linux-kernel - Re: [PATCH 02/19] perf: Add ability to attach user level registers dump to sample

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Date:	Wed, 13 Jun 2012 15:12:40 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	acme@...hat.com, a.p.zijlstra@...llo.nl, mingo@...e.hu,
	paulus@...ba.org, cjashfor@...ux.vnet.ibm.com, fweisbec@...il.com,
	gorcunov@...nvz.org, tzanussi@...il.com, mhiramat@...hat.com,
	robert.richter@....com, fche@...hat.com,
	linux-kernel@...r.kernel.org, masami.hiramatsu.pt@...achi.com,
	drepper@...il.com, asharma@...com, benjamin.redelings@...cent.org
Subject: Re: [PATCH 02/19] perf: Add ability to attach user level registers
 dump to sample

On Wed, Jun 13, 2012 at 01:16:44PM +0200, Stephane Eranian wrote:
> On Mon, Jun 11, 2012 at 3:19 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> > Introducing sample_regs_user bitmask into perf_event_attr
> > struct to define the user level registers we want to attach
> > to the sample. The dump itself is triggered once the
> > sample_regs_user is not empty.
> >
> > Only user level registers are dump at the moment. Meaning the
> > register values of the user space context as it was before the
> > user entered the kernel for whatever reason (syscall, irq,
> > exception, or a PMI happening in userspace).
> >
> > The layout of the sample_regs_user bitmap is described in
> > asm/perf_regs.h for archs that support register dump.
> >
> > This is going to be useful to bring Dwarf CFI based stack
> > unwinding on top of samples.
> >
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Signed-off-by: Jiri Olsa <jolsa@...hat.com>
> > ---
> >  include/linux/perf_event.h |   10 ++++++-
> >  kernel/events/core.c       |   61 ++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 70 insertions(+), 1 deletions(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 1ce887a..d66cbeb 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -271,7 +271,13 @@ struct perf_event_attr {
> >                __u64           bp_len;
> >                __u64           config2; /* extension of config1 */
> >        };
> > -       __u64   branch_sample_type; /* enum branch_sample_type */
> > +       __u64   branch_sample_type; /* enum perf_branch_sample_type */
> > +
> > +       /*
> > +        * Defines set of user regs to dump on samples.
> > +        * See asm/perf_regs.h for details.
> > +        */
> > +       __u64   sample_regs_user;
> >  };
> That's not enough. You also need to define PERF_SAMPLE_USER_REGS
> for sample_type. Although the sample_regs_users might look like it's enough
> to capture regs, there is a problem when it comes to parsing the record. You
> need an ordering guarantee that is explicitly spelled out in the API (the header
> file). In your current patch, I have no way of knowing that sample_regs_users
> are saved after BRANCH_STACK (should you have that enabled). Remember
> that you can turn on/off sampled infos at will in sample_type. Yet to find the
> infos when parsing, you need to know the order.

Well, the sample_regs_user != 0 substitute the PERF_SAMPLE_USER_REGS bit.
The behaviour is the same as if there was that bit defined..

After last discussion the idea was to keep this just with sample_regs_user != 0.

I dont see any limitation except for being incosistent with the rest of
the sample dumps. I'm all for having that PERF_SAMPLE_USER_REGS bit and
the user stack bit as well.

jirka
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