linux-kernel - Re: [PATCH 22/23] sysctl arm: Remove binary sysctl support

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Date:	Mon, 9 Nov 2009 16:46:21 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Andi Kleen <andi@...stfloor.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 22/23] sysctl arm: Remove binary sysctl support

> Can you name one binary sysctl value that gets accessed more
> than a few times during the execution of a vaguely common
> application? We're talking about microseconds for typically
> write-once or read-once settings.

For example shell scripts tend to execute programs quite a lot.

> The question is just how many sysctl values you regard as both
> common and performance critical.

Very little, I suspect in fact it's only one.

> > > in glibc-ports for the support of arm inb/outb.  The only other
> > > use in older glibc was checking to see if we ran on an SMP kernel.
> > 
> > That older glibc is widely deployed. And it won't go away next year.
> 
> So? Most users of old glibc are also using old kernels, and they

How do you know? At least here it's quite common to use new kernels
with old user land.

> can still use the  config option for the compatibility code.
> There wouldn't even be a performance penalty over new glibc with
> new kernels which already use procfs.

When he drops the sysctl(2) API completely the old userland will
be unhappy.

> > I just think you should have two flavours of emulation layer:
> > full and "common sysctls". This can be probably done with the same
> > code and some strategic ifdefs.
> 
> If it's just about code size, I totally wouldn't bother. Just put the
> emulation code in loadable module and add a 'printk("Warning, %s is
> using sysctl %s, wasting %d kb of kernel memory")' to it's module_init
> function.

That means non modular kernels can't support old userland.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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