[PATCH 5/5] perf docs: arm_spe: Document new discard mode

[Ian Rogers]

On Tue, Dec 17, 2024 at 3:56 AM James Clark <james.clark at linaro.org> wrote:
>
> Document the flag, hint what it's used for and give an example with
> other useful options to get minimal output.
>
> Signed-off-by: James Clark <james.clark at linaro.org>
> ---
>  tools/perf/Documentation/perf-arm-spe.txt | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/tools/perf/Documentation/perf-arm-spe.txt b/tools/perf/Documentation/perf-arm-spe.txt
> index de2b0b479249..588eead438bc 100644
> --- a/tools/perf/Documentation/perf-arm-spe.txt
> +++ b/tools/perf/Documentation/perf-arm-spe.txt
> @@ -150,6 +150,7 @@ arm_spe/load_filter=1,min_latency=10/'
>    pct_enable=1        - collect physical timestamp instead of virtual timestamp (PMSCR.PCT) - requires privilege
>    store_filter=1      - collect stores only (PMSFCR.ST)
>    ts_enable=1         - enable timestamping with value of generic timer (PMSCR.TS)
> +  discard=1           - enable SPE PMU events but don't collect sample data - see 'Discard mode' (PMBLIMITR.FM = DISCARD)
>
>  +++*+++ Latency is the total latency from the point at which sampling started on that instruction, rather
>  than only the execution latency.
> @@ -220,6 +221,16 @@ Common errors
>
>     Increase sampling interval (see above)
>
> +Discard mode
> +~~~~~~~~~~~~
> +
> +SPE PMU events can be used without the overhead of collecting sample data if
> +discard mode is supported (optional from Armv8.6). First run a system wide SPE
> +session (or on the core of interest) using options to minimize output. Then run
> +perf stat:
> +
> +  perf record -e arm_spe/discard/ -a -N -B --no-bpf-event -o - > /dev/null &
> +  perf stat -e SAMPLE_FEED_LD

Perhaps clarify this should be an ARM SPE event? It seems strange to
have one perf command affect a later one, the purpose of things like
event multiplexing is to hide the hardware limits. I'd prefer if the
last bit was like:
```
Then run perf stat with an SPE event on the same PMU:

perf record -e arm_spe/discard/ -a -N -B --no-bpf-event -o - > /dev/null &
perf stat -e arm_spe/SAMPLE_FEED_LD/
``

Thanks,
Ian