* [PATCH bpf-next v4 1/2] bpftool: Use batch lookups for bounded hash map dumps
2026-09-11 3:47 [PATCH bpf-next v4 0/2] bpftool: Batch bounded hash map dumps Tianyi Chen
@ 2026-09-11 3:47 ` Tianyi Chen
2026-09-11 3:47 ` [PATCH bpf-next v4 2/2] selftests/bpf: Check bpftool batch map dump contents Tianyi Chen
1 sibling, 0 replies; 3+ messages in thread
From: Tianyi Chen @ 2026-09-11 3:47 UTC (permalink / raw)
To: qmo, bpf; +Cc: andrii, eddyz87, ihor.solodrai, linux-kselftest
From: Tianyi Chen <hi@tychen.cc>
Use BPF_MAP_LOOKUP_BATCH when dumping hash maps to reduce the number
of BPF syscalls while preserving plain, JSON and BTF formatting.
For a 100,000-entry hash map in an x86-64 KVM guest, BPF syscall counts
fell from 200,004 to 395. The median of five untraced runs fell from
0.774013 s to 0.733844 s, about 5.2% in this measurement. Syscall counts
were collected separately with strace.
Hash batch lookup must fit an entire bucket. Restrict eligibility to
maps whose maximum key/value storage fits in 4 MiB, so even a worst-case
bucket can fit without restarting a partially printed dump. Start with
up to 256 entries and grow on ENOSPC using the same input cursor.
Fall back to individual lookups only when the initial batch operation
is unsupported. Restarting after output has begun would duplicate
entries. Process the final partial batch on ENOENT, but do not trust
count or output buffers after other errors. Keep fatal diagnostics on
stderr so JSON element arrays contain only map entries.
Link: https://github.com/libbpf/bpftool/issues/63
Assisted-by: LLM
Signed-off-by: Tianyi Chen <hi@tychen.cc>
---
tools/bpf/bpftool/map.c | 117 +++++++++++++++++++++++++++++++++++++---
1 file changed, 109 insertions(+), 8 deletions(-)
diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
index 684a8fb7241..971da173600 100644
--- a/tools/bpf/bpftool/map.c
+++ b/tools/bpf/bpftool/map.c
@@ -740,15 +740,10 @@ static int do_show(int argc, char **argv)
return errno == ENOENT ? 0 : -1;
}
-static int dump_map_elem(int fd, void *key, void *value,
- struct bpf_map_info *map_info, struct btf *btf,
- json_writer_t *btf_wtr)
+static void print_map_elem(void *key, void *value,
+ struct bpf_map_info *map_info, struct btf *btf,
+ json_writer_t *btf_wtr)
{
- if (bpf_map_lookup_elem(fd, key, value)) {
- print_entry_error(map_info, key, errno);
- return -1;
- }
-
if (json_output) {
print_entry_json(map_info, key, value, btf);
} else if (btf) {
@@ -762,10 +757,112 @@ static int dump_map_elem(int fd, void *key, void *value,
} else {
print_entry_plain(map_info, key, value);
}
+}
+
+static int dump_map_elem(int fd, void *key, void *value,
+ struct bpf_map_info *map_info, struct btf *btf,
+ json_writer_t *btf_wtr)
+{
+ if (bpf_map_lookup_elem(fd, key, value)) {
+ print_entry_error(map_info, key, errno);
+ return -1;
+ }
+ print_map_elem(key, value, map_info, btf, btf_wtr);
return 0;
}
+#define MAP_DUMP_BATCH_FALLBACK 1
+#define MAP_DUMP_BATCH_SIZE 256U
+#define MAP_DUMP_BATCH_MAX_BYTES (4 * 1024 * 1024)
+
+/* Return MAP_DUMP_BATCH_FALLBACK only before batch traversal starts. */
+static int dump_map_batch(int fd, void *key, void *value,
+ struct bpf_map_info *info, struct btf *btf,
+ json_writer_t *wtr, unsigned int *num_elems)
+{
+ __u32 capacity, count, batch = 0, next_batch = 0, i;
+ void *keys = NULL, *values = NULL, *buf;
+ bool first = true, can_fallback = true;
+ int err;
+
+ /*
+ * Hash lookup batches must accommodate a whole bucket. Restrict the
+ * optimization to maps whose worst-case bucket fits the memory budget,
+ * so a later ENOSPC never forces a restart after printing some entries.
