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

Ingo Molnar
2009-06-11T10:22:23Z

* Peter Zijlstra <peterz@infradead.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@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.

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