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

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Date:	Wed, 10 Jun 2009 14:22:40 +0100
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>
CC:	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: RE: [tip:tracing/core] x86, bts: reenable ptrace branch trace
	support

>-----Original Message-----
>From: Ingo Molnar [mailto:mingo@...e.hu]
>Sent: Wednesday, June 10, 2009 2:51 PM
>To: Peter Zijlstra
>Cc: mingo@...hat.com; hpa@...or.com; linux-kernel@...r.kernel.org; Metzger, Markus T;
>tglx@...utronix.de; oleg@...hat.com; linux-tip-commits@...r.kernel.org
>Subject: Re: [tip:tracing/core] x86, bts: reenable ptrace branch trace support
>
>
>* Peter Zijlstra <peterz@...radead.org> wrote:
>
>> On Thu, 2009-05-07 at 09:25 +0000, tip-bot for Markus Metzger wrote:
>> > Commit-ID:  7e0bfad24d85de7cf2202a7b0ce51de11a077b21
>> > Gitweb:     http://git.kernel.org/tip/7e0bfad24d85de7cf2202a7b0ce51de11a077b21
>> > Author:     Markus Metzger <markus.t.metzger@...el.com>
>> > AuthorDate: Fri, 24 Apr 2009 09:44:48 +0200
>> > Committer:  Ingo Molnar <mingo@...e.hu>
>> > CommitDate: Fri, 24 Apr 2009 10:18:51 +0200
>> >
>> > x86, bts: reenable ptrace branch trace support
>> >
>> > The races found by Oleg Nesterov have been fixed.
>> >
>> > Reenable branch trace support.
>> >
>> > Signed-off-by: Markus Metzger <markus.t.metzger@...el.com>
>> > Acked-by: Oleg Nesterov <oleg@...hat.com>
>> > LKML-Reference: <20090424094448.A30216@...ona.ch.intel.com>
>> > Signed-off-by: Ingo Molnar <mingo@...e.hu>
>>
>> OK, this whole BTS thing worries me because it seems to expose too
>> much of the Intel Debug Store thingy to the outside world.
>>
>> The thing is, once we do PEBS from inside the kernel, we'll have
>> to share the debug store, and the way this thing sets it up isn't
>> really going to work.
>>
>> So what I'd like to see is having this debug store removed from
>> the interface and abstracted away as cpu resource -- a single page
>> when in use should suffice I think.
>>
>> If you want to expose a buffer to userspace, use the regular
>> mmap() interface and fill those pages from the DS interrupt
>> handler.
>
>hm, that makes sense indeed. We are in the merge window now -
>Markus, what's your suggestion?

The Debug Store interface is completely in-kernel. It does not expose
anything to the outside world.

What we expose is a ptrace interface for branch tracing.
That this is built on top of Debug Store is completely hidden.
The Debug Store interface may be changed without impacting the
user-visible part at any time.

I do think that a ptrace interface makes sense since debuggers are the
targeted users for branch tracing.

I don't see why we should not merge the fixes now and then rework the
in-kernel parts as needed for supporting PEBS.


regards,
markus.

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