linux-kernel - Re: [patch 10/10] perf_event_open.2: 4.0 update rdpmc documentation

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Date:	Mon, 20 Apr 2015 10:42:56 -0400 (EDT)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	Andy Lutomirski <luto@...capital.net>
cc:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	linux-man@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	Andy Lutomirski <luto@...capital.net>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"hillf.zj" <hillf.zj@...baba-inc.com>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 10/10] perf_event_open.2: 4.0 update rdpmc
 documentation

On Fri, 17 Apr 2015, Andy Lutomirski wrote:        
> On 04/16/2015 11:20 AM, Vince Weaver wrote:
> > Probably a better change would have been to add "2" to mean per-process
> > and make that the default setting.  Probably too late to fix that now.
> 
> Good point.  I wish you'd thought of that sooner :(

Well it's been a busy semester and so I've had less time than normal to 
play perf_event ABI police.

I also was surprised this particular patch managed to get into 4.0 so 
quickly; I thought it was going to be a 4.1 change when I added it to my 
"changesets to look at more closely" list.

In any case most people won't notice this change.  It will be a minor 
inconvenience for HPC people doing complicated homebrew self monitoring
(especially if RHEL backports the patch and then suddenly things break and 
a simple "well are you running 4.0 or newer" is meaningless). Using
the value in /sys/devices/cpu/rdpmc to help debug the problem wouldn't
have worked anyway because it looks like non-root can't do that.

Vince
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