linux-kernel - Re: [PATCH 0/2] x86: Speed up ioremap operations

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Date:	Fri, 29 Aug 2014 13:52:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mike Travis <travis@....com>
Cc:	mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
	msalter@...hat.com, dyoung@...hat.com, riel@...hat.com,
	peterz@...radead.org, mgorman@...e.de,
	linux-kernel@...r.kernel.org, x86@...nel.org, linux-mm@...ck.org,
	Alex Thorlton <athorlton@....com>, Cliff Wickman <cpw@....com>,
	Russ Anderson <rja@....com>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH 0/2] x86: Speed up ioremap operations

On Fri, 29 Aug 2014 13:44:31 -0700 Mike Travis <travis@....com> wrote:

> 
> 
> On 8/29/2014 1:16 PM, Andrew Morton wrote:
> > On Fri, 29 Aug 2014 14:53:28 -0500 Mike Travis <travis@....com> wrote:
> > 
> >>
> >> We have a large university system in the UK that is experiencing
> >> very long delays modprobing the driver for a specific I/O device.
> >> The delay is from 8-10 minutes per device and there are 31 devices
> >> in the system.  This 4 to 5 hour delay in starting up those I/O
> >> devices is very much a burden on the customer.
> >>
> >> There are two causes for requiring a restart/reload of the drivers.
> >> First is periodic preventive maintenance (PM) and the second is if
> >> any of the devices experience a fatal error.  Both of these trigger
> >> this excessively long delay in bringing the system back up to full
> >> capability.
> >>
> >> The problem was tracked down to a very slow IOREMAP operation and
> >> the excessively long ioresource lookup to insure that the user is
> >> not attempting to ioremap RAM.  These patches provide a speed up
> >> to that function.
> >>
> > 
> > Really would prefer to have some quantitative testing results in here,
> > as that is the entire point of the patchset.  And it leaves the reader
> > wondering "how much of this severe problem remains?".
> 
> Okay, I have some results from testing.  The modprobe time appears to
> be affected quite a bit by previous activity on the ioresource list,
> which I suspect is due to cache preloading.  While the overall
> improvement is impacted by other overhead of starting the devices,
> this drastically improves the modprobe time.
> 
> Also our system is considerably smaller so the percentages gained
> will not be the same.  Best case improvement with the modprobe
> on our 20 device smallish system was from 'real    5m51.913s' to
> 'real    0m18.275s'.

Thanks, I slurped that into the changelog.

> > Also, the -stable backport is a big ask, isn't it?  It's arguably
> > notabug and the affected number of machines is small.
> > 
> 
> Ingo had suggested this.  We are definitely pushing it to our distro
> suppliers for our customers.  Whether it's a big deal for smaller
> systems is up in the air.  Note that the customer system has 31 devices
> on an SSI that includes a large number of other IB and SAS devices
> as well as a number of nodes which all which have discontiguous memory
> segments.  I'm envisioning an ioresource list that numbers at least
> several hundred entries.  While that's somewhat indicative of typical
> UV systems it is generally not that common otherwise.
> 
> So I guess the -stable is merely a suggestion, not a request.

Cc Greg for his thoughts!
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