Commit ba113eca authored by James Clark's avatar James Clark Committed by Will Deacon
Browse files

perf docs: arm_spe: Document new discard mode 



Document the flag along with PMU events to hint what it's used for and
give an example with other useful options to get minimal output.

Reviewed-by: default avatarYeoreum Yun <yeoreum.yun@arm.com>
Signed-off-by: default avatarJames Clark <james.clark@linaro.org>
Link: https://lore.kernel.org/r/20250108142904.401139-3-james.clark@linaro.org


Signed-off-by: default avatarWill Deacon <will@kernel.org>
parent d28d95bc

tools/perf/Documentation/perf-arm-spe.txt

+26 −0
Original line number Diff line number Diff line
@@ -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 +221,31 @@ Common errors 


   Increase sampling interval (see above)



PMU events

~~~~~~~~~~



SPE has events that can be counted on core PMUs. These are prefixed with

SAMPLE_, for example SAMPLE_POP, SAMPLE_FEED, SAMPLE_COLLISION and

SAMPLE_FEED_BR.



These events will only count when an SPE event is running on the same core that

the PMU event is opened on, otherwise they read as 0. There are various ways to

ensure that the PMU event and SPE event are scheduled together depending on the

way the event is opened. For example opening both events as per-process events

on the same process, although it's not guaranteed that the PMU event is enabled

first when context switching. For that reason it may be better to open the PMU

event as a systemwide event and then open SPE on the process of interest.



Discard mode

~~~~~~~~~~~~



SPE related (SAMPLE_* etc) core 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



SEE ALSO

--------