linux-kernel - Re: [RFC][PATCH] drivers: base: dynamic memory block creation

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Message-ID: <20130816184100.GA7265@variantweb.net>
Date:	Fri, 16 Aug 2013 13:41:00 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Dave Hansen <dave@...1.net>,
	Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH] drivers: base: dynamic memory block creation

On Thu, Aug 15, 2013 at 02:01:09AM +0200, Rafael J. Wysocki wrote:
> On Wednesday, August 14, 2013 02:31:45 PM Seth Jennings wrote:
> > Large memory systems (~1TB or more) experience boot delays on the order
> > of minutes due to the initializing the memory configuration part of
> > sysfs at /sys/devices/system/memory/.
> > 
> > ppc64 has a normal memory block size of 256M (however sometimes as low
> > as 16M depending on the system LMB size), and (I think) x86 is 128M.  With
> > 1TB of RAM and a 256M block size, that's 4k memory blocks with 20 sysfs
> > entries per block that's around 80k items that need be created at boot
> > time in sysfs.  Some systems go up to 16TB where the issue is even more
> > severe.
> > 
> > This patch provides a means by which users can prevent the creation of
> > the memory block attributes at boot time, yet still dynamically create
> > them if they are needed.
> > 
> > This patch creates a new boot parameter, "largememory" that will prevent
> > memory_dev_init() from creating all of the memory block sysfs attributes
> > at boot time.  Instead, a new root attribute "show" will allow
> > the dynamic creation of the memory block devices.
> > Another new root attribute "present" shows the memory blocks present in
> > the system; the valid inputs for the "show" attribute.
> 
> I wonder how this is going to work with the ACPI device object binding to
> memory blocks that's in 3.11-rc.

Thanks for pointing this out.  Yes the walking of the memory blocks in
this code will present a problem for the dynamic creation of memory
blocks :/  Sounds like there are some other challenges (backward
compatibility, no boot paramater) that I'll have to look at as well.

Seth

> 
> That stuff will only work if the memory blocks are already there when
> acpi_memory_enable_device() runs and that is called from the ACPI namespace
> scanning code executed (1) during boot and (2) during hotplug.  So I don't
> think you can just create them on the fly at run time as a result of a
> sysfs write.
> 
> Thanks,
> Rafael
> 

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