null_syscall.c

  1// SPDX-License-Identifier: GPL-2.0-or-later
  2/*
  3 * Test null syscall performance
  4 *
  5 * Copyright (C) 2009-2015 Anton Blanchard, IBM
  6 */
  7
  8#define NR_LOOPS 10000000
  9
 10#include <string.h>
 11#include <stdio.h>
 12#include <stdlib.h>
 13#include <unistd.h>
 14#include <time.h>
 15#include <sys/types.h>
 16#include <sys/time.h>
 17#include <sys/syscall.h>
 18#include <signal.h>
 19
 20static volatile int soak_done;
 21unsigned long long clock_frequency;
 22unsigned long long timebase_frequency;
 23double timebase_multiplier;
 24
 25static inline unsigned long mftb(void)
 26{
 27	unsigned long low;
 28
 29	asm volatile("mftb %0" : "=r" (low));
 30
 31	return low;
 32}
 33
 34static void sigalrm_handler(int unused)
 35{
 36	soak_done = 1;
 37}
 38
 39/*
 40 * Use a timer instead of busy looping on clock_gettime() so we don't
 41 * pollute profiles with glibc and VDSO hits.
 42 */
 43static void cpu_soak_usecs(unsigned long usecs)
 44{
 45	struct itimerval val;
 46
 47	memset(&val, 0, sizeof(val));
 48	val.it_value.tv_usec = usecs;
 49
 50	signal(SIGALRM, sigalrm_handler);
 51	setitimer(ITIMER_REAL, &val, NULL);
 52
 53	while (1) {
 54		if (soak_done)
 55			break;
 56	}
 57
 58	signal(SIGALRM, SIG_DFL);
 59}
 60
 61/*
 62 * This only works with recent kernels where cpufreq modifies
 63 * /proc/cpuinfo dynamically.
 64 */
 65static void get_proc_frequency(void)
 66{
 67	FILE *f;
 68	char line[128];
 69	char *p, *end;
 70	unsigned long v;
 71	double d;
 72	char *override;
 73
 74	/* Try to get out of low power/low frequency mode */
 75	cpu_soak_usecs(0.25 * 1000000);
 76
 77	f = fopen("/proc/cpuinfo", "r");
 78	if (f == NULL)
 79		return;
 80
 81	timebase_frequency = 0;
 82
 83	while (fgets(line, sizeof(line), f) != NULL) {
 84		if (strncmp(line, "timebase", 8) == 0) {
 85			p = strchr(line, ':');
 86			if (p != NULL) {
 87				v = strtoull(p + 1, &end, 0);
 88				if (end != p + 1)
 89					timebase_frequency = v;
 90			}
 91		}
 92
 93		if (((strncmp(line, "clock", 5) == 0) ||
 94		     (strncmp(line, "cpu MHz", 7) == 0))) {
 95			p = strchr(line, ':');
 96			if (p != NULL) {
 97				d = strtod(p + 1, &end);
 98				if (end != p + 1) {
 99					/* Find fastest clock frequency */
100					if ((d * 1000000ULL) > clock_frequency)
101						clock_frequency = d * 1000000ULL;
102				}
103			}
104		}
105	}
106
107	fclose(f);
108
109	override = getenv("FREQUENCY");
110	if (override)
111		clock_frequency = strtoull(override, NULL, 10);
112
113	if (timebase_frequency)
114		timebase_multiplier = (double)clock_frequency
115					/ timebase_frequency;
116	else
117		timebase_multiplier = 1;
118}
119
120static void do_null_syscall(unsigned long nr)
121{
122	unsigned long i;
123
124	for (i = 0; i < nr; i++)
125		syscall(__NR_gettid);
126}
127
128#define TIME(A, STR) \
129
130int main(void)
131{
132	unsigned long tb_start, tb_now;
133	struct timespec tv_start, tv_now;
134	unsigned long long elapsed_ns, elapsed_tb;
135
136	get_proc_frequency();
137
138	clock_gettime(CLOCK_MONOTONIC, &tv_start);
139	tb_start = mftb();
140
141	do_null_syscall(NR_LOOPS);
142
143	clock_gettime(CLOCK_MONOTONIC, &tv_now);
144	tb_now = mftb();
145
146	elapsed_ns = (tv_now.tv_sec - tv_start.tv_sec) * 1000000000ULL +
147			(tv_now.tv_nsec - tv_start.tv_nsec);
148	elapsed_tb = tb_now - tb_start;
149
150	printf("%10.2f ns %10.2f cycles\n", (float)elapsed_ns / NR_LOOPS,
151			(float)elapsed_tb * timebase_multiplier / NR_LOOPS);
152
153	return 0;
154}