+ * Division also bounds the allocation multiplications on 32-bit hosts.
+ */
+ if (info->type != BPF_MAP_TYPE_HASH || !info->max_entries ||
+ (__u64)info->key_size + info->value_size >
+ MAP_DUMP_BATCH_MAX_BYTES / info->max_entries)
+ return MAP_DUMP_BATCH_FALLBACK;
+
+ capacity = min(info->max_entries, MAP_DUMP_BATCH_SIZE);
+resize:
+ buf = realloc(keys, (size_t)capacity * info->key_size);
+ if (!buf) {
+ err = ENOMEM;
+ goto error;
+ }
+ keys = buf;
+ buf = realloc(values, (size_t)capacity * info->value_size);
+ if (!buf) {
+ err = ENOMEM;
+ goto error;
+ }
+ values = buf;
+
+ while (true) {
+ count = capacity;
+ err = bpf_map_lookup_batch(fd, first ? NULL : &batch,
+ &next_batch, keys, values, &count, NULL);
+ err = err ? errno : 0;
+ /*
+ * Older kernels reject the command before updating count. Do not
+ * inspect the buffers on these errors, or fall back after progress.
+ */
+ if (can_fallback && (err == EINVAL || err == EOPNOTSUPP ||
+ err == 524 /* ENOTSUPP */)) {
+ err = MAP_DUMP_BATCH_FALLBACK;
+ goto out;
+ }
+ can_fallback = false;
+ if (err == ENOSPC) {
+ if (capacity == info->max_entries)
+ goto error;
+ capacity += min(capacity, info->max_entries - capacity);
+ /* Preserve the input cursor: the oversized bucket was not read. */
+ goto resize;
+ }
+ /* In particular, EFAULT can leave count and the buffers invalid. */
+ if (err && err != ENOENT)
+ goto error;
+ for (i = 0; i < count; i++) {
+ /*
+ * Keep the alignment provided by individual lookups, including
+ * for BTF types whose map key/value size is not aligned.
+ */
+ memcpy(key, keys + (size_t)i * info->key_size, info->key_size);
+ memcpy(value, values + (size_t)i * info->value_size, info->value_size);
+ print_map_elem(key, value, info, btf, wtr);
+ (*num_elems)++;
+ }
+ if (err == ENOENT) {
+ err = 0;
+ goto out;
+ }
+ first = false;
+ batch = next_batch;
+ }
+error:
+ fprintf(stderr, "Error: can't lookup map batch: %s\n", strerror(err));
+ err = -1;
+out:
+ free(keys);
+ free(values);
+ return err;
+}
+
static int maps_have_btf(int *fds, int nb_fds)
{
struct bpf_map_info info = {};
@@ -869,6 +966,9 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
p_info("Warning: cannot read values from %s map with value_size != 8",
map_type_str);
}
+ err = dump_map_batch(fd, key, value, info, btf, wtr, &num_elems);
+ if (err != MAP_DUMP_BATCH_FALLBACK)
+ goto end_dump;
while (true) {
err = bpf_map_get_next_key(fd, prev_key, key);
if (err) {
@@ -881,6 +981,7 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
prev_key = key;
}
+end_dump:
if (wtr) {
jsonw_end_array(wtr); /* elements */
if (show_header)
--
2.55.0
^ permalink raw reply related [flat|nested] 3+ messages in thread* [PATCH bpf-next v4 2/2] selftests/bpf: Check bpftool batch map dump contents
2026-09-11 3:47 [PATCH bpf-next v4 0/2] bpftool: Batch bounded hash map dumps Tianyi Chen
2026-09-11 3:47 ` [PATCH bpf-next v4 1/2] bpftool: Use batch lookups for " Tianyi Chen
@ 2026-09-11 3:47 ` Tianyi Chen
1 sibling, 0 replies; 3+ messages in thread
From: Tianyi Chen @ 2026-09-11 3:47 UTC (permalink / raw)
To: qmo, bpf; +Cc: andrii, eddyz87, ihor.solodrai, linux-kselftest
From: Tianyi Chen <hi@tychen.cc>
Exercise hash map dumps around the initial batch size and across
multiple batches, including empty and single-entry maps. Compare each
complete unordered key/value set with the input data in plain, JSON
and pretty JSON output.
