Re: [PATCH] perf evsel: Don't set sample_regs_intr/sample_regs_user for dummy event

[Jin, Yao]

Hi Jiri,

On 7/3/2020 7:00 PM, Jiri Olsa wrote:

On Fri, Jul 03, 2020 at 08:42:15AM +0800, Jin Yao wrote:

Since commit 0a892c1c9472 ("perf record: Add dummy event during system wide 
synthesis"),
a dummy event is added to capture mmaps.

But if we run perf-record as,

  # perf record -e cycles:p -IXMM0 -a -- sleep 1
  Error:
  dummy:HG: PMU Hardware doesn't support sampling/overflow-interrupts. Try 
'perf stat'

The issue is, if we enable the extended regs (-IXMM0), but the
pmu->capabilities is not set with PERF_PMU_CAP_EXTENDED_REGS, the kernel
will return -EOPNOTSUPP error.

See following code pieces.

/* in kernel/events/core.c */
static int perf_try_init_event(struct pmu *pmu, struct perf_event *event)

{
        ....
        if (!(pmu->capabilities & PERF_PMU_CAP_EXTENDED_REGS) &&
            has_extended_regs(event))
                ret = -EOPNOTSUPP;
        ....
}

For software dummy event, the PMU should be not set with
PERF_PMU_CAP_EXTENDED_REGS. But unfortunately in current code, the dummy
event has possibility to be set with PERF_REG_EXTENDED_MASK bit.

In evsel__config, /* tools/perf/util/evsel.c */

if (opts->sample_intr_regs) {
        attr->sample_regs_intr = opts->sample_intr_regs;
}

If we use -IXMM0, the attr>sample_regs_intr will be set with
PERF_REG_EXTENDED_MASK bit.

It doesn't make sense to set attr->sample_regs_intr for a
software dummy event.

This patch adds dummy event checking before setting
attr->sample_regs_intr.

After:
   # ./perf record -e cycles:p -IXMM0 -a -- sleep 1
   [ perf record: Woken up 1 times to write data ]
   [ perf record: Captured and wrote 0.413 MB perf.data (45 samples) ]

LGTM, Adrian (cc-ed) just added another check to the same place,
but it looks like both of them should be there:

   https://lore.kernel.org/lkml/20200630133935.11150-2-adrian.hun...@intel.com/

jirka

Thanks Jiri! Yes, it looks like both of checks should be added here.

So do I post v2 (just rebase) once Adrian's patch gets merged?

Thanks
Jin Yao

Fixes: 0a892c1c9472 ("perf record: Add dummy event during system wide 
synthesis")
Signed-off-by: Jin Yao <yao....@linux.intel.com>
---
  tools/perf/util/evsel.c | 4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 96e5171dce41..df3315543e86 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -1020,12 +1020,12 @@ void evsel__config(struct evsel *evsel, struct 
record_opts *opts,
        if (callchain && callchain->enabled && !evsel->no_aux_samples)
                evsel__config_callchain(evsel, opts, callchain);

- if (opts->sample_intr_regs) {

+       if (opts->sample_intr_regs && !is_dummy_event(evsel)) {
                attr->sample_regs_intr = opts->sample_intr_regs;
                evsel__set_sample_bit(evsel, REGS_INTR);
        }

- if (opts->sample_user_regs) {

+       if (opts->sample_user_regs && !is_dummy_event(evsel)) {
                attr->sample_regs_user |= opts->sample_user_regs;
                evsel__set_sample_bit(evsel, REGS_USER);
        }
--
2.17.1