linux-kernel - [PATCH] perf evsel: Don't set sample_regs_intr/sample_regs

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Message-Id: <20200703004215.24418-1-yao.jin@linux.intel.com>
Date:   Fri,  3 Jul 2020 08:42:15 +0800
From:   Jin Yao <yao.jin@...ux.intel.com>
To:     acme@...nel.org, jolsa@...nel.org, peterz@...radead.org,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com
Cc:     Linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kan.liang@...el.com, yao.jin@...el.com, irogers@...gle.com,
        Jin Yao <yao.jin@...ux.intel.com>
Subject: [PATCH] perf evsel: Don't set sample_regs_intr/sample_regs_user for dummy event

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) ]

Fixes: 0a892c1c9472 ("perf record: Add dummy event during system wide synthesis")
Signed-off-by: Jin Yao <yao.jin@...ux.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