Add a small selftest that stresses pipe->mutex contention by spawning N
writer threads that hammer a single pipe with multi-page writes, plus M
reader threads that drain. Each writer records its own write() latency
samples into a log2-bucketed histogram; main aggregates and prints
total writes, throughput, average and percentile (p50/p99/p99.9)
latencies, and the maximum observed latency.
This was used to validate "fs/pipe: bulk pre-allocate pages outside
pipe->mutex in anon_pipe_write". By default it sweeps over
writers={1,2,5} x readers={1,5,10} using 64KB writes for 3s on a 1 MB
pipe (~27s total); -w/-r switch to a single configuration and -s/-d/-p
tune msgsize/duration/pipe size. Output is one-line-per-metric with a
"---" separator between configurations so two runs (e.g. baseline vs
patched) can be diffed directly.
Pass --memory-pressure to fork stress-ng (--vm 4 --vm-bytes 80%
--vm-method all) for the duration of the run, so alloc_page() in
anon_pipe_write() routinely hits direct reclaim. The flag fails
fast if stress-ng is not on $PATH.
The program exits 0 on a clean run and reports its results to stdout,
so it integrates with the kselftest framework via TEST_GEN_PROGS.
Signed-off-by: Breno Leitao <[email protected]>
---
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/pipe/.gitignore | 1 +
tools/testing/selftests/pipe/Makefile | 9 +
tools/testing/selftests/pipe/pipe_bench.c | 351 ++++++++++++++++++++++++++++++
4 files changed, 362 insertions(+)
diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 6e59b8f63e416..bcd9db9d292ca 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -91,6 +91,7 @@ TARGETS += pcie_bwctrl
TARGETS += perf_events
TARGETS += pidfd
TARGETS += pid_namespace
+TARGETS += pipe
TARGETS += power_supply
TARGETS += powerpc
TARGETS += prctl
diff --git a/tools/testing/selftests/pipe/.gitignore
b/tools/testing/selftests/pipe/.gitignore
new file mode 100644
index 0000000000000..20b549361a152
--- /dev/null
+++ b/tools/testing/selftests/pipe/.gitignore
@@ -0,0 +1 @@
+pipe_bench
diff --git a/tools/testing/selftests/pipe/Makefile
b/tools/testing/selftests/pipe/Makefile
new file mode 100644
index 0000000000000..1810c680117b3
--- /dev/null
+++ b/tools/testing/selftests/pipe/Makefile
@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 Meta Platforms, Inc. and affiliates
+# Copyright (c) 2026 Breno Leitao <[email protected]>
+
+CFLAGS += -O2 -Wall -Wextra -pthread
+
+TEST_GEN_PROGS := pipe_bench
+
+include ../lib.mk
diff --git a/tools/testing/selftests/pipe/pipe_bench.c
b/tools/testing/selftests/pipe/pipe_bench.c
new file mode 100644
index 0000000000000..4b4ee6c8c0ced
--- /dev/null
+++ b/tools/testing/selftests/pipe/pipe_bench.c
@@ -0,0 +1,351 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * pipe_bench - exercise pipe->mutex contention under concurrent writers.
+ *
+ * N writer threads hammer a single pipe with multi-page writes; M reader
+ * threads drain it. Each writer records its own write() latency histogram.
+ * Multi-page writes (msgsize >= PAGE_SIZE) force the loop in
+ * anon_pipe_write() to call alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT) under
+ * pipe->mutex, which is the critical section the patch shrinks.
+ *
+ * By default the benchmark sweeps writers in {1, 2, 5} x readers in
+ * {1, 5, 10} and prints one block per configuration so two runs (e.g.
+ * baseline vs patched) can be diffed directly. Pass -w and -r to run a
+ * single configuration instead. Pass --memory-pressure to spawn stress-ng
+ * alongside the sweep so the per-page alloc_page() path under pipe->mutex
+ * has to dip into reclaim.
