tools/perf: Allow to select the number of hash buckets

Add the -b/ --buckets argument to specify the number of hash buckets for
the private futex hash. This is directly passed to
    prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_SET_SLOTS, buckets, immutable)

and must return without an error if specified. The `immutable' is 0 by
default and can be set to 1 via the -I/ --immutable argument.
The size of the private hash is verified with PR_FUTEX_HASH_GET_SLOTS.
If PR_FUTEX_HASH_GET_SLOTS failed then it is assumed that an older
kernel was used without the support and that the global hash is used.

Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

1 files changed, 65 insertions, 0 deletions

diff --git a/tools/perf/bench/futex.c b/tools/perf/bench/futex.c
new file mode 100644
index 000000000000..02ae6c52ba88
--- /dev/null
+++ b/tools/perf/bench/futex.c
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <err.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <linux/prctl.h>
+#include <sys/prctl.h>
+
+#include "futex.h"
+
+void futex_set_nbuckets_param(struct bench_futex_parameters *params)
+{
+	int ret;
+
+	if (params->nbuckets < 0)
+		return;
+
+	ret = prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_SET_SLOTS, params->nbuckets, params->buckets_immutable);
+	if (ret) {
+		printf("Requesting %d hash buckets failed: %d/%m\n",
+		       params->nbuckets, ret);
+		err(EXIT_FAILURE, "prctl(PR_FUTEX_HASH)");
+	}
+}
+
+void futex_print_nbuckets(struct bench_futex_parameters *params)
+{
+	char *futex_hash_mode;
+	int ret;
+
+	ret = prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_GET_SLOTS);
+	if (params->nbuckets >= 0) {
+		if (ret != params->nbuckets) {
+			if (ret < 0) {
+				printf("Can't query number of buckets: %m\n");
+				err(EXIT_FAILURE, "prctl(PR_FUTEX_HASH)");
+			}
+			printf("Requested number of hash buckets does not currently used.\n");
+			printf("Requested: %d in usage: %d\n", params->nbuckets, ret);
+			err(EXIT_FAILURE, "prctl(PR_FUTEX_HASH)");
+		}
+		if (params->nbuckets == 0) {
+			ret = asprintf(&futex_hash_mode, "Futex hashing: global hash");
+		} else {
+			ret = prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_GET_IMMUTABLE);
+			if (ret < 0) {
+				printf("Can't check if the hash is immutable: %m\n");
+				err(EXIT_FAILURE, "prctl(PR_FUTEX_HASH)");
+			}
+			ret = asprintf(&futex_hash_mode, "Futex hashing: %d hash buckets %s",
+				       params->nbuckets,
+				       ret == 1 ? "(immutable)" : "");
+		}
+	} else {
+		if (ret <= 0) {
+			ret = asprintf(&futex_hash_mode, "Futex hashing: global hash");
+		} else {
+			ret = asprintf(&futex_hash_mode, "Futex hashing: auto resized to %d buckets",
+				       ret);
+		}
+	}
+	if (ret < 0)
+		err(EXIT_FAILURE, "ENOMEM, futex_hash_mode");
+	printf("%s\n", futex_hash_mode);
+	free(futex_hash_mode);
+}