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Message-ID: <Z8JRC2oSs8i53t_s@google.com>
Date: Fri, 28 Feb 2025 16:12:59 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Thomas Richter <tmricht@...ux.ibm.com>, Ian Rogers <irogers@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
linux-perf-users@...r.kernel.org, acme@...nel.org,
agordeev@...ux.ibm.com, gor@...ux.ibm.com, sumanthk@...ux.ibm.com,
hca@...ux.ibm.com
Subject: Re: [PATCH] perf/test: Skip leader sampling for s390
Hello,
On Fri, Feb 28, 2025 at 07:22:41AM +0100, Thomas Richter wrote:
> In tree linux-next
> the perf test case 114 'perf record tests' has a subtest
> named 'Basic leader sampling test' which always fails on s390.
> Root cause is this invocation
>
> # perf record -vv -e '{cycles,cycles}:Su' -- perf test -w brstack
>
> ...
> In the debug output the following 2 event are installed:
>
> ------------------------------------------------------------
> perf_event_attr:
> type 0 (PERF_TYPE_HARDWARE)
> size 136
> config 0 (PERF_COUNT_HW_CPU_CYCLES)
> { sample_period, sample_freq } 4000
> sample_type IP|TID|TIME|READ|CPU|PERIOD|IDENTIFIER
> read_format ID|GROUP|LOST
> disabled 1
> exclude_kernel 1
> exclude_hv 1
> freq 1
> sample_id_all 1
> ------------------------------------------------------------
> sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8 = 5
> ------------------------------------------------------------
> perf_event_attr:
> type 0 (PERF_TYPE_HARDWARE)
> size 136
> config 0 (PERF_COUNT_HW_CPU_CYCLES)
> sample_type IP|TID|TIME|READ|CPU|PERIOD|IDENTIFIER
> read_format ID|GROUP|LOST
> exclude_kernel 1
> exclude_hv 1
> sample_id_all 1
> ------------------------------------------------------------
> sys_perf_event_open: pid -1 cpu 0 group_fd 5 flags 0x8 = 6
> ...
>
> The first event is the group leader and is installed as sampling event.
> The secound one is group member and is installed as counting event.
>
> Namhyung Kim confirms this observation:
> > Yep, the syntax '{event1,event2}:S' is for group leader sampling which
> > reduces the overhead of PMU interrupts. The idea is that those events
> > are scheduled together so sampling is enabled only for the leader
> > (usually the first) event and it reads counts from the member events
> > using PERF_SAMPLE_READ.
> >
> > So they should have the same counts if it uses the same events in a
> > group.
>
> However this does not work on s390. s390 has one dedicated sampling PMU
> which supports only one event. A different PMU is used for counting.
> Both run concurrently using different setups and frequencies.
>
> On s390x a sampling event is setup using a preset trigger and a large
> buffer. The hardware
> - writes a samples (64 bytes) into this buffer
> when a given number of CPU instructions has been executed.
> - and triggers an interrupt when the buffer gets full.
> The trigger has just a few possible values.
>
> On s390x the counting event cycles is used to read out the numer of
> CPU cycles executed.
>
> On s390 above invocation created 2 events executed on 2 different
> PMU and the result are diffent values from two independently running
> PMUs which do not match in a consistent and reliably as on Intel:
>
> # ./perf record -e '{cycles,cycles}:Su' -- perf test -w brstack
> ...
> # ./perf script
> perf 2799437 92568.845118: 5508000 cycles: 3ffbcb898b6 do_lookup_x+0x196
> perf 2799437 92568.845119: 1377000 cycles: 3ffbcb898b6 do_lookup_x+0x196
> perf 2799437 92568.845120: 4131000 cycles: 3ffbcb897e8 do_lookup_x+0xc8
> perf 2799437 92568.845121: 1377000 cycles: 3ffbcb8a37c _dl_lookup_symbol
> perf 2799437 92568.845122: 1377000 cycles: 3ffbcb89558 check_match+0x18
> perf 2799437 92568.845123: 2754000 cycles: 3ffbcb89b2a do_lookup_x+0x40a
> perf 2799437 92568.845124: 1377000 cycles: 3ffbcb89b1e do_lookup_x+0x3fe
>
> As can be seen the result match very often but not all the time
> make this test on s390 failing very, very often.
>
> This patch bypasses this test on s390.
>
> Output before:
> # ./perf test 114
> 114: perf record tests : FAILED!
> #
>
> Output after:
> # ./perf test 114
> 114: perf record tests : Ok
> #
>
> Signed-off-by: Thomas Richter <tmricht@...ux.ibm.com>
> Acked-by: Sumanth Korikkar <sumanthk@...ux.ibm.com>
Thanks for the fix. I think Ian saw the same problem on other archs
too. Maybe we need to enable it on supported archs only.
Thanks,
Namhyung
> ---
> tools/perf/tests/shell/record.sh | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/tools/perf/tests/shell/record.sh b/tools/perf/tests/shell/record.sh
> index ba8d873d3ca7..98b69820bc5f 100755
> --- a/tools/perf/tests/shell/record.sh
> +++ b/tools/perf/tests/shell/record.sh
> @@ -231,6 +231,12 @@ test_cgroup() {
>
> test_leader_sampling() {
> echo "Basic leader sampling test"
> + if [ "$(uname -m)" = s390x ]
> + then
> + echo "Leader sampling skipped"
> + ((skipped+=1))
> + return
> + fi
> if ! perf record -o "${perfdata}" -e "{cycles,cycles}:Su" -- \
> perf test -w brstack 2> /dev/null
> then
> --
> 2.45.2
>
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