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

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Message-ID: <CABPqkBRdVMzocJu_SJbU81wg6Ef=KgbXw-18tb+bZEon79fVDA@mail.gmail.com>
Date:	Wed, 13 Jun 2012 15:18:54 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Jiri Olsa <jolsa@...hat.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 3:12 PM, Jiri Olsa <jolsa@...hat.com> wrote:
> 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..
>
No it's not the same.  Looking at sample_regs_user != 0, do you know in which
order the regs array is going to appear RELATIVE to the other captured
information?

Take sample_type = IP|CPU|PERIOD, sample_regs_users = EAX

Now, I get the raw record, want to parse it. Which comes first the user_regs
or the IP, CPU, PERIOD?

Worst, I add more entries to PERF_SAMPLE_*, are they laid out before or
after the regs?

If you look carefully at perf_output_sample(), you will notice that data is
written in the exact order of the enum perf_event_sample_format. Otherwise
there is no way to parse this in the right order without looking at the kernel
source code, which is not the right way....


> 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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