G'Day perf users,
I just debugged something and wanted to share in case others hit
this... perf CPU profiling didn't work for this Linux 4.1 guest (AWS
EC2):
# ./perf record -F 99 -a -g -- sleep 5
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.300 MB perf.data ]
# ./perf script
#
There are no CPU samples recorded (perf.data just has
PERF_RECORD_MMAP, etc). I believe it's because it's using the "cycles"
PMC event, and not "cpu-clock". And cycles is bogus:
# ./perf stat -e cycles -a sleep 1 2>&1 | grep cycles
68,719,476,704 cycles
# ./perf stat -e cycles -a sleep 2 2>&1 | grep cycles
68,719,476,704 cycles
I can fix it by profiling cpu-clock:
# ./perf record -e cpu-clock -a -g -- sleep 5
[ perf record: Woken up 27 times to write data ]
[ perf record: Captured and wrote 9.884 MB perf.data (105989 samples) ]
# ./perf script
[...data...]
Which is what I think all my other systems are doing anyway; some perf
-v -v output from those other systems:
[...]
sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8
sys_perf_event_open failed, error -2
Warning:
The cycles event is not supported, trying to fall back to cpu-clock-ticks
[...]
This 4.1 system has these:
# cat /sys/devices/cpu/events/cpu-cycles
event=0x3c
Which other systems don't. Profiling these (eg, -e r003c) also doesn't work.
So I can use "-e cpu-clock", which is fine. It even lets me set the
profile frequency:
# ./perf record -F 99 -e cpu-clock -a -- sleep 1
Maybe this issue is already known and fixed in latest. I couldn't find
anything in a search...
Brendan
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