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

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Date:	Wed, 14 Aug 2013 12:43:48 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
Cc:	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 Wed, Aug 14, 2013 at 02:31:45PM -0500, 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/.

Are you sure that is the problem area?  Have you run perf on it?

> 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.

The x86 developers are working with larger memory sizes and they haven't
seen the problem in this area, for them it's in other places, as I
referred to in my other email.

> 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.

You never documented any of these abi changes, which is a requirement
(not that I'm agreeing that a boot parameter is ok...)

> There was a significant amount of refactoring to allow for this but
> IMHO, the code is much easier to understand now.

Care to refactor things first, with no logical changes, and then make
your changes in a follow-on patch, so that people can actually find what
you changed in the patch?

Remember, a series of patches please, not one big "refactor and change
it all" patch.

thanks,

greg k-h
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