Question on Intel® Xeon® uncore performance counter IIO_CREDITS_REJECT

k1234 — Tue, 12 Aug 2025 04:12:46 GMT

Hello,

I am trying to understand peer-to-peer (P2P) transmission behavior on an Intel® Xeon® Scalable processor (Intel® Xeon® Gold 6526Y) using uncore performance counters. I am focusing on the IIO_CREDITS_REJECT event (EventCode=0x34).

According to the document and the GitHub repo, there are 3 documented umasks for this event:

UMask=0x08 → UNC_M2P_IIO_CREDITS_REJECT.DRS
UMask=0x10 → UNC_M2P_IIO_CREDITS_REJECT.NCB
UMask=0x20 → UNC_M2P_IIO_CREDITS_REJECT.NCS

I ran an application on socket 1 that generates P2P traffic and collected performance counter data with the following command:

$ sudo perf stat -a --per-socket \ -e 'uncore_m2pcie_1/event=0x34,umask=0x08/' \ -e 'uncore_m2pcie_1/event=0x34,umask=0x10/' \ -e 'uncore_m2pcie_1/event=0x34,umask=0x20/' \ -e 'uncore_m2pcie_1/event=0x34,umask=0xff/' \ -- sleep 10 Performance counter stats for 'system wide': S0 1 0 uncore_m2pcie_1/event=0x34,umask=0x08/ S0 1 0 uncore_m2pcie_1/event=0x34,umask=0x10/ S0 1 0 uncore_m2pcie_1/event=0x34,umask=0x20/ S0 1 0 uncore_m2pcie_1/event=0x34,umask=0xff/ S1 1 0 uncore_m2pcie_1/event=0x34,umask=0x08/ S1 1 0 uncore_m2pcie_1/event=0x34,umask=0x10/ S1 1 0 uncore_m2pcie_1/event=0x34,umask=0x20/ S1 1 44,001,744,822 uncore_m2pcie_1/event=0x34,umask=0xff/ 10.000831997 seconds time elapsed

I was trying to use umask=0xff to aggregate all sub-events. After narrowing things down, I found that umask=0x02 alone reproduces the large count:

$ sudo perf stat -a --per-socket \ -e 'uncore_m2pcie_1/event=0x34,umask=0x08/' \ -e 'uncore_m2pcie_1/event=0x34,umask=0x10/' \ -e 'uncore_m2pcie_1/event=0x34,umask=0x20/' \ -e 'uncore_m2pcie_1/event=0x34,umask=0x02/' \ -- sleep 10 Performance counter stats for 'system wide': S0 1 0 uncore_m2pcie_1/event=0x34,umask=0x08/ S0 1 0 uncore_m2pcie_1/event=0x34,umask=0x10/ S0 1 0 uncore_m2pcie_1/event=0x34,umask=0x20/ S0 1 0 uncore_m2pcie_1/event=0x34,umask=0x02/ S1 1 0 uncore_m2pcie_1/event=0x34,umask=0x08/ S1 1 0 uncore_m2pcie_1/event=0x34,umask=0x10/ S1 1 0 uncore_m2pcie_1/event=0x34,umask=0x20/ S1 1 44,004,918,932 uncore_m2pcie_1/event=0x34,umask=0x02/ 10.000857627 seconds time elapsed

Does the subevent uncore_m2pcie_1/event=0x34,umask=0x02 mean something? Is there any other document related with it?

Thanks!

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Question on Intel® Xeon® uncore performance counter IIO_CREDITS_REJECT