linux-kernel - Re: [GIT PULL] perf changes for v3.8

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Date:	Thu, 13 Dec 2012 09:24:20 -0700
From:	David Ahern <dsahern@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] perf changes for v3.8

On 12/13/12 9:03 AM, Linus Torvalds wrote:
> On Thu, Dec 13, 2012 at 6:30 AM, David Ahern <dsahern@...il.com> wrote:
>>
>> One of the problems is that existing binaries set the exclude_guest flag
>> (https://lkml.org/lkml/2012/7/9/292).
>
> [ to zero ]
>
> Yeah. And it apparently *never* worked. So it's not a regression.

The flag works. It does have a purpose. I did not write the original 
code; I am not defending its design. It is what is. We now have a 
catastrophic problem that needs to be fixed.

> So instead, you expect everybody else - for whom things *used* to work
> - to upgrade their binary, or their scripts, or just start using an
> insane command line flag that makes no sense for them? Forcing
> non-virtualization users to use a "only trace the host" flag is crazy.
>
> Either way, somebody will be unhappy. No question about that. But our
> rule in the kernel is "no regressions".

...

> But that whole "no regressions" really is important. I can work around
> things very easily, but the "no regressions" rule really means that I
> should never *need* to work around things.

I get the regressions point. I have seen that statement from you enough 
I think you have it on a permanent copy-and-paste shortcut.

Without the kernel side restriction existing perf binaries will crash 
all running VMs. I could write the patch to completely invert the 
exclude_guest logic -- make it include_guest. That breaks all existing 
perf binaries as well - just a different syntax that gets broken. That 
regression is acceptable?

David


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