perf_self_monitor: improve output

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

1 files changed, 11 insertions, 4 deletions

diff --git a/tight_loop/perf_self_monitor.c b/tight_loop/perf_self_monitor.c
index 33c2be2..378d0fd 100644
--- a/tight_loop/perf_self_monitor.c
+++ b/tight_loop/perf_self_monitor.c
@@ -98,9 +98,9 @@ struct psm_atomic {
 	uint64_t initial_raw_count;
 
 	/*
-	 * This is here to improve code generation.  struct psm_atomic is
-	 * unlikely to exist as escaped and hence be affected by a "memory"
-	 * clobber.
+	 * This is here to improve code generation.  struct psm_atomic
+	 * is unlikely to be referenced by an escapted pointer and hence
+	 * be affected by a "memory" clobber.
 	 */
 	unsigned int rdpmc_ecx;
 };
@@ -227,8 +227,15 @@ int main()
 
 	psm_settle(ctr, psm_atomic_sample_empty, NULL);
 
+	uint64_t baseline =
+		psm_integer_quantile(ctr, psm_atomic_sample_empty,
+				     NULL, 250, 500);
+	printf("An empty sample takes %llu cycles\n",
+	       (unsigned long long)baseline);
+
 	for (int i = 0; i < 20; i++) {
-		printf("%llu\n", (unsigned long long)psm_integer_quantile(ctr, psm_atomic_sample_enosys, NULL, 0, 500));
+		uint64_t cost = psm_integer_quantile(ctr, psm_atomic_sample_enosys, NULL, 250, 500) - baseline;
+		printf("%llu\n", (unsigned long long)cost);
 	}
 
 	psm_counter_destroy(ctr);