perf_self_monitor: Make the number of reps configurable

Merely adding the loop adds about six cycles to ENOSYS.

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

1 files changed, 38 insertions, 21 deletions

diff --git a/tight_loop/perf_self_monitor.c b/tight_loop/perf_self_monitor.c
index 51c3338..323e91f 100644
--- a/tight_loop/perf_self_monitor.c
+++ b/tight_loop/perf_self_monitor.c
@@ -82,6 +82,7 @@ static inline uint64_t psm_rdpmc(unsigned int ecx)
 
 typedef bool (*psm_sample_fn)(uint64_t *count,
 			      const struct psm_counter *counter,
+			      int reps,
 			      void *opaque);
 
 /*
@@ -146,40 +147,46 @@ static inline bool psm_atomic_elapsed(uint64_t *elapsed_count,
 }
 
 bool psm_atomic_sample_empty(uint64_t *count,
-			     const struct psm_counter *ctr, void *opaque)
+			     const struct psm_counter *ctr,
+			     int reps, void *opaque)
 {
 	struct psm_atomic duration = psm_atomic_start(ctr);
 	return psm_atomic_elapsed(count, &duration, ctr);
 }
 
 bool psm_atomic_sample_enosys(uint64_t *count,
-			      const struct psm_counter *ctr, void *opaque)
+			      const struct psm_counter *ctr,
+			      int reps, void *opaque)
 {
 	struct psm_atomic duration = psm_atomic_start(ctr);
 #ifdef __x86_64__
 	unsigned long rax;
-	rax = 0xbfffffff;
-	asm volatile ("syscall" : "+a" (rax) : : "rcx", "r11");
+	for (int i = 0; i < reps; i++) {
+		rax = 0xbfffffff;
+		asm volatile ("syscall" : "+a" (rax) : : "rcx", "r11");
+	}
 #else
-	syscall(0x3fffffff);
+	for (int i = 0; i < reps; i++)
+		syscall(0x3fffffff);
 #endif
 	return psm_atomic_elapsed(count, &duration, ctr);
 }
 
-bool psm_atomic_sample_bad_prctl_x1000(uint64_t *count,
-			      const struct psm_counter *ctr, void *opaque)
+bool psm_atomic_sample_bad_prctl(uint64_t *count,
+				 const struct psm_counter *ctr,
+				 int reps, void *opaque)
 {
 	struct psm_atomic duration = psm_atomic_start(ctr);
 #ifdef __x86_64__
 	unsigned long rax;
 	register unsigned long rdi asm("rdi");
-	for (int i = 0; i < 1000; i++) {
+	for (int i = 0; i < reps; i++) {
 		rax = SYS_prctl;
 		rdi = -1;
 		asm volatile ("syscall" : "+a" (rax), "+r" (rdi) : : "rcx", "r11");
 	}
 #else
-	for (int i = 0; i < 1000; i++)
+	for (int i = 0; i < reps; i++)
 		syscall(SYS_prctl, -1);
 #endif
 	return psm_atomic_elapsed(count, &duration, ctr);
@@ -198,7 +205,8 @@ static int compare_ui64(const void *a_void, const void *b_void)
 }
 
 uint64_t psm_integer_quantile(const struct psm_counter *ctr, psm_sample_fn fn,
-			      void *opaque, size_t q, size_t n)
+			      int reps, void *opaque,
+			      size_t q, size_t n)
 {
 	if (q >= n)
 		abort();
@@ -206,7 +214,7 @@ uint64_t psm_integer_quantile(const struct psm_counter *ctr, psm_sample_fn fn,
 	uint64_t *array = calloc(sizeof(uint64_t), n);
 	for (size_t i = 0; i < n; ) {
 		uint64_t sample;
-		if (!fn(&sample, ctr, opaque))
+		if (!fn(&sample, ctr, reps, opaque))
 			continue;
 		array[i++] = sample;
 	}
@@ -218,7 +226,8 @@ uint64_t psm_integer_quantile(const struct psm_counter *ctr, psm_sample_fn fn,
 	return ret;
 }
 
-void psm_settle(const struct psm_counter *ctr, psm_sample_fn fn, void *opaque)
+void psm_settle(const struct psm_counter *ctr, psm_sample_fn fn,
+		int reps, void *opaque)
 {
 	uint64_t val = UINT64_MAX;
 	int good_iters = 0;
@@ -226,7 +235,7 @@ void psm_settle(const struct psm_counter *ctr, psm_sample_fn fn, void *opaque)
 		uint64_t best = UINT64_MAX;
 		for (int inner = 0; inner < 10; ) {
 			uint64_t count;
-			if (!fn(&count, ctr, opaque))
+			if (!fn(&count, ctr, reps, opaque))
 				continue;  /* Rejected sample */
 			if (count < best)
 				best = count;
@@ -244,26 +253,34 @@ void psm_settle(const struct psm_counter *ctr, psm_sample_fn fn, void *opaque)
 	}
 }
 
+int reparray[] = {1, 200};
+
 int main()
 {
 	struct psm_counter *ctr = psm_counter_create();
 
-	psm_settle(ctr, psm_atomic_sample_empty, NULL);
+	psm_settle(ctr, psm_atomic_sample_empty, 1, NULL);
 
 	uint64_t baseline =
 		psm_integer_quantile(ctr, psm_atomic_sample_empty,
-				     NULL, 250, 500);
+				     1, NULL, 250, 500);
 	printf("An empty sample takes %llu cycles\n",
 	       (unsigned long long)baseline);
 
-	for (int i = 0; i < 20; i++) {
-		uint64_t cost = psm_integer_quantile(ctr, psm_atomic_sample_enosys, NULL, 250, 500) - baseline;
-		printf("1x ENOSYS: %llu\n", (unsigned long long)cost);
+	for (int repidx = 0; repidx < 2; repidx++) {
+		int reps = reparray[repidx];
+		for (int i = 0; i < 10; i++) {
+			uint64_t cost = psm_integer_quantile(ctr, psm_atomic_sample_enosys, reps, NULL, 250, 500) - baseline;
+			printf("%dx ENOSYS: %llu\n", reps, (unsigned long long)cost/reps);
+		}
 	}
 
-	for (int i = 0; i < 20; i++) {
-		uint64_t cost = psm_integer_quantile(ctr, psm_atomic_sample_bad_prctl_x1000, NULL, 250, 500) - baseline;
-		printf("1000x bad prctl: %llu\n", (unsigned long long)cost/1000);
+	for (int repidx = 0; repidx < 2; repidx++) {
+		int reps = reparray[repidx];
+		for (int i = 0; i < 10; i++) {
+			uint64_t cost = psm_integer_quantile(ctr, psm_atomic_sample_bad_prctl, reps, NULL, 250, 500) - baseline;
+			printf("%dx bad prctl: %llu\n", reps, (unsigned long long)cost/reps);
+		}
 	}
 
 	psm_counter_destroy(ctr);