perf tools: sched-pipe bench: add (-n) nonblocking benchmark

The -n mode will benchmark pipes in a non-blocking mode using
epoll_wait.

This specific mode was added to demonstrate the broken sync nature
of epoll: https://lore.kernel.org/lkml/20240426-zupfen-jahrzehnt-5be786bcdf04@brauner

Signed-off-by: Brian Geffon <[e-mail]>
Reviewed-by: Ian Rogers <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Namhyung Kim <[e-mail]>

tools/perf/bench/sched-pipe.c

···
 #include <errno.h>
 #include <fcntl.h>
 #include <assert.h>
+#include <sys/epoll.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/syscall.h>
···
 	int			nr;
 	int			pipe_read;
 	int			pipe_write;
+	struct epoll_event      epoll_ev;
+	int			epoll_fd;
 	bool			cgroup_failed;
 	pthread_t		pthread;
 };
···
 /* Use processes by default: */
 static bool			threaded;
 
+static bool			nonblocking;
 static char			*cgrp_names[2];
 static struct cgroup		*cgrps[2];
 
···
 }
 
 static const struct option options[] = {
+	OPT_BOOLEAN('n', "nonblocking",	&nonblocking,	"Use non-blocking operations"),
 	OPT_INTEGER('l', "loop",	&loops,		"Specify number of loops"),
 	OPT_BOOLEAN('T', "threaded",	&threaded,	"Specify threads/process based task setup"),
 	OPT_CALLBACK('G', "cgroups", NULL, "SEND,RECV",
···
 	free(cgrp_names[nr]);
 }
 
+static inline int read_pipe(struct thread_data *td)
+{
+	int ret, m;
+retry:
+	if (nonblocking) {
+		ret = epoll_wait(td->epoll_fd, &td->epoll_ev, 1, -1);
+		if (ret < 0)
+			return ret;
+	}
+	ret = read(td->pipe_read, &m, sizeof(int));
+	if (nonblocking && ret < 0 && errno == EWOULDBLOCK)
+		goto retry;
+	return ret;
+}
+
 static void *worker_thread(void *__tdata)
 {
 	struct thread_data *td = __tdata;
-	int m = 0, i;
-	int ret;
+	int i, ret, m = 0;
 
 	ret = enter_cgroup(td->nr);
 	if (ret < 0) {
···
 		return NULL;
 	}
 
+	if (nonblocking) {
+		td->epoll_ev.events = EPOLLIN;
+		td->epoll_fd = epoll_create(1);
+		BUG_ON(td->epoll_fd < 0);
+		BUG_ON(epoll_ctl(td->epoll_fd, EPOLL_CTL_ADD, td->pipe_read, &td->epoll_ev) < 0);
+	}
+
 	for (i = 0; i < loops; i++) {
 		if (!td->nr) {
-			ret = read(td->pipe_read, &m, sizeof(int));
+			ret = read_pipe(td);
 			BUG_ON(ret != sizeof(int));
 			ret = write(td->pipe_write, &m, sizeof(int));
 			BUG_ON(ret != sizeof(int));
 		} else {
 			ret = write(td->pipe_write, &m, sizeof(int));
 			BUG_ON(ret != sizeof(int));
-			ret = read(td->pipe_read, &m, sizeof(int));
+			ret = read_pipe(td);
 			BUG_ON(ret != sizeof(int));
 		}
 	}
···
 	 * discarding returned value of read(), write()
 	 * causes error in building environment for perf
 	 */
-	int __maybe_unused ret, wait_stat;
+	int __maybe_unused ret, wait_stat, flags = 0;
 	pid_t pid, retpid __maybe_unused;
 
 	argc = parse_options(argc, argv, options, bench_sched_pipe_usage, 0);
 
-	BUG_ON(pipe(pipe_1));
-	BUG_ON(pipe(pipe_2));
+	if (nonblocking)
+		flags |= O_NONBLOCK;
+
+	BUG_ON(pipe2(pipe_1, flags));
+	BUG_ON(pipe2(pipe_2, flags));
 
 	gettimeofday(&start, NULL);