Re: [PATCH V4 01/13] perf/core: Add new branch sample type for LBR TOS

[Liang, Kan]

On 11/19/2019 2:02 PM, Stephane Eranian wrote:
> On Tue, Nov 19, 2019 at 6:35 AM<kan.liang@xxxxxxxxxxxxxxx>  wrote:
> > From: Kan Liang<kan.liang@xxxxxxxxxxxxxxx>
> >
> > In LBR call stack mode, the depth of reconstructed LBR call stack limits
> > to the number of LBR registers. With LBR Top-of-Stack (TOS) information,
> > perf tool may stitch the stacks of two samples. The reconstructed LBR
> > call stack can break the HW limitation.
> >
> > Add a new branch sample type to retrieve LBR TOS. The new type is PMU
> > specific. Add it at the end of enum perf_branch_sample_type.
> > Add a macro to retrieve defined bits of branch sample type.
> > Update perf_copy_attr() to handle the new bit.
> >
> > Only when the new branch sample type is set, the TOS information is
> > dumped into the PERF_SAMPLE_BRANCH_STACK output.
> > Perf tool should check the attr.branch_sample_type, and apply the
> > corresponding format for PERF_SAMPLE_BRANCH_STACK samples.
> > Otherwise, some user case may be broken. For example, users may parse a
> > perf.data, which include the new branch sample type, with an old version
> > perf tool (without the check). Users probably get incorrect information
> > without any warning.
> >
> > Signed-off-by: Kan Liang<kan.liang@xxxxxxxxxxxxxxx>
> > ---
> >   include/linux/perf_event.h      |  2 ++
> >   include/uapi/linux/perf_event.h | 16 ++++++++++++++--
> >   kernel/events/core.c            | 13 ++++++++++++-
> >   3 files changed, 28 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 011dcbdbccc2..761021c7ee8a 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -93,6 +93,7 @@ struct perf_raw_record {
> >   /*
> >    * branch stack layout:
> >    *  nr: number of taken branches stored in entries[]
> > + *  tos: Top-of-Stack (TOS) information. PMU specific data.
> >    *
> >    * Note that nr can vary from sample to sample
> >    * branches (to, from) are stored from most recent
> > @@ -101,6 +102,7 @@ struct perf_raw_record {
> >    */
> >   struct perf_branch_stack {
> >          __u64                           nr;
> > +       __u64                           tos; /* PMU specific data */
> >          struct perf_branch_entry        entries[0];
> >   };
> Same remark as with the other patch. You need to abstract this.
> The TOS and PMU specific data should be limited to  x86/event/intel/*.[ch].

If we change tos to a generic name, e.g. pmu_specific_data, can we still 
keep it here?

If not, I think the only way is to introduce a new method, e.g. 
output_br_pmu_data(), at struct pmu.
When outputting the sample data, the generic code will call 
event->pmu->output_br_pmu_data() to retrieve the TOS in Intel code.
I think it's too complicated.

Thanks,
Kan