Re: Invalid event (cycles:pp) in per-thread mode, enable system wide with '-a'.

[Ravi Bangoria]

On 12-Oct-22 9:36 AM, Ravi Bangoria wrote:
> On 12-Oct-22 3:02 AM, Nick Desaulniers wrote:
>> On Thu, Oct 6, 2022 at 8:56 PM Ravi Bangoria <ravi.bangoria@xxxxxxx> 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@xxxxxxx
>>>>>
>>>>> 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@xxxxxxxxxxxxxx/
>>>>> 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@xxxxxxx
>>
>> Hi Ravi,
>> I didn't see the above diff in
>> https://lore.kernel.org/lkml/20221008062424.313-1-ravi.bangoria@xxxxxxx/
>> 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.

Hi Nick,

Since you have practical use case, would it be possible to run your workflow
with perf rewrite and IBS patches applied? It will help us in finding/fixing
more bugs and upstreaming these changes.

Thanks,
Ravi