Re: perf stat issue with 7.0.0rc3

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Tue, Mar 17, 2026 at 04:56:51PM -0300, Arnaldo Carvalho de Melo wrote:
> It is not trying PERF_COUNT_HW_STALLED_CYCLES_FRONTEND, is asking for
> PERF_COUNT_HW_STALLED_CYCLES_BACKEND instead...

If I instead ask just for stalled-cycles-frontend and
stalled-cycles-backend:

root@number:~# strace -e perf_event_open perf stat -e stalled-cycles-frontend,stalled-cycles-backend sleep 1
perf_event_open({type=PERF_TYPE_RAW, size=PERF_ATTR_SIZE_VER9, config=0xa9, sample_period=0, sample_type=PERF_SAMPLE_IDENTIFIER, read_format=PERF_FORMAT_TOTAL_TIME_ENABLED|PERF_FORMAT_TOTAL_TIME_RUNNING, disabled=1, inherit=1, enable_on_exec=1, precise_ip=0 /* arbitrary skid */, ...}, 250619, -1, -1, PERF_FLAG_FD_CLOEXEC) = 3
perf_event_open({type=PERF_TYPE_HARDWARE, size=PERF_ATTR_SIZE_VER9, config=PERF_COUNT_HW_STALLED_CYCLES_BACKEND, sample_period=0, sample_type=PERF_SAMPLE_IDENTIFIER, read_format=PERF_FORMAT_TOTAL_TIME_ENABLED|PERF_FORMAT_TOTAL_TIME_RUNNING, disabled=1, inherit=1, enable_on_exec=1, precise_ip=0 /* arbitrary skid */, ...}, 250619, -1, -1, PERF_FLAG_FD_CLOEXEC) = -1 ENOENT (No such file or directory)
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=250619, si_uid=0, si_status=0, si_utime=0, si_stime=0} ---

 Performance counter stats for 'sleep 1':

           409,276      stalled-cycles-frontend
   <not supported>      stalled-cycles-backend

       1.000428804 seconds time elapsed

       0.000439000 seconds user
       0.000000000 seconds sys


--- SIGCHLD {si_signo=SIGCHLD, si_code=SI_USER, si_pid=250618, si_uid=0} ---
+++ exited with 0 +++
root@number:~#

It used type=PERF_TYPE_RAW, config=0xa9 for  stalled-cycles-frontend but
type=PERF_TYPE_HARDWARE, config=PERF_COUNT_HW_STALLED_CYCLES_BACKEND.

⬢ [acme@toolbx perf-tools]$ git grep stalled-cycles-frontend tools
tools/bpf/bpftool/link.c:       [PERF_COUNT_HW_STALLED_CYCLES_FRONTEND] = "stalled-cycles-frontend",
tools/perf/builtin-stat.c:     3,856,436,920 stalled-cycles-frontend   #   74.09% frontend cycles idle
tools/perf/pmu-events/arch/common/common/legacy-hardware.json:    "EventName": "stalled-cycles-frontend",
tools/perf/pmu-events/empty-pmu-events.c:/* offset=122795 */ "stalled-cycles-frontend\000legacy hardware\000Stalled cycles during issue [This event is an alias of idle-cycles-frontend]\000legacy-hardware-config=7\000\00000\000\000\000\000\000"
tools/perf/pmu-events/empty-pmu-events.c:/* offset=122945 */ "idle-cycles-frontend\000legacy hardware\000Stalled cycles during issue [This event is an alias of stalled-cycles-frontend]\000legacy-hardware-config=7\000\00000\000\000\000\000\000"
tools/perf/pmu-events/empty-pmu-events.c:{ 122795 }, /* stalled-cycles-frontend\000legacy hardware\000Stalled cycles during issue [This event is an alias of idle-cycles-frontend]\000legacy-hardware-config=7\000\00000\000\000\000\000\000 */
tools/perf/tests/shell/stat+std_output.sh:event_name=(cpu-clock task-clock context-switches cpu-migrations page-faults stalled-cycles-frontend stalled-cycles-backend cycles instructions branches branch-misses)
tools/perf/util/evsel.c:        "stalled-cycles-frontend",
⬢ [acme@toolbx perf-tools]$

This machine is:

⬢ [acme@toolbx perf-tools]$ grep -m1 "model name" /proc/cpuinfo
model name	: AMD Ryzen 9 9950X3D 16-Core Processor
⬢ [acme@toolbx perf-tools]

And doesn't have PERF_COUNT_HW_STALLED_CYCLES_BACKEND, but has
PERF_COUNT_HW_STALLED_CYCLES_FRONTEND, that gets configured using
PERF_TYPE_RAW and 0xa9 because:

root@number:~# cat /sys/devices/cpu/events/stalled-cycles-frontend
event=0xa9
root@number:~#

But I couldn't so far explain why in the default case it is asking for
PERF_COUNT_HW_STALLED_CYCLES_BACKEND, when it should be asking for
PERF_COUNT_HW_STALLED_CYCLES_FRONTEND or PERF_TYPE_RAW+config=0xa9...

- Arnaldo