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

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Date:	Mon, 12 Jan 2009 23:40:37 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Frederik Deweerdt <frederik.deweerdt@...og.eu>, tglx@...utronix.de,
	hpa@...or.com, linux-kernel@...r.kernel.org
Subject: Re: [patch] tlb flush_data: replace per_cpu with an array


* Andi Kleen <andi@...stfloor.org> wrote:

> On Mon, Jan 12, 2009 at 10:35:42PM +0100, Frederik Deweerdt 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).
> 
> Nope it doesn't save memory on most systems because per cpu is only 
> allocated based on the CPUs that are actually there. And if you have 
> more than 8 cores you can likely afford a few bytes per CPU.

No distro kernel will build with less than 8 CPUs anyway so this point is 
moot.

> You would need to cache line pad each entry then, otherwise you risk 
> false sharing. [...]

They are already cache line padded.

> [...] That would make the array 1K on 128 bytes cache line system.

512 bytes.

> [...]  This means on small systems this would actually waste much more 
> memory.

Really small systems will be UP and wont do cross-CPU TLB flushes, so if 
they are a worry the flush code can be UP optimized. (Nobody bothered so 
far.)

	Ingo
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