On 7/29/15 1:04 PM, Brendan Gregg wrote:
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...
It's a known problem with VMs (Solaris based ldoms and apparently xen guests as examples). The hypervisor tells perf it has a PMU (see dmesg | grep Performance) but does not really implement it (no NMIs for example).
David -- To unsubscribe from this list: send the line "unsubscribe linux-perf-users" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
