perf record -e branch-misses ls > /dev/null

perf record -e branch-misses ls > /dev/null

[wangxiaolei]

hi

When I use the perf tool on nxp-imx6sx, the CPU is armv7 cortex-A9 to 
test, I execute the perf record -e branch-misses ls > /dev/null command, 
but the perf report result is indeed:
perf report
Error:
The perf.data data has no samples!
# To display the perf.data header info, please use 
--header/--header-only options.

root@nxp-imx6:~# perf list hardware

List of pre-defined events (to be used in -e):

    branch-instructions OR branches [Hardware event]
    branch-misses [Hardware event]
    bus-cycles [Hardware event]
    cache-misses [Hardware event]
    cache-references [Hardware event]
    cpu-cycles OR cycles [Hardware event]
    instructions [Hardware event]

And not only this one hardware event, only cycles are working normally 
in the following supported hardware time, other hardware events are not 
interrupted and reported, and the value in the read PMXEVCNTR register 
is always -1, and the PMCR register E, bit[0 ] it will be written to 0 
before reading the PMXEVCNTR register. I don’t know if the value in the 
PMXEVCNTR register is always -1 for this reason. Does anyone have any 
good suggestions for debugging this problem?


thanks

xiaolei

Re: perf record -e branch-misses ls > /dev/null

[Mark Rutland]

On Wed, Mar 08, 2023 at 03:56:41PM +0800, wangxiaolei wrote:
> hi
> 
> When I use the perf tool on nxp-imx6sx, the CPU is armv7 cortex-A9 to test,
> I execute the perf record -e branch-misses ls > /dev/null command, but the
> perf report result is indeed:
> perf report
> Error:
> The perf.data data has no samples!
> # To display the perf.data header info, please use --header/--header-only
> options.

Which kernel version are you seeing this with?

Is anything printed regarding the PMU in dmesg?

> root@nxp-imx6:~# perf list hardware
> 
> List of pre-defined events (to be used in -e):
> 
>    branch-instructions OR branches [Hardware event]
>    branch-misses [Hardware event]
>    bus-cycles [Hardware event]
>    cache-misses [Hardware event]
>    cache-references [Hardware event]
>    cpu-cycles OR cycles [Hardware event]
>    instructions [Hardware event]

That's just listing the generic HW events; does the PMU actually show up under
sysfs?

What do you see if you run:

$ ls /sys/bus/event_source/devices/

> And not only this one hardware event, only cycles are working normally in
> the following supported hardware time, other hardware events are not
> interrupted and reported

Can you explain what you mean by "not interrupted and reported"?

Do you mean the PMU interrupt isn't firing?

> , and the value in the read PMXEVCNTR register is
> always -1, and the PMCR register E, bit[0 ] it will be written to 0 before
> reading the PMXEVCNTR register.

How are you reading PMXEVCNTR?

Have you instrumented the PMU driver, or are you using other code?

> I don’t know if the value in the PMXEVCNTR register is always -1 for this
> reason. Does anyone have any good suggestions for debugging this problem?

There are a number of potential reasons for this; if you can answer some of my
questions above it'd help to narrow this down.

Thanks,
Mark.