Re: [GIT PULL] perf changes for v3.8 [LWN.net]

From:		Linus Torvalds <torvalds-AT-linux-foundation.org>
To:		David Ahern <dsahern-AT-gmail.com>
Subject:		Re: [GIT PULL] perf changes for v3.8
Date:		Thu, 13 Dec 2012 08:03:58 -0800
Message-ID:		<CA+55aFzmDooC3tYzvP1YGv9g4ej1hBUQBPV6R+ukrfxuEELBaQ@mail.gmail.com>
Cc:		Ingo Molnar <mingo-AT-kernel.org>, Linux Kernel Mailing List <linux-kernel-AT-vger.kernel.org>, Arnaldo Carvalho de Melo <acme-AT-infradead.org>, Peter Zijlstra <a.p.zijlstra-AT-chello.nl>, Thomas Gleixner <tglx-AT-linutronix.de>, Andrew Morton <akpm-AT-linux-foundation.org>
Archive‑link:		Article

On Thu, Dec 13, 2012 at 6:30 AM, David Ahern <dsahern@gmail.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.

> So, requesting users to update their binaries if they want to use precise
> sampling is not acceptable. A 100% catastrophic failure of all running VMs
> is acceptable? All VMs will crash and there is no direct causal
> relationship.

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".

Now, I do agree that for "perf", it's fairly easy to say "just
recompile". I can do it in seconds, and it would presumably solve my
problem by just making the "host only" case the default, and I don't
need the "H" any more.

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.

So when I see a regression, I consider it a major bug, even if the
workaround is trivial.

                     Linus