Cover one-byte keys, three-byte values and BTF-formatted maps to catch
cursor sizing, buffer alignment and formatting regressions.
Assisted-by: LLM
Signed-off-by: Tianyi Chen <hi@tychen.cc>
---
.../bpf/prog_tests/bpftool_map_batch.c | 187 ++++++++++++++++++
1 file changed, 187 insertions(+)
create mode 100644 tools/testing/selftests/bpf/prog_tests/bpftool_map_batch.c
diff --git a/tools/testing/selftests/bpf/prog_tests/bpftool_map_batch.c b/tools/testing/selftests/bpf/prog_tests/bpftool_map_batch.c
new file mode 100644
index 00000000000..b4216ed778e
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/bpftool_map_batch.c
@@ -0,0 +1,187 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <bpftool_helpers.h>
+#include <bpf/btf.h>
+#include <ctype.h>
+
+#define MAX_ENTRIES 1025
+#define RECORD_SIZE 256
+#define OUTPUT_SIZE (MAX_ENTRIES * RECORD_SIZE + 1024)
+
+struct dump_case {
+ const char *name;
+ unsigned int count;
+ unsigned int key_size;
+ unsigned int value_size;
+ bool btf;
+};
+
+static void hex_bytes(char *out, const void *data, unsigned int size, bool json)
+{
+ const unsigned char *bytes = data;
+ unsigned int i;
+
+ if (json)
+ *out++ = '[';
+ for (i = 0; i < size; i++) {
+ if (json && i)
+ *out++ = ',';
+ out += sprintf(out, json ? "\"0x%02x\"" : "%02x", bytes[i]);
+ }
+ if (json)
+ *out++ = ']';
+ *out = '\0';
+}
+
+static void expected_record(char *record, const struct dump_case *test,
+ unsigned int index, bool json)
+{
+ __u32 key = index, value = index * 37 + 11;
+ unsigned char short_key = index;
+ char key_hex[64], value_hex[64], formatted[96];
+
+ hex_bytes(key_hex, test->key_size == 1 ? (void *)&short_key : &key,
+ test->key_size, json);
+ hex_bytes(value_hex, &value, test->value_size, json);
+ snprintf(formatted, sizeof(formatted), "{\"key\":%u,\"value\":%u}",
+ key, value);
+ if (json && test->btf)
+ snprintf(record, RECORD_SIZE,
+ "{\"key\":%s,\"value\":%s,\"formatted\":%s}",
+ key_hex, value_hex, formatted);
+ else if (json)
+ snprintf(record, RECORD_SIZE, "{\"key\":%s,\"value\":%s}",
+ key_hex, value_hex);
+ else if (test->btf)
+ snprintf(record, RECORD_SIZE, "%s", formatted);
+ else
+ snprintf(record, RECORD_SIZE, "key:%svalue:%s", key_hex, value_hex);
+}
+
+static void check_dump(const struct dump_case *test, __u32 id, bool json, bool pretty)
+{
+ bool array = json || test->btf;
+ char command[MAX_BPFTOOL_CMD_LEN], expected[RECORD_SIZE], footer[64];
+ bool seen[MAX_ENTRIES] = {};
+ char *output, *src, *dst, *cursor;
+ unsigned int i, n;
+ int err;
+
+ output = calloc(1, OUTPUT_SIZE);
+ if (!ASSERT_OK_PTR(output, "alloc_output"))
+ return;
+ snprintf(command, sizeof(command), "%smap dump id %u",
+ pretty ? "-p " : json ? "-j " : "", id);
+ err = get_bpftool_command_output(command, output, OUTPUT_SIZE);
+ if (!ASSERT_OK(err, "map_dump"))
+ goto out;
+ /*
+ * Ignore presentation whitespace, but compare complete records and all
+ * punctuation. Expected contents come only from the input data, never
+ * from another map walk or bpftool invocation.
