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

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Date:   Tue, 19 Nov 2019 17:25:45 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        vitaly.slobodskoy@...el.com, pavel.gerasimov@...el.com,
        Andi Kleen <ak@...ux.intel.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH V4 01/13] perf/core: Add new branch sample type for LBR
 TOS



On 11/19/2019 2:02 PM, Stephane Eranian wrote:
> On Tue, Nov 19, 2019 at 6:35 AM<kan.liang@...ux.intel.com>  wrote:
>> From: Kan Liang<kan.liang@...ux.intel.com>
>>
>> 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@...ux.intel.com>
>> ---
>>   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