linux-kernel - Re: perf events ring buffer memory barrier on powerpc

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Message-ID: <alpine.DEB.2.02.1310291524240.6608@pianoman.cluster.toy>
Date:	Tue, 29 Oct 2013 15:27:05 -0400 (EDT)
From:	Vince Weaver <vince@...ter.net>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Victor Kaplansky <VICTORK@...ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Anton Blanchard <anton@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux PPC dev <linuxppc-dev@...abs.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Michael Ellerman <michael@...erman.id.au>,
	Michael Neuling <mikey@...ling.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: perf events ring buffer memory barrier on powerpc

On Tue, 29 Oct 2013, Peter Zijlstra wrote:

> On Tue, Oct 29, 2013 at 11:21:31AM +0100, Peter Zijlstra wrote:
> --- linux-2.6.orig/include/uapi/linux/perf_event.h
> +++ linux-2.6/include/uapi/linux/perf_event.h
> @@ -479,13 +479,15 @@ struct perf_event_mmap_page {
>  	/*
>  	 * Control data for the mmap() data buffer.
>  	 *
> -	 * User-space reading the @data_head value should issue an rmb(), on
> -	 * SMP capable platforms, after reading this value -- see
> -	 * perf_event_wakeup().
> +	 * User-space reading the @data_head value should issue an smp_rmb(),
> +	 * after reading this value.

so where's the patch fixing perf to use the new recommendations?

Is this purely a performance thing or a correctness change?

A change like this a bit of a pain, especially as userspace doesn't really 
have nice access to smb_mb() defines so a lot of cut-and-pasting will 
ensue for everyone who's trying to parse the mmap buffer.

Vince
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