linux-kernel - Re: [PATCH] Use uname not sysctl to get the kernel revision

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Date:	Fri, 14 Jul 2006 21:09:34 +0200
From:	Andi Kleen <ak@...e.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Steve Munroe <sjmunroe@...ibm.com>, Theodore Tso <tytso@....edu>,
	libc-alpha@...rceware.org, linux-kernel@...r.kernel.org,
	akpm@...l.org, Arjan van de Ven <arjan@...radead.org>,
	Ulrich Drepper <drepper@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Randy.Dunlap" <rdunlap@...otime.net>
Subject: Re: [PATCH] Use uname not sysctl to get the kernel revision

On Friday 14 July 2006 20:49, Benjamin Herrenschmidt wrote:
> > We will need an implementation that will fall back to sys_sysctl for older
> > kernels. This is already common practice in glibc. I don't really
> > understand the performance concern because it seems to me that
> > _is_smp_system() is only called once per process.
> > 
> > But isn't this the kind of thing that the Aux Vector is for? I like vDSO
> > too, but I think it is best deployed for information of a more dynamic
> > nature and performance sensitive.
> 
> For a simple "is_smp" kind of flag, I would tend to agree with the
> above... for more complex NUMA topology and/or cache characteristics,
> which is quite a bigger amount of information, I'm not sure it's worth
> copying all of that data on every process exec (and making the initial
> AT_ parsing slower). Especially since very few processes actually care
> about those.

I've actually spent some thought on that recently. The motivation
came from someone who wanted the number of CPUs in a fast way 
to tune AMD64 memcpy etc. better.

My proposal was to supply four new count:
number of cores, number of siblings, number of sockets, number of nodes

These all fit easily in 16bit so it would be 2 new entries in the
aux vector (128 bit total). Shouldn't be much overhead to write this.

If you need more exact topology you can probably eat the overhead
of parsing /proc/cpuinfo or read it from sysfs (or just use libnuma
which supplies most of this in an easy way) 

Doing it in a vDSO would be in theory ok for me too, except that x86-64
doesn't have one so far. Even in vDSO I wouldn't add much more than this
(like bitmaps and similar) because otherwise cpu/node hotplug could be racy.
Also I'm reluctant to redo /proc/cpuinfo and /sys for this.

-Andi
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