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

On 3/13/20 4:08 AM, Kim Phillips wrote:
> On 3/11/20 11:00 AM, Ravi Bangoria wrote:
>> Hi Kim,
> Hi Ravi,
>> On 3/6/20 3:36 AM, Kim Phillips wrote:
>>>> On 3/3/20 3:55 AM, Kim Phillips wrote:
>>>>> On 3/2/20 2:21 PM, Stephane Eranian wrote:
>>>>>> On Mon, Mar 2, 2020 at 2:13 AM 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?
>>>>> It's not really 1:1, we don't have these separations of stages
>>>>> and reasons, for example: we have missed in L2 cache, for example.
>>>>> So IBS output is flatter, with more cycle latency figures than
>>>>> IBM's AFAICT.
>>>> AMD IBS captures pipeline latency data incase Fetch sampling like the
>>>> Fetch latency, tag to retire latency, completion to retire latency and
>>>> so on. Yes, Ops sampling do provide more data on load/store centric
>>>> information. But it also captures more detailed data for Branch instructions.
>>>> And we also looked at ARM SPE, which also captures more details pipeline
>>>> data and latency information.
>>>>>> 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.
>>>>> Right, IBS doesn't have a filter to only count stalled or otherwise
>>>>> bad events.  IBS' PPR descriptions has one occurrence of the
>>>>> word stall, and no penalty.  The way I read IBS is it's just
>>>>> reporting more sample data than just the precise IP: things like
>>>>> hits, misses, cycle latencies, addresses, types, etc., so words
>>>>> like 'extended', or the 'auxiliary' already used today even
>>>>> are more appropriate for IBS, although I'm the last person to
>>>>> bikeshed.
>>>> We are thinking of using "pipeline" word instead of Hazard.
>>> Hm, the word 'pipeline' occurs 0 times in IBS documentation.
>> NP. We thought pipeline is generic hw term so we proposed "pipeline"
>> word. We are open to term which can be generic enough.
>>> I realize there are a couple of core pipeline-specific pieces
>>> of information coming out of it, but the vast majority
>>> are addresses, latencies of various components in the memory
>>> hierarchy, and various component hit/miss bits.
>> Yes. we should capture core pipeline specific details. For example,
>> IBS generates Branch unit information(IbsOpData1) and Icahce related
>> data(IbsFetchCtl) which is something that shouldn't be extended as
>> part of perf-mem, IMO.
> Sure, IBS Op-side output is more 'perf mem' friendly, and so it
> should populate perf_mem_data_src fields, just like POWER9 can:
> union perf_mem_data_src {
> ...
> __u64 mem_rsvd:24,
> mem_snoopx:2, /* snoop mode, ext */
> mem_remote:1, /* remote */
> mem_lvl_num:4, /* memory hierarchy level number */
> mem_dtlb:7, /* tlb access */
> mem_lock:2, /* lock instr */
> mem_snoop:5, /* snoop mode */
> mem_lvl:14, /* memory hierarchy level */
> mem_op:5; /* type of opcode */
> E.g., SIER[LDST] SIER[A_XLATE_SRC] can be used to populate
> mem_lvl[_num], SIER_TYPE can be used to populate 'mem_op',
> 'mem_lock', and the Reload Bus Source Encoding bits can
> be used to populate mem_snoop, right?
Hi Kim,

Yes. We do expose these data as part of perf-mem for POWER.

> used for the ld/st target addresses, too.
>>> What's needed here is a vendor-specific extended
>>> sample information that all these technologies gather,
>>> of which things like e.g., 'L1 TLB cycle latency' we
>>> all should have in common.
>> Yes. We will include fields to capture the latency cycles (like Issue
>> latency, Instruction completion latency etc..) along with other pipeline
>> details in the proposed structure.
> Latency figures are just an example, and from what I
> can tell, struct perf_sample_data already has a 'weight' member,
> used with PERF_SAMPLE_WEIGHT, that is used by intel-pt to
> transfer memory access latency figures. Granted, that's
> a bad name given all other vendors don't call latency
> 'weight'.
> I didn't see any latency figures coming out of POWER9,
> and do not expect this patchseries to implement those
> of other vendors, e.g., AMD's IBS; leave each vendor
> to amend perf to suit their own h/w output please.

Reference structure proposed in this patchset did not have members
to capture latency info for that exact reason. But idea here is to
abstract  as vendor specific as possible. So if we include u16 array,
then this format can also capture data from IBS since it provides
few latency details.

> My main point there, however, was that each vendor should
> use streamlined record-level code to just copy the data
> in the proprietary format that their hardware produces,
> and then then perf tooling can synthesize the events
> from the raw data at report/script/etc. time.
>>> I'm not sure why a new PERF_SAMPLE_PIPELINE_HAZ is needed
>>> either.  Can we use PERF_SAMPLE_AUX instead?
>> We took a look at PERF_SAMPLE_AUX. IIUC, PERF_SAMPLE_AUX is intended when
>> large volume of data needs to be captured as part of without
>> frequent PMIs. But proposed type is to address the capture of pipeline
> SAMPLE_AUX shouldn't care whether the volume is large, or how frequent
> PMIs are, even though it may be used in those environments.
>> information on each sample using PMI at periodic intervals. Hence proposing
> And that's fine for any extra bits that POWER9 has to convey
> to its users beyond things already represented by other sample
> types like PERF_SAMPLE_DATA_SRC, but the capturing of both POWER9
> and other vendor e.g., AMD IBS data can be made vendor-independent
> at record time by using SAMPLE_AUX, or SAMPLE_RAW even, which is
> what IBS currently uses.

My bad. Not sure what you mean by this. We are trying to abstract
as much vendor specific data as possible with this (like perf-mem).

>>>  Take a look at
>>> commit 98dcf14d7f9c "perf tools: Add kernel AUX area sampling
>>> definitions".  The sample identifier can be used to determine
>>> which vendor's sampling IP's data is in it, and events can
>>> be recorded just by copying the content of the SIER, etc.
>>> registers, and then events get synthesized from the aux
>>> sample at report/inject/annotate etc. time.  This allows
>>> for less sample recording overhead, and moves all the vendor
>>> specific decoding and common event conversions for userspace
>>> to figure out.
>> When AUX buffer data is structured, tool side changes added to present the
>> pipeline data can be re-used.
> Not sure I understand: AUX data would be structured on
> each vendor's raw h/w register formats.
> Thanks,
> Kim
>>>>>> 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.
>>>>> I thought Intel's various LBR, PEBS, and PT supported providing
>>>>> similar sample data in perf already, like with perf mem/c2c?
>>>> perf-mem is more of data centric in my opinion. It is more towards
>>>> memory profiling. So proposal here is to expose pipeline related
>>>> details like stalls and latencies.
>>> Like I said, I don't see it that way, I see it as "any particular
>>> vendor's event's extended details', and these pipeline details
>>> have overlap with existing infrastructure within perf, e.g., L2
>>> cache misses.
>>> Kim

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