RE: [tip:tracing/core] Revert "x86, bts: reenable ptrace branch trace support"

Peter Zijlstra
2009-06-11T06:36:35Z

On Thu, 2009-06-11 at 07:30 +0100, Metzger, Markus T wrote:
> >-----Original Message-----
> >From: tip-bot for Ingo Molnar [mailto:mingo@elte.hu]
> >Sent: Thursday, June 11, 2009 1:37 AM
> >To: linux-tip-commits@vger.kernel.org
> >Cc: hpa@zytor.com; mingo@redhat.com; peterz@infradead.org; Metzger, Markus T; oleg@redhat.com;
> >tglx@linutronix.de; mingo@elte.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@elte.hu>
> >AuthorDate: Thu, 11 Jun 2009 00:32:00 +0200
> >Committer: Ingo Molnar <mingo@elte.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.