Re: Invalid event (cycles:pp) in per-thread mode, enable system wide with '-a'.

[Nick Desaulniers]

On Thu, Oct 6, 2022 at 8:56 PM Ravi Bangoria <ravi.bangoria@xxxxxxx> wrote:
>
> +cc: PeterZ
>
> >>>>> +Ravi who may be able to say if there are any issues with the precise
> >>>>> sampling on AMD.
> >>>>
> >>>> Afaik cvcles:pp will use IBS but it doesn't support per-task profiling
> >>>> since it has no task context.  Ravi is working on it..
> >>>
> >>> Right.
> >>> https://lore.kernel.org/lkml/20220829113347.295-1-ravi.bangoria@xxxxxxx
> >>
> >> Cool, thanks for working on this Ravi.
> >>
> >> I'm not sure yet whether I may replace the kernel on my corporate
> >> provided workstation, so I'm not sure yet I can help test that patch.
> >>
> >> Can you confirm that
> >> $ perf record -e cycles:pp --freq=128 --call-graph lbr -- <command to profile>
> >>
> >> works with just that patch applied? Or is there more work required?
> >> What is the status of that patch?
> >>
> >> For context, we had difficulty upstreaming support for instrumentation
> >> based profile guided optimizations in the Linux kernel.
> >> https://lore.kernel.org/lkml/CAHk-=whqCT0BeqBQhW8D-YoLLgp_eFY=8Y=9ieREM5xx0ef08w@xxxxxxxxxxxxxx/
> >> We'd like to be able to use either instrumentation or sampling to
> >> optimize our builds.  The major barrier to sample based approaches are
> >> architecture / micro architecture issues with sample based profile
> >> data collection, and bitrot of data processing utilities.
> >> https://github.com/google/autofdo/issues/144
> >
> > On existing AMD Zen2, Zen3 the following cmdline:
> > $ perf record -e cycles:pp --freq=128 --call-graph lbr -- <command to profile>
> >
> > does not work. I see two reasons:
> >
> > 1. cycles:pp is likely converted into IBS op in cycle mode.
> >     Current kernels do not support IBS in per-thread mode.
> >     This is purely a kernel limitation
>
> Right, it's purely a kernel limitation. And below simple patch on top
> of event-context rewrite patch[1] should be sufficient to make cycles:pp
> working in per-process mode on AMD Zen.
>
> ---
> diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
> index c251bc44c088..de01b5d27e40 100644
> --- a/arch/x86/events/amd/ibs.c
> +++ b/arch/x86/events/amd/ibs.c
> @@ -665,7 +665,7 @@ static struct perf_ibs perf_ibs_fetch = {
>
>  static struct perf_ibs perf_ibs_op = {
>         .pmu = {
> -               .task_ctx_nr    = perf_invalid_context,
> +               .task_ctx_nr    = perf_hw_context,
>
>                 .event_init     = perf_ibs_init,
>                 .add            = perf_ibs_add,
> ---
>
> [1]: https://lore.kernel.org/lkml/20220829113347.295-1-ravi.bangoria@xxxxxxx

Hi Ravi,
I didn't see the above diff in
https://lore.kernel.org/lkml/20221008062424.313-1-ravi.bangoria@xxxxxxx/
Was there another distinct patch you were going to send for the above?

-- 
Thanks,
~Nick Desaulniers