linux-kernel - Re: x86: Is there still value in having a special tlb flush IPI vector?

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Date:	Fri, 1 Aug 2008 11:32:32 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Jens Axboe <jens.axboe@...cle.com>, Andi Kleen <ak@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: x86: Is there still value in having a special tlb flush IPI vector?

On Friday 01 August 2008 02:48, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@...radead.org> wrote:

> > The overhead of that is a smp_mb() and a list_empty() check in
> > generic_smp_call_function_single_interrupt() if there is indeed no
> > work to do.
>
> that would be a miniscule cost - cacheline is read-shared amongst cpus
> so there's no real bouncing there. So i'm all for it ...

smp_mb would cost some cycles. So would the branch mispredict because
list_empty would otherwise normally be taken I think. q likely is not
in cache either.

I'm not in favour.
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