Re: [PATCH 02/19] perf: Add ability to attach user level registersdump to sample

[Stephane Eranian]

On Wed, Jun 13, 2012 at 3:23 PM, Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> On Wed, Jun 13, 2012 at 03:18:54PM +0200, Stephane Eranian wrote:
>> On Wed, Jun 13, 2012 at 3:12 PM, Jiri Olsa <jolsa@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxx>
>> >> > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
>> >> > ---
>> >> > Âinclude/linux/perf_event.h | Â 10 ++++++-
>> >> > Âkernel/events/core.c    |  61 ++++++++++++++++++++++++++++++++++++++++++++
>> >> > Â2 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 {
>> >> > Â Â Â Â Â Â Â Â__u64 Â Â Â Â Â bp_len;
>> >> > Â Â Â Â Â Â Â Â__u64 Â Â Â Â Â config2; /* extension of config1 */
>> >> > Â Â Â Â};
>> >> > - Â Â Â __u64 Â branch_sample_type; /* enum branch_sample_type */
>> >> > + Â Â Â __u64 Â branch_sample_type; /* enum perf_branch_sample_type */
>> >> > +
>> >> > + Â Â Â /*
>> >> > + Â Â Â Â* Defines set of user regs to dump on samples.
>> >> > + Â Â Â Â* See asm/perf_regs.h for details.
>> >> > + Â Â Â Â*/
>> >> > + Â Â Â __u64 Â sample_regs_user;
>> >> > Â};
>> >> 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. ÂLooking 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?
>
> after.. but only because I know that.. yep, I think you're right,
> we should track it in the sample_type enum.. I'll add those 2 bits
>
Good thanks.

> jirka
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