+ *
+ * Copyright (c) 2026 Meta Platforms, Inc. and affiliates
+ * Copyright (c) 2026 Breno Leitao <[email protected]>
+ */
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <getopt.h>
+#include <pthread.h>
+#include <signal.h>
+#include <stdatomic.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/wait.h>
+#include <time.h>
+#include <unistd.h>
+
+#define ARRAY_SIZE(a) (sizeof(a) / sizeof((a)[0]))
+#define HIST_BUCKETS 32
+
+static size_t g_msgsize = 16 * 4096;
+static int g_duration = 3;
+static int g_pipe_size = 1024 * 1024;
+static int g_memory_pressure;
+
+static atomic_int g_stop;
+static int g_pipe[2];
+
+struct wstats {
+ uint64_t writes;
+ uint64_t bytes;
+ uint64_t lat_sum_ns;
+ uint64_t lat_max_ns;
+ uint64_t lat_hist[HIST_BUCKETS];
+};
+
+static inline uint64_t now_ns(void)
+{
+ struct timespec ts;
+
+ clock_gettime(CLOCK_MONOTONIC, &ts);
+ return (uint64_t)ts.tv_sec * 1000000000ull + ts.tv_nsec;
+}
+
+static inline int log2_bucket(uint64_t v)
+{
+ int b = 0;
+
+ if (!v)
+ return 0;
+ while (v >>= 1)
+ b++;
+ return b < HIST_BUCKETS ? b : HIST_BUCKETS - 1;
+}
+
+static void *writer(void *arg)
+{
+ struct wstats *s = arg;
+ char *buf = aligned_alloc(4096, g_msgsize);
+
+ if (!buf)
+ return NULL;
+ memset(buf, 0xAA, g_msgsize);
+
+ while (!atomic_load_explicit(&g_stop, memory_order_relaxed)) {
+ uint64_t t0 = now_ns();
+ ssize_t n = write(g_pipe[1], buf, g_msgsize);
+ uint64_t dt = now_ns() - t0;
+
+ if (n > 0) {
+ s->writes++;
+ s->bytes += n;
+ s->lat_sum_ns += dt;
+ if (dt > s->lat_max_ns)
+ s->lat_max_ns = dt;
+ s->lat_hist[log2_bucket(dt)]++;
+ } else if (n < 0 && errno == EPIPE) {
+ break;
+ }
+ }
+ free(buf);
+ return NULL;
+}
+
+static void *reader(void *arg)
+{
+ char *buf = aligned_alloc(4096, g_msgsize);
+
+ (void)arg;
+ if (!buf)
+ return NULL;
+ /*
+ * Drain until EOF (write end closed by main). g_stop is not checked
+ * here on purpose: writers may be blocked in write() with the pipe
+ * full when g_stop is set, so the reader must keep draining until
+ * main closes the write end.
+ */
+ for (;;) {
+ ssize_t n = read(g_pipe[0], buf, g_msgsize);
+
+ if (n <= 0)
+ break;
+ }
+ free(buf);
+ return NULL;
+}
+
+static void summarize(struct wstats *all, int nw, int nr)
+{
+ uint64_t total_writes = 0, total_bytes = 0, total_lat = 0;
+ uint64_t max_lat = 0;
+ uint64_t agg[HIST_BUCKETS] = {0};
+ uint64_t p50_target, p99_target, p999_target;
+ uint64_t cum = 0, p50 = 0, p99 = 0, p999 = 0;
+ uint64_t avg_ns;
+ double sec;
+
+ for (int i = 0; i < nw; i++) {
+ total_writes += all[i].writes;
+ total_bytes += all[i].bytes;
+ total_lat += all[i].lat_sum_ns;
+ if (all[i].lat_max_ns > max_lat)
+ max_lat = all[i].lat_max_ns;
+ for (int b = 0; b < HIST_BUCKETS; b++)
+ agg[b] += all[i].lat_hist[b];
+ }
+
+ p50_target = total_writes * 50 / 100;
+ p99_target = total_writes * 99 / 100;
+ p999_target = total_writes * 999 / 1000;
+
+ for (int b = 0; b < HIST_BUCKETS; b++) {
+ cum += agg[b];
+ if (!p50 && cum >= p50_target)
+ p50 = 1ULL << b;
+ if (!p99 && cum >= p99_target)
+ p99 = 1ULL << b;
+ if (!p999 && cum >= p999_target)
+ p999 = 1ULL << b;
+ }
+
+ sec = g_duration;
+ avg_ns = total_writes ? total_lat / total_writes : 0;
+
+ printf("config: writers=%d readers=%d msgsize=%zu duration=%d
pipe_size=%d memory_pressure=%s\n",
+ nw, nr, g_msgsize, g_duration, g_pipe_size,
+ g_memory_pressure ? "yes" : "no");
+ printf("writes: total=%llu rate=%.0f/s\n",
+ (unsigned long long)total_writes, total_writes / sec);
+ printf("throughput_MBps: %.2f\n",
+ (total_bytes / sec) / (1024.0 * 1024.0));
+ printf("lat_avg_ns: %llu\n", (unsigned long long)avg_ns);
+ printf("lat_p50_ns_upper: %llu\n", (unsigned long long)p50);
+ printf("lat_p99_ns_upper: %llu\n", (unsigned long long)p99);
+ printf("lat_p999_ns_upper: %llu\n", (unsigned long long)p999);
