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Date:   Wed, 12 Oct 2022 09:36:22 +0530
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Subject: Re: Invalid event (cycles:pp) in per-thread mode, enable system wide
 with '-a'.
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To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Stephane Eranian <eranian@google.com>,
        Slade Watkins <srw@sladewatkins.net>,
        linux-perf-users <linux-perf-users@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ian Rogers <irogers@google.com>,
        Namhyung Kim <namhyung@gmail.com>,
        Kees Cook <keescook@chromium.org>, sandipan.das@amd.com,
        Bill Wendling <morbo@google.com>,
        clang-built-linux <llvm@lists.linux.dev>,
        Yonghong Song <yhs@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ravi Bangoria <ravi.bangoria@amd.com>
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From:   Ravi Bangoria <ravi.bangoria@amd.com>
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On 12-Oct-22 3:02 AM, Nick Desaulniers wrote:
> On Thu, Oct 6, 2022 at 8:56 PM Ravi Bangoria <ravi.bangoria@amd.com> wrote:
>>
>> +cc: PeterZ
>>
>>>>>>> +Ravi who may be able to say if there are any issues with the precise
>>>>>>> sampling on AMD.
>>>>>>
>>>>>> Afaik cvcles:pp will use IBS but it doesn't support per-task profiling
>>>>>> since it has no task context.  Ravi is working on it..
>>>>>
>>>>> Right.
>>>>> https://lore.kernel.org/lkml/20220829113347.295-1-ravi.bangoria@amd.com
>>>>
>>>> Cool, thanks for working on this Ravi.
>>>>
>>>> I'm not sure yet whether I may replace the kernel on my corporate
>>>> provided workstation, so I'm not sure yet I can help test that patch.
>>>>
>>>> Can you confirm that
>>>> $ perf record -e cycles:pp --freq=128 --call-graph lbr -- <command to profile>
>>>>
>>>> works with just that patch applied? Or is there more work required?
>>>> What is the status of that patch?
>>>>
>>>> For context, we had difficulty upstreaming support for instrumentation
>>>> based profile guided optimizations in the Linux kernel.
>>>> https://lore.kernel.org/lkml/CAHk-=whqCT0BeqBQhW8D-YoLLgp_eFY=8Y=9ieREM5xx0ef08w@mail.gmail.com/
>>>> We'd like to be able to use either instrumentation or sampling to
>>>> optimize our builds.  The major barrier to sample based approaches are
>>>> architecture / micro architecture issues with sample based profile
>>>> data collection, and bitrot of data processing utilities.
>>>> https://github.com/google/autofdo/issues/144
>>>
>>> On existing AMD Zen2, Zen3 the following cmdline:
>>> $ perf record -e cycles:pp --freq=128 --call-graph lbr -- <command to profile>
>>>
>>> does not work. I see two reasons:
>>>
>>> 1. cycles:pp is likely converted into IBS op in cycle mode.
>>>     Current kernels do not support IBS in per-thread mode.
>>>     This is purely a kernel limitation
>>
>> Right, it's purely a kernel limitation. And below simple patch on top
>> of event-context rewrite patch[1] should be sufficient to make cycles:pp
>> working in per-process mode on AMD Zen.
>>
>> ---
>> diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
>> index c251bc44c088..de01b5d27e40 100644
>> --- a/arch/x86/events/amd/ibs.c
>> +++ b/arch/x86/events/amd/ibs.c
>> @@ -665,7 +665,7 @@ static struct perf_ibs perf_ibs_fetch = {
>>
>>  static struct perf_ibs perf_ibs_op = {
>>         .pmu = {
>> -               .task_ctx_nr    = perf_invalid_context,
>> +               .task_ctx_nr    = perf_hw_context,
>>
>>                 .event_init     = perf_ibs_init,
>>                 .add            = perf_ibs_add,
>> ---
>>
>> [1]: https://lore.kernel.org/lkml/20220829113347.295-1-ravi.bangoria@amd.com
> 
> Hi Ravi,
> I didn't see the above diff in
> https://lore.kernel.org/lkml/20221008062424.313-1-ravi.bangoria@amd.com/
> Was there another distinct patch you were going to send for the above?

Yes Nick. I was planning to send it once the rewrite stuff goes in.

Thanks,
Ravi