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

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Date:	Thu, 13 Dec 2012 18:31:01 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	David Ahern <dsahern@...il.com>,
	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


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, Dec 13, 2012 at 8:59 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> >>
> >> It's not a regression since THAT CODE NEVER WORKED, for
> >> chissake! The case of people actually profiling into virtual
> >> machines crashes the running VMs, as you say. There's no way
> >> in hell we can call it a regression to say "you now have to
> >> use a flag if you profile a load with virtualization", since
> >> there wasn't any working case to begin with.
> >
> > Correct.
> >
> > ::include_guest looks like the more logical flag direction to
> > use in any case.
> 
> See the email I just sent. The *non*-precise case presumably used to
> work (and included the virtualized environment). No?
> 
> So the default shouldn't necessarily be "include guest". The default
> should presumably be "the user didn't say", and then the kernel does
> whatever works best.
> 
> If the user actually explicitly says one or the other, we should try
> to honor that (and then EOPNOTSUPP may be a "sorry, I really cannot do
> that particular combination that you explicitly asked for").
> 
> That should make everybody happy. Doing a non-PEBS virtualized perf
> run should still work with the old binary.
> 
> So there should be two bits: "include guest" (V in the event specifier
> unless you already used that for something else) and "host only" (H),
> and they should both default to off. Then the kernel can see the three
> actual cases.
> 
> (Or four cases, if you really want to: you may or may not want to make
> the "both V and H set means both, and _only_ V set means 'no host at
> all, _only_ virtual environment'. So then ":ppV" would mean
> "cycle-accurate for virtual box _only_", while ":ppVH" would mean
> "cycle-accurate for both the host and the virtual box". Of course,
> considering the PEBS interface, right now neither of those can
> actually work, but plain ":V" and ":HV" could work).
> 
> The important thing, I think, is that if the user doesn't know 
> or care about the VM case (because he's not running any!) and 
> doesn't specify, then the kernel should not say EOPNOTSUPP, 
> and should do whatever works for that cpu.

Agreed.

David, wanna send a patch for this?

Thanks,

	Ingo
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