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Subject[PATCH v2] perf test x86: address multiplexing in rdpmc test
From
Counters may be being used for pinned or other events which inhibit the
instruction counter in the test from being scheduled - time_enabled > 0
but time_running == 0. This causes the test to fail with division by 0.
Make time_running an out parameter of mmap_read_self and add a sleep loop
to ensure that the counter is running before computing the delta.

v2. Address review feedback from Peter Zijlstra.

Signed-off-by: Ian Rogers <irogers@google.com>
---
tools/perf/arch/x86/tests/rdpmc.c | 57 +++++++++++++++++++++++--------
1 file changed, 42 insertions(+), 15 deletions(-)

diff --git a/tools/perf/arch/x86/tests/rdpmc.c b/tools/perf/arch/x86/tests/rdpmc.c
index 1ea916656a2d..64145d4e3d4d 100644
--- a/tools/perf/arch/x86/tests/rdpmc.c
+++ b/tools/perf/arch/x86/tests/rdpmc.c
@@ -34,20 +34,33 @@ static u64 rdtsc(void)
return low | ((u64)high) << 32;
}

-static u64 mmap_read_self(void *addr)
+/*
+ * Return a user rdpmc result. The kernel may be multiplexing events and so the
+ * result is scaled to the period of time the counter was enabled. The
+ * time_running out parameter is set to the amount of time the counter has been
+ * running. The result will be zero if time_running is zero.
+ */
+static u64 mmap_read_self(void *addr, u64 *running)
{
struct perf_event_mmap_page *pc = addr;
u32 seq, idx, time_mult = 0, time_shift = 0;
- u64 count, cyc = 0, time_offset = 0, enabled, running, delta;
+ u64 count, cyc = 0, time_offset = 0, enabled, delta;

do {
seq = pc->lock;
barrier();

enabled = pc->time_enabled;
- running = pc->time_running;
-
- if (enabled != running) {
+ *running = pc->time_running;
+
+ if (*running == 0) {
+ /*
+ * Counter won't have a value as due to multiplexing the
+ * event wasn't scheduled.
+ */
+ return 0;
+ }
+ if (enabled != *running) {
cyc = rdtsc();
time_mult = pc->time_mult;
time_shift = pc->time_shift;
@@ -62,7 +75,7 @@ static u64 mmap_read_self(void *addr)
barrier();
} while (pc->lock != seq);

- if (enabled != running) {
+ if (enabled != *running) {
u64 quot, rem;

quot = (cyc >> time_shift);
@@ -72,11 +85,11 @@ static u64 mmap_read_self(void *addr)

enabled += delta;
if (idx)
- running += delta;
+ *running += delta;

- quot = count / running;
- rem = count % running;
- count = quot * enabled + (rem * enabled) / running;
+ quot = count / *running;
+ rem = count % *running;
+ count = quot * enabled + (rem * enabled) / *running;
}

return count;
@@ -104,7 +117,7 @@ static int __test__rdpmc(void)
.config = PERF_COUNT_HW_INSTRUCTIONS,
.exclude_kernel = 1,
};
- u64 delta_sum = 0;
+ u64 delta_sum = 0, sleep_count = 0;
struct sigaction sa;
char sbuf[STRERR_BUFSIZE];

@@ -130,14 +143,23 @@ static int __test__rdpmc(void)
}

for (n = 0; n < 6; n++) {
- u64 stamp, now, delta;
-
- stamp = mmap_read_self(addr);
+ u64 stamp, now, delta, running;
+
+ for (;;) {
+ stamp = mmap_read_self(addr, &running);
+ if (running != 0)
+ break;
+ /* Try to wait for event to be running. */
+ sleep_count++;
+ if (sleep_count > 60)
+ goto out_never_run;
+ sleep(1);
+ }

for (i = 0; i < loops; i++)
tmp++;

- now = mmap_read_self(addr);
+ now = mmap_read_self(addr, &running);
loops *= 10;

delta = now - stamp;
@@ -155,6 +177,11 @@ static int __test__rdpmc(void)
return -1;

return 0;
+
+out_never_run:
+ close(fd);
+ pr_err("Event counter failed to multiplexed in. Are higher priority counters being sampled by a different process?\n");
+ return -1;
}

int test__rdpmc(struct test *test __maybe_unused, int subtest __maybe_unused)
--
2.25.1.696.g5e7596f4ac-goog
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