linux-kernel - Re: [patch] tlb flush_data: replace per

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Message-ID: <20090112214643.GD7320@elte.hu>
Date:	Mon, 12 Jan 2009 22:46:43 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederik Deweerdt <frederik.deweerdt@...og.eu>
Cc:	andi@...stfloor.org, tglx@...utronix.de, hpa@...or.com,
	linux-kernel@...r.kernel.org, Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [patch] tlb flush_data: replace per_cpu with an array


* Frederik Deweerdt <frederik.deweerdt@...og.eu> wrote:

> Hi,
> 
> On x86_64 flush tlb data is stored in per_cpu variables. This is 
> unnecessary because only the first NUM_INVALIDATE_TLB_VECTORS entries 
> are accessed.
>
> This patch aims at making the code less confusing (there's nothing 
> really "per_cpu") by using a plain array. It also would save some memory 
> on most distros out there (Ubuntu x86_64 has NR_CPUS=64 by default).

indeed, well spotted!

This construct was indeed an abuse of the per_cpu facilities.

Note that beyond the obvious memory savings, your patch should make the 
code a bit faster and a bit smaller as well in the SMP TLB flush codepath: 
the smp_flush_state data structure is 64 (or 128) bytes so static array 
arithmetics will be faster than the per_cpu indirection.

I have applied your patch to tip/x86/mm, thanks Frederik!

	Ingo
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