Re: [BUG] perf/x86/intel: HitM false-positives on Ice Lake / Tiger Lake (I think?)

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2024-02-22 3:07 p.m., Jann Horn wrote:
> On Thu, Feb 22, 2024 at 9:05 PM Liang, Kan <kan.liang@linux.intel.com> wrote:
>>
>> Hi Jann,
>>
>> Sorry for the late response.
>>
>> On 2024-02-20 10:42 a.m., Arnaldo Carvalho de Melo wrote:
>>> Just adding Joe Mario to the CC list.
>>>
>>> On Mon, Feb 19, 2024 at 03:20:00PM -0800, Ian Rogers wrote:
>>>> On Mon, Feb 19, 2024 at 5:01 AM Jann Horn <jannh@google.com> wrote:
>>>>>
>>>>> Hi!
>>>>>
>>>>> From what I understand, "perf c2c" shows bogus HitM events on Ice Lake
>>>>> (and newer) because Intel added some feature where *clean* cachelines
>>>>> can get snoop-forwarded ("cross-core FWD"), and the PMU apparently
>>>>> treats this mostly the same as snoop-forwarding of modified cache
>>>>> lines (HitM)? On a Tiger Lake CPU, I can see addresses from the kernel
>>>>> rodata section in "perf c2c report".
>>>>>
>>>>> This is mentioned in the SDM, Volume 3B, section "20.9.7 Load Latency
>>>>> Facility", table "Table 20-101. Data Source Encoding for Memory
>>>>> Accesses (Ice Lake and Later Microarchitectures)", encoding 07H:
>>>>> "XCORE FWD. This request was satisfied by a sibling core where either
>>>>> a modified (cross-core HITM) or a non-modified (cross-core FWD)
>>>>> cache-line copy was found."
>>>>>
>>>>> I don't see anything about this in arch/x86/events/intel/ds.c - if I
>>>>> understand correctly, the kernel's PEBS data source decoding assumes
>>>>> that 0x07 means "L3 hit, snoop hitm" on these CPUs. I think this needs
>>>>> to be adjusted somehow - and maybe it just isn't possible to actually
>>>>> distinguish between HitM and cross-core FWD in PEBS events on these
>>>>> CPUs (without big-hammer chicken bit trickery)? Maybe someone from
>>>>> Intel can clarify?
>>>>>
>>>>> (The SDM describes that E-cores on the newer 12th Gen have more
>>>>> precise PEBS encodings that distinguish between "L3 HITM" and "L3
>>>>> HITF"; but I guess the P-cores there maybe still don't let you
>>>>> distinguish HITM/HITF?)
>>
>> Right, there is no way to distinguish HITM/HITF on Tiger Lake.
> 
> Aah, okay, thank you very much for the clarification!
> 
>> I think what we can do is to add both HITM and HITF for the 0x07 to
>> match the SDM description.
>>
>> How about the below patch (not tested yet)?
>> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
>> index d49d661ec0a7..8c966b5b23cb 100644
>> --- a/arch/x86/events/intel/ds.c
>> +++ b/arch/x86/events/intel/ds.c
>> @@ -84,7 +84,7 @@ static u64 pebs_data_source[] = {
>>         OP_LH | P(LVL, L3)  | LEVEL(L3) | P(SNOOP, NONE),  /* 0x04: L3 hit */
>>         OP_LH | P(LVL, L3)  | LEVEL(L3) | P(SNOOP, MISS),  /* 0x05: L3 hit,
>> snoop miss */
>>         OP_LH | P(LVL, L3)  | LEVEL(L3) | P(SNOOP, HIT),   /* 0x06: L3 hit,
>> snoop hit */
>> -       OP_LH | P(LVL, L3)  | LEVEL(L3) | P(SNOOP, HITM),  /* 0x07: L3 hit,
>> snoop hitm */
>> +       OP_LH | P(LVL, L3)  | LEVEL(L3) | P(SNOOP, HITM) | P(SNOOPX, FWD),  /*
>> 0x07: L3 hit, snoop hitm & fwd */
>>         OP_LH | P(LVL, REM_CCE1) | REM | LEVEL(L3) | P(SNOOP, HIT),  /* 0x08:
>> L3 miss snoop hit */
>>         OP_LH | P(LVL, REM_CCE1) | REM | LEVEL(L3) | P(SNOOP, HITM), /* 0x09:
>> L3 miss snoop hitm*/
>>         OP_LH | P(LVL, LOC_RAM)  | LEVEL(RAM) | P(SNOOP, HIT),       /* 0x0a:
>> L3 miss, shared */
> 
> (I'm not familiar enough with the perf semantics to know how the event
> encoding works, maybe someone else can have a look?)
>

I can do the test to verify the settings and perf c2c. But I don't have
a benchmark. Could you please share your benchmark with me?
For example, the data you used in your example.
# perf record -e mem_load_l3_hit_retired.xsnp_fwd:ppp --all-kernel -c
100 --data

Thanks,
Kan
>>
>>
>>>>>
>>>>>
>>>>> I think https://perfmon-events.intel.com/tigerLake.html is also
>>>>> outdated, or at least it uses ambiguous grammar: The
>>>>> MEM_LOAD_L3_HIT_RETIRED.XSNP_FWD event (EventSel=D2H UMask=04H) is
>>>>> documented as "Counts retired load instructions where a cross-core
>>>>> snoop hit in another cores caches on this socket, the data was
>>>>> forwarded back to the requesting core as the data was modified
>>>>> (SNOOP_HITM) or the L3 did not have the data(SNOOP_HIT_WITH_FWD)" -
>>>>> from what I understand, a "cross-core FWD" should be a case where the
>>>>> L3 does have the data, unless L3 has become non-inclusive on Ice Lake?
>>>>>
>>
>> For the event, the BriefDescription in the event list json file gives a
>> more accurate description.
>> "BriefDescription": "Snoop hit a modified(HITM) or clean line(HIT_W_FWD)
>> in another on-pkg core which forwarded the data back due to a retired
>> load instruction.",
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/pmu-events/arch/x86/tigerlake/cache.json#n286
> 
> Ah, right, that's clearer.
>