perf_self_monitor: Wait for a count to settle down at startup

Signed-off-by: Andy Lutomirski <luto@kernel.org>

1 files changed, 39 insertions, 0 deletions

diff --git a/tight_loop/perf_self_monitor.c b/tight_loop/perf_self_monitor.c
index 52d338b..f3de608 100644
--- a/tight_loop/perf_self_monitor.c
+++ b/tight_loop/perf_self_monitor.c
@@ -79,6 +79,10 @@ static inline uint64_t psm_rdpmc(unsigned int ecx)
 	return (((uint64_t)edx << 32) | eax);
 }
 
+typedef bool (*psm_sample_fn)(uint64_t *count,
+			      const struct psm_counter *counter,
+			      void *opaque);
+
 /*
  * psm_atomic is a mechanism for very precise counter measurement of
  * very short operations.  A psm_atomic interval will fail if the
@@ -140,10 +144,45 @@ static inline bool psm_atomic_elapsed(uint64_t *elapsed_count,
 	return true;
 }
 
+bool psm_atomic_sample_empty(uint64_t *count,
+			     const struct psm_counter *ctr, void *opaque)
+{
+	struct psm_atomic duration = psm_atomic_start(ctr);
+	return psm_atomic_elapsed(count, &duration, ctr);
+}
+
+void psm_settle(const struct psm_counter *ctr, psm_sample_fn fn, void *opaque)
+{
+	uint64_t val = UINT64_MAX;
+	int good_iters = 0;
+	for (int total_iters = 0; total_iters < 500; total_iters++) {
+		uint64_t best = UINT64_MAX;
+		for (int inner = 0; inner < 10; ) {
+			uint64_t count;
+			if (!fn(&count, ctr, opaque))
+				continue;  /* Rejected sample */
+			if (count < best)
+				best = count;
+			inner++;
+		}
+
+		if (best != val) {
+			good_iters = 1;
+			val = best;
+		} else {
+			good_iters++;
+			if (good_iters == 10)
+				return;
+		}
+	}
+}
+
 int main()
 {
 	struct psm_counter *ctr = psm_counter_create();
 
+	psm_settle(ctr, psm_atomic_sample_empty, NULL);
+
 	for (int i = 0; i < 20; i++) {
 		struct psm_atomic duration = psm_atomic_start(ctr);
 		unsigned long rax;