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

[Ravi Bangoria <ravi.bangoria@linux.ibm.com>]

Hi Andi,

Sorry for being bit late.

On 3/3/20 7:03 AM, Andi Kleen wrote:
> On Mon, Mar 02, 2020 at 11:13:32AM +0100, Peter Zijlstra 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?
> 
> Intel PEBS has a similar concept for annotation of memory accesses,
> which is already exported through perf_mem_data_src. This is essentially
> an extension. It would be better to have something unified here.
> Right now it seems to duplicate at least part of the PEBS facility.

IIUC there is a distinction from perf mem vs exposing the pipeline details.
perf-mem/perf_mem_data_src is more of memory accesses profiling. And proposal
here is to expose pipeline related details like stalls and latencies. Would
prefer/suggest not to extend the current structure further to capture pipeline
details.

Ravi