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

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Message-ID: <20080731164826.GH26393@elte.hu>
Date:	Thu, 31 Jul 2008 18:48:26 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	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?


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, 2008-07-29 at 20:00 +1000, Nick Piggin wrote:
> > On Tuesday 29 July 2008 19:54, Peter Zijlstra wrote:
> > > On Tue, 2008-07-29 at 14:30 +1000, Nick Piggin wrote:
> > > > Not to mention the minor problem that it still deadlocks when called with
> > > > interrupts disabled ;)
> > >
> > > __smp_call_function_single has potential though..
> > 
> > For reschedule interrupt? I don't really agree.
> 
> How about using just arch_send_call_function_single_ipi() to implement 
> smp_send_reschedule() ?

agreed, that's just a single IPI which kicks the need_resched logic on 
return-from-interrupt.

> 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 ...

	Ingo
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