[PATCH 2/3] perf bench futex: synchronize 'bench futex wake-parallel' wakers

[Kim Phillips]

From: James Yang <james.yang@xxxxxxx>

Waker threads in the futex wake-parallel benchmark are started by a
loop using pthread_create().  However, there is no synchronization
for when the waker threads wake the waiting threads.  Comparison of
the waker threads' measurement timestamps show they are not all
running concurrently because older waker threads finish their task
before newer waker threads even start.

This patch uses a barrier to better synchronize the waker threads.

Additionally, unlike the waiter threads, the waker threads' processor
affinity is not specified, so the result has run-to-run variability as
the scheduler decides on which CPUs they are to run.  So we add a
-W/--affine-wakers flag to stripe the affinity of the waker threads
across the online CPUs instead of having the scheduler place them.

Signed-off-by: James Yang <james.yang@xxxxxxx>
Signed-off-by: Kim Phillips <kim.phillips@xxxxxxx>
---
 tools/perf/bench/futex-wake-parallel.c | 44 ++++++++++++++++++++++++++++++----
 1 file changed, 40 insertions(+), 4 deletions(-)

diff --git a/tools/perf/bench/futex-wake-parallel.c b/tools/perf/bench/futex-wake-parallel.c
index 5617bcd17e55..65a3a3466dce 100644
--- a/tools/perf/bench/futex-wake-parallel.c
+++ b/tools/perf/bench/futex-wake-parallel.c
@@ -8,6 +8,8 @@
  * it can be used to measure futex_wake() changes.
  */
 
+#include "debug.h"
+
 /* For the CLR_() macros */
 #include <string.h>
 #include <pthread.h>
@@ -31,6 +33,8 @@ struct thread_data {
 	pthread_t worker;
 	unsigned int nwoken;
 	struct timeval runtime;
+	struct timeval start;
+	struct timeval end;
 };
 
 static unsigned int nwakes = 1;
@@ -39,10 +43,11 @@ static unsigned int nwakes = 1;
 static u_int32_t futex = 0;
 
 static pthread_t *blocked_worker;
-static bool done = false, silent = false, fshared = false;
+static bool done = false, silent = false, fshared = false, affine_wakers = false;
 static unsigned int nblocked_threads = 0, nwaking_threads = 0;
 static pthread_mutex_t thread_lock;
 static pthread_cond_t thread_parent, thread_worker;
+static pthread_barrier_t barrier;
 static struct stats waketime_stats, wakeup_stats;
 static unsigned int ncpus, threads_starting;
 static int futex_flag = 0;
@@ -52,6 +57,7 @@ static const struct option options[] = {
 	OPT_UINTEGER('w', "nwakers", &nwaking_threads, "Specify amount of waking threads"),
 	OPT_BOOLEAN( 's', "silent",  &silent,   "Silent mode: do not display data/details"),
 	OPT_BOOLEAN( 'S', "shared",  &fshared,  "Use shared futexes instead of private ones"),
+	OPT_BOOLEAN( 'W', "affine-wakers", &affine_wakers, "Stripe affinity of waker threads across CPUs"),
 	OPT_END()
 };
 
@@ -65,6 +71,8 @@ static void *waking_workerfn(void *arg)
 	struct thread_data *waker = (struct thread_data *) arg;
 	struct timeval start, end;
 
+	pthread_barrier_wait(&barrier);
+
 	gettimeofday(&start, NULL);
 
 	waker->nwoken = futex_wake(&futex, nwakes, futex_flag);
@@ -75,31 +83,59 @@ static void *waking_workerfn(void *arg)
 	gettimeofday(&end, NULL);
 	timersub(&end, &start, &waker->runtime);
 
+	waker->start = start;
+	waker->end   = end;
+
 	pthread_exit(NULL);
 	return NULL;
 }
 
-static void wakeup_threads(struct thread_data *td, pthread_attr_t thread_attr)
+static void wakeup_threads(struct thread_data *td, pthread_attr_t thread_attr,
+			   int *cpu_map)
 {
 	unsigned int i;
 
 	pthread_attr_setdetachstate(&thread_attr, PTHREAD_CREATE_JOINABLE);
 
+	pthread_barrier_init(&barrier, NULL, nwaking_threads + 1);
+
 	/* create and block all threads */
 	for (i = 0; i < nwaking_threads; i++) {
 		/*
 		 * Thread creation order will impact per-thread latency
 		 * as it will affect the order to acquire the hb spinlock.
-		 * For now let the scheduler decide.
 		 */
+
+		if (affine_wakers) {
+			cpu_set_t cpu;
+			CPU_ZERO(&cpu);
+			CPU_SET(cpu_map[(i + 1) % ncpus], &cpu);
+
+			if (pthread_attr_setaffinity_np(&thread_attr,
+							sizeof(cpu_set_t),
+							&cpu))
+				err(EXIT_FAILURE, "pthread_attr_setaffinity_np");
+		}
+
 		if (pthread_create(&td[i].worker, &thread_attr,
 				   waking_workerfn, (void *)&td[i]))
 			err(EXIT_FAILURE, "pthread_create");
 	}
 
+	pthread_barrier_wait(&barrier);
+
 	for (i = 0; i < nwaking_threads; i++)
 		if (pthread_join(td[i].worker, NULL))
 			err(EXIT_FAILURE, "pthread_join");
+
+	pthread_barrier_destroy(&barrier);
+
+	for (i = 0; i < nwaking_threads; i++) {
+		pr_debug("%6ld.%06ld\t"
+			 "%6ld.%06ld\n",
+			 td[i].start.tv_sec, td[i].start.tv_usec,
+			 td[i].end.tv_sec, td[i].end.tv_usec);
+	}
 }
 
 static void *blocked_workerfn(void *arg __maybe_unused)
@@ -281,7 +317,7 @@ int bench_futex_wake_parallel(int argc, const char **argv)
 		usleep(100000);
 
 		/* Ok, all threads are patiently blocked, start waking folks up */
-		wakeup_threads(waking_worker, thread_attr);
+		wakeup_threads(waking_worker, thread_attr, cpu_map);
 
 		for (i = 0; i < nblocked_threads; i++) {
 			ret = pthread_join(blocked_worker[i], NULL);
-- 
2.15.0