+ */
+ for (src = output, dst = output; *src; src++)
+ if (!isspace((unsigned char)*src))
+ *dst++ = *src;
+ *dst = '\0';
+ cursor = output;
+ if (array) {
+ if (!ASSERT_EQ(*cursor, '[', "array_start"))
+ goto out;
+ cursor++;
+ }
+ for (n = 0; n < test->count; n++) {
+ if (array && n) {
+ if (!ASSERT_EQ(*cursor, ',', "record_separator"))
+ goto out;
+ cursor++;
+ }
+ for (i = 0; i < test->count; i++) {
+ if (seen[i])
+ continue;
+ expected_record(expected, test, i, json);
+ if (!strncmp(cursor, expected, strlen(expected)))
+ break;
+ }
+ if (!ASSERT_LT(i, test->count, "unique_expected_record"))
+ goto out;
+ seen[i] = true;
+ cursor += strlen(expected);
+ }
+ if (array) {
+ ASSERT_STREQ(cursor, "]", "array_end_and_count");
+ } else {
+ snprintf(footer, sizeof(footer), "Found%uelement%s", test->count,
+ test->count == 1 ? "" : "s");
+ ASSERT_STREQ(cursor, footer, "plain_count");
+ }
+out:
+ free(output);
+}
+
+static void run_dump_case(const struct dump_case *test)
+{
+ LIBBPF_OPTS(bpf_map_create_opts, opts);
+ struct bpf_map_info info = {};
+ __u32 info_len = sizeof(info);
+ struct btf *btf = NULL;
+ unsigned int i;
+ int fd = -1;
+
+ if (test->btf) {
+ btf = btf__new_empty();
+ if (!ASSERT_OK_PTR(btf, "btf_new"))
+ return;
+ if (!ASSERT_EQ(btf__add_int(btf, "unsigned int", 4, 0), 1,
+ "btf_int") ||
+ !ASSERT_OK(btf__load_into_kernel(btf), "btf_load"))
+ goto out;
+ opts.btf_fd = btf__fd(btf);
+ opts.btf_key_type_id = 1;
+ opts.btf_value_type_id = 1;
+ }
+ fd = bpf_map_create(BPF_MAP_TYPE_HASH, "dump_batch", test->key_size,
+ test->value_size, test->count ?: 1, &opts);
+ if (!ASSERT_OK_FD(fd, "map_create"))
+ goto out;
+ for (i = 0; i < test->count; i++) {
+ __u32 key = i, value = i * 37 + 11;
+ unsigned char short_key = i;
+ void *key_ptr = test->key_size == 1 ? (void *)&short_key : &key;
+
+ if (!ASSERT_OK(bpf_map_update_elem(fd, key_ptr, &value, BPF_ANY),
+ "map_update"))
+ goto out;
+ }
+ if (!ASSERT_OK(bpf_map_get_info_by_fd(fd, &info, &info_len), "map_info"))
+ goto out;
+ check_dump(test, info.id, false, false);
+ check_dump(test, info.id, true, false);
+ check_dump(test, info.id, true, true);
+out:
+ if (fd >= 0)
+ close(fd);
+ btf__free(btf);
+}
+
+void test_bpftool_map_batch(void)
+{
+ static const struct dump_case cases[] = {
+ { "empty", 0, 4, 4 },
+ { "single", 1, 4, 4 },
+ { "below_batch", 255, 4, 4 },
+ { "exact_batch", 256, 4, 4 },
+ { "above_batch", 257, 4, 4 },
+ { "multiple_batches", 1025, 4, 4 },
+ { "one_byte_key", 256, 1, 4 },
+ { "odd_value_size", 257, 4, 3 },
+ { "btf_empty", 0, 4, 4, true },
+ { "btf_single", 1, 4, 4, true },
+ { "btf_multiple_batches", 1025, 4, 4, true },
+ };
+ unsigned int i;
+
+ for (i = 0; i < ARRAY_SIZE(cases); i++)
+ if (test__start_subtest(cases[i].name))
+ run_dump_case(&cases[i]);
+}
--
2.55.0
^ permalink raw reply related [flat|nested] 3+ messages in thread