Re: [PATCH] perf record: enable multiplexing scaling via -R

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Aug 30, 2017 at 11:21:23PM -0700, Stephane Eranian wrote:
> 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.

hum, how about we introduce new modifier to attach timing info, like:
  $ perf record -e cycles:T ....

modifiers might be scares resource, but we don't add them every day,
and this requirement looks generic

jirka