> option to enable/disable USB in the BIOS.

Heh, then I would suggest not loading this driver at all.  It sounds
like you have an internal USB controller that probably does not have
properly terminated connectors.

thanks,

greg k-h