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

Jann Horn
2024-02-22T20:08:01Z

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 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.