linux-kernel - Re: [PATCH] Allow per-cpu variables to be page-aligned

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Date:	Wed, 21 Mar 2007 22:44:16 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Allow per-cpu variables to be page-aligned

On Wed, 2007-03-21 at 03:21 -0600, Eric W. Biederman wrote:
> Do we really want to allow modules to be able to allocate page sized
> per cpu memory.

Hi Eric!

	They always could, of course, they just wouldn't get correct alignment.
I think the principle of least surprise says that if we support this, it
will also work in modules...

>   If my memory servers on how this code works we will wind
> up allocating 1 page of per cpu memory for every module that allocates a
> per cpu variable.  128 bytes sucks 4k is an order of magnitude worse.

Not quite.  We allocate a total amount of per-cpu memory at boot, then
anything left over gets used for per-cpu vars in modules.

Looking at the module per-cpu code again, the rounding up of the memory
used by the kernel seems unnecessary though.  I'll try ripping that
out...

> On x86_64 we are only reserving 8K for modules...

Really?  I can't see that.

It did look like the x86-64 setup_per_cpu_areas should be moved into
common code though (it's numa-aware).  Maybe that breaks some platforms.

It means the x86 cpu_pda initialization would have to be done in
smp_prepare_boot_cpu tho...

Cheers!
Rusty.

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