Re: [RFC 00/11] perf: Enhancing perf to export processor hazard information

[maddy <maddy@linux.ibm.com>]

On 3/3/20 1:51 AM, Stephane Eranian wrote:
> On Mon, Mar 2, 2020 at 2:13 AM Peter Zijlstra <peterz@infradead.org> wrote:
>> On Mon, Mar 02, 2020 at 10:53:44AM +0530, Ravi Bangoria wrote:
>>> Modern processors export such hazard data in Performance
>>> Monitoring Unit (PMU) registers. Ex, 'Sampled Instruction Event
>>> Register' on IBM PowerPC[1][2] and 'Instruction-Based Sampling' on
>>> AMD[3] provides similar information.
>>>
>>> Implementation detail:
>>>
>>> A new sample_type called PERF_SAMPLE_PIPELINE_HAZ is introduced.
>>> If it's set, kernel converts arch specific hazard information
>>> into generic format:
>>>
>>>    struct perf_pipeline_haz_data {
>>>           /* Instruction/Opcode type: Load, Store, Branch .... */
>>>           __u8    itype;
>>>           /* Instruction Cache source */
>>>           __u8    icache;
>>>           /* Instruction suffered hazard in pipeline stage */
>>>           __u8    hazard_stage;
>>>           /* Hazard reason */
>>>           __u8    hazard_reason;
>>>           /* Instruction suffered stall in pipeline stage */
>>>           __u8    stall_stage;
>>>           /* Stall reason */
>>>           __u8    stall_reason;
>>>           __u16   pad;
>>>    };
>> Kim, does this format indeed work for AMD IBS?
>
> Personally, I don't like the term hazard. This is too IBM Power
> specific. We need to find a better term, maybe stall or penalty.

Yes, names can be reworked and thinking more on it, how about these
as "pipeline" data instead of "hazard" data.

> Also worth considering is the support of ARM SPE (Statistical
> Profiling Extension) which is their version of IBS.
> Whatever gets added need to cover all three with no limitations.

Thanks for pointing this out. We looked at the ARM SPE spec and it does
provides information like issue latency, translation latency so on.
And AMD IBS provides data like fetch latency, tag to retire latency,
completion to retire latency and so on when using Fetch sampling.
  So yes, will rework the struct definition to include data from ARM SPE
and AMD IBS also. Will post out a newer version soon.

Thanks for the comments
Maddy