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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Ingo Molnar <mingo@elte.hu>
Subject: Re: x86: Is there still value in having a special tlb flush IPI vector?
Date: Fri, 1 Aug 2008 11:32:32 +1000
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Cc: Peter Zijlstra <peterz@infradead.org>,
       Jeremy Fitzhardinge <jeremy@goop.org>,
       Jens Axboe <jens.axboe@oracle.com>, Andi Kleen <ak@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>
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On Friday 01 August 2008 02:48, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@infradead.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.