Date:	Fri, 29 Aug 2014 15:31:01 -0700
From:	Greg KH <greg@...ah.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mike Travis <travis@....com>, 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>
Subject: Re: [PATCH 0/2] x86: Speed up ioremap operations

On Fri, Aug 29, 2014 at 01:52:00PM -0700, Andrew Morton wrote:
> 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!

Sounds like a good thing for stable.

thanks,

greg k-h
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