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SubjectRe: [PATCH] perf record: enable multiplexing scaling via -R
Hi,


On Mon, Aug 28, 2017 at 1:41 PM, Andi Kleen <andi@firstfloor.org> wrote:
>> So I think we are good to go. to capture multiplexing scaling factor
>> when sampling simply use the S
>> modifier.
>> But to my surprise, newer kernels are not happy with the cmdline:
>> $ perf record -e cycles:S noploop 1
>> Error:
>> The sys_perf_event_open() syscall returned with 22 (Invalid argument)
>> for event (cycles:Su).
>> /bin/dmesg may provide additional information.
>> No CONFIG_PERF_EVENTS=y kernel support configured?
>
> Likely due to
>
> ba5213ae6b88 perf/core: Correct event creation with PERF_FORMAT_GROUP
>
> It's not supported with inherited events.
>
Yes, and other things have changed as well. I did a bit of research to
figure out how
to make this work out-of the-box with the latest perf (v4.13). It
turns out you need to
combine multiple options and an event modifier. This is quite cumbersome
but here it is:

$ perf record --no-inherit --running-time -e cycles:S ........

You need:
- no-inherit: the kernel does not know how to deal with multiplexing
when events are inherited
- running-time: this used to be automatic for PERF_SAMPLE_READ with
perf record, now it is not
This includes TIME_ENABLED/TIME_RUNNING in the sample_read format.
- :S : to add a PERF_SAMPLE_READ to each sample, it encapsulates the
event value + timings.
We do not care about the value but are only interested in the timings.

The kernel cannot record the timings without a PERF_SAMPLE_READ.

I am also surprised to see that perf record keep inherit=1 in
system-wide mode. I don't think this
is relavant in this mode. But the kernel this fails in this case,
which I think is a bug. In system-wide
mode, the attr-.no_inherit should be ignored. We can fix perf record
to avoid this in system-wide.

The cmdline above works for both per-thread and system-wide modes.

So I think we do not need my patch or variations thereof, everything
is there, though a bit difficult
to combine.

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