+ printf("lat_max_ns: %llu\n", (unsigned long long)max_lat);
+}
+
+static pid_t spawn_stress_ng(void)
+{
+ pid_t pid = fork();
+
+ if (pid < 0) {
+ perror("fork");
+ return -1;
+ }
+ if (pid == 0) {
+ execlp("stress-ng", "stress-ng",
+ "--vm", "4", "--vm-bytes", "80%",
+ "--vm-method", "all",
+ (char *)NULL);
+ fprintf(stderr, "exec stress-ng failed: %s\n",
+ strerror(errno));
+ _exit(127);
+ }
+ /* Give stress-ng a moment to map its VM regions before measuring. */
+ sleep(1);
+ return pid;
+}
+
+static void kill_stress_ng(pid_t pid)
+{
+ int status;
+
+ if (pid <= 0)
+ return;
+ kill(pid, SIGTERM);
+ for (int i = 0; i < 20; i++) {
+ if (waitpid(pid, &status, WNOHANG) > 0)
+ return;
+ usleep(100 * 1000);
+ }
+ kill(pid, SIGKILL);
+ waitpid(pid, &status, 0);
+}
+
+static int run_one(int nw, int nr)
+{
+ pthread_t *wt, *rt;
+ struct wstats *ws;
+
+ atomic_store(&g_stop, 0);
+
+ if (pipe(g_pipe) < 0) {
+ perror("pipe");
+ return -1;
+ }
+ if (fcntl(g_pipe[1], F_SETPIPE_SZ, g_pipe_size) < 0)
+ perror("F_SETPIPE_SZ (continuing)");
+
+ wt = calloc(nw, sizeof(*wt));
+ rt = calloc(nr, sizeof(*rt));
+ ws = calloc(nw, sizeof(*ws));
+
+ if (!wt || !rt || !ws) {
+ fprintf(stderr, "alloc failed\n");
+ free(wt);
+ free(rt);
+ free(ws);
+ close(g_pipe[0]);
+ close(g_pipe[1]);
+ return -1;
+ }
+
+ for (int i = 0; i < nr; i++)
+ pthread_create(&rt[i], NULL, reader, NULL);
+ for (int i = 0; i < nw; i++)
+ pthread_create(&wt[i], NULL, writer, &ws[i]);
+
+ sleep(g_duration);
+ atomic_store(&g_stop, 1);
+
+ /*
+ * Close write end first so any writer blocked in write() gets EPIPE
+ * and exits, and so the readers see EOF after draining.
+ */
+ close(g_pipe[1]);
+ for (int i = 0; i < nw; i++)
+ pthread_join(wt[i], NULL);
+ for (int i = 0; i < nr; i++)
+ pthread_join(rt[i], NULL);
+ close(g_pipe[0]);
+
+ summarize(ws, nw, nr);
+ fflush(stdout);
+
+ free(wt);
+ free(rt);
+ free(ws);
+ return 0;
+}
+
+int main(int argc, char **argv)
+{
+ static const int writers_sweep[] = {1, 2, 5};
+ static const int readers_sweep[] = {1, 5, 10};
+ static const struct option long_opts[] = {
+ {"memory-pressure", no_argument, NULL, 'M'},
+ {0, 0, 0, 0},
+ };
+ int writers_override = 0, readers_override = 0;
+ pid_t stress_pid = -1;
+ int rc = 0, opt;
+
+ while ((opt = getopt_long(argc, argv, "w:r:s:d:p:",
+ long_opts, NULL)) != -1) {
+ switch (opt) {
+ case 'w':
+ writers_override = atoi(optarg);
+ break;
+ case 'r':
+ readers_override = atoi(optarg);
+ break;
+ case 's':
+ g_msgsize = atol(optarg);
+ break;
+ case 'd':
+ g_duration = atoi(optarg);
+ break;
+ case 'p':
+ g_pipe_size = atoi(optarg);
+ break;
+ case 'M':
+ g_memory_pressure = 1;
+ break;
+ default:
+ fprintf(stderr,
+ "usage: %s [-w writers] [-r readers] [-s
msgsize] [-d secs] [-p pipe_size] [--memory-pressure]\n"
+ " default: sweep writers={1,2,5} x
readers={1,5,10}\n"
+ " --memory-pressure: spawn stress-ng (--vm 4
--vm-bytes 80%% --vm-method all) for the run\n",
+ argv[0]);
+ return 1;
+ }
+ }
+
+ signal(SIGPIPE, SIG_IGN);
+ setvbuf(stdout, NULL, _IOLBF, 0);
+ setvbuf(stderr, NULL, _IOLBF, 0);
+
+ fprintf(stderr, "pid=%d\n", getpid());
+ fflush(stderr);
+
+ if (g_memory_pressure) {
+ stress_pid = spawn_stress_ng();
+ if (stress_pid < 0) {
+ fprintf(stderr,
+ "memory_pressure requested but stress-ng could
not be spawned\n");
+ return 1;
+ }
+ }
+
+ if (writers_override > 0 || readers_override > 0) {
+ int nw = writers_override > 0 ? writers_override : 1;
+ int nr = readers_override > 0 ? readers_override : 1;
+
+ rc = run_one(nw, nr) < 0 ? 1 : 0;
+ goto out;
+ }
+
+ for (size_t i = 0; i < ARRAY_SIZE(writers_sweep); i++) {
+ for (size_t j = 0; j < ARRAY_SIZE(readers_sweep); j++) {
+ printf("---\n");
+ if (run_one(writers_sweep[i], readers_sweep[j]) < 0) {
+ rc = 1;
+ goto out;
+ }
+ }
+ }
+out:
+ kill_stress_ng(stress_pid);
+ return rc;
+}
--
2.53.0-Meta
