linux-kernel - Re: [tip:tracing/core] Revert "x86, bts: reenable ptrace branch trace support"

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Date:	Thu, 11 Jun 2009 12:21:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Metzger, Markus T" <markus.t.metzger@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:tracing/core] Revert "x86, bts: reenable ptrace branch
	trace support"


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, 2009-06-11 at 07:30 +0100, Metzger, Markus T wrote:
> > >-----Original Message-----
> > >From: tip-bot for Ingo Molnar [mailto:mingo@...e.hu]
> > >Sent: Thursday, June 11, 2009 1:37 AM
> > >To: linux-tip-commits@...r.kernel.org
> > >Cc: hpa@...or.com; mingo@...hat.com; peterz@...radead.org; Metzger, Markus T; oleg@...hat.com;
> > >tglx@...utronix.de; mingo@...e.hu
> > >Subject: [tip:tracing/core] Revert "x86, bts: reenable ptrace branch trace support"
> > >
> > >Commit-ID:  511b01bdf64ad8a38414096eab283c7784aebfc4
> > >Gitweb:     http://git.kernel.org/tip/511b01bdf64ad8a38414096eab283c7784aebfc4
> > >Author:     Ingo Molnar <mingo@...e.hu>
> > >AuthorDate: Thu, 11 Jun 2009 00:32:00 +0200
> > >Committer:  Ingo Molnar <mingo@...e.hu>
> > >CommitDate: Thu, 11 Jun 2009 00:32:00 +0200
> > >
> > >Revert "x86, bts: reenable ptrace branch trace support"
> > >
> > >This reverts commit 7e0bfad24d85de7cf2202a7b0ce51de11a077b21.
> > >
> > >A late objection to the ABI has arrived:
> > >
> > >   http://lkml.org/lkml/2009/6/10/253
> > 
> > I thought that this has been resolved. See for example http://lkml.org/lkml/2009/6/10/257.
> > 
> > Peters concerns were that Debug Store details are exposed to user space, which is
> > not the case. Debug Store itself is fully in-kernel and the expectation of a 
> > user-defined buffer can be implemented on top of the Debug Store changes that
> > Peter expects are needed to support PEBS.
> > 
> > A user-defined trace buffer size is required to support 
> > different usage models. Some users only need a small amount of 
> > trace, whereas others need a big amount. The interface will have 
> > to reflect that in some way.
> 
> Right, your last email did explain how we could keep per task 
> in-kernel buffers and fill them from the DS and still have them of 
> user-specified size.
> 
> That would indeed keep the proposed ABI workable, what I'm still 
> not liking is that this buffer is in-kernel, but I guess that 
> might be something for other people to have an opinion on.

Hm. Wrt. the ABI, wouldnt it make more sense to expose this PMU 
feature via perfcounters: a sampling hw-branch-executions counter, 
with interval=1.

That would give the exact existing semantics, plus a lot lot more. 
Markus?

	Ingo
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