[PATCH v2] perf test x86: address multiplexing in rdpmc test

[Ian Rogers]

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@xxxxxxxxxx>
---
 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