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Message-Id: <20260116090809.25290-1-realwujing@gmail.com>
Date: Fri, 16 Jan 2026 17:08:09 +0800
From: Qiliang Yuan <realwujing@...il.com>
To: andrii.nakryiko@...il.com,
eddyz87@...il.com
Cc: andrii@...nel.org,
ast@...nel.org,
bpf@...r.kernel.org,
daniel@...earbox.net,
haoluo@...gle.com,
jolsa@...nel.org,
kpsingh@...nel.org,
linux-kernel@...r.kernel.org,
martin.lau@...ux.dev,
realwujing@...il.com,
sdf@...ichev.me,
song@...nel.org,
yonghong.song@...ux.dev,
yuanql9@...natelecom.cn
Subject: [PATCH v2] bpf/verifier: optimize ID mapping reset in states_equal
The verifier uses an ID mapping table (struct bpf_idmap) during state
equivalence checks. Currently, reset_idmap_scratch performs a full memset
on the entire map (~4.7KB) in every call to states_equal.
Following suggestions from Eduard Zingerman and Andrii Nakryiko, this
patch optimizes the reset logic to avoid unnecessary memory operations:
1. Rename 'map_cnt' to 'cnt' for brevity.
2. Replace the O(N) memset() in reset_idmap_scratch() with a simple
O(1) counter reset (idmap->cnt = 0).
3. Update check_ids() to search only up to 'idmap->cnt' entries. If no
mapping is found, append the new mapping and increment 'cnt'.
This ensures that reset overhead is minimal and the search loop is
bounded by the number of IDs actually encountered in the current
equivalence check.
Benchmark results (system-wide 'perf stat' during high-concurrency 'veristat'
stress test, 60s):
The following results, captured using perf while running veristat in parallel
across all CPU cores, show a significant reduction in instruction overhead
(~9.3%) and branch executions (~11%), confirming that the O(1) reset logic
significantly reduces the verifier's workload during state equivalence
checks.
Metric | Baseline | Patched | Delta
----------------|---------------|---------------|----------
Iterations | 5710 | 5731 | +0.37%
Instructions | 1.714 T | 1.555 T | -9.28%
Inst/Iter | 300.2 M | 271.3 M | -9.63%
Cycles | 1.436 T | 1.335 T | -7.03%
Branches | 350.4 B | 311.9 B | -10.99%
Migrations | 25,977 | 23,524 | -9.44%
Test Command:
seq 1 2000000 | sudo perf stat -a -- \
timeout 60s xargs -P $(nproc) -I {} ./veristat access_map_in_map.bpf.o
Detailed Performance Stats:
Baseline:
Performance counter stats for 'system wide':
6,735,538 context-switches # 3505.5 cs/sec cs_per_second
1,921,431.27 msec cpu-clock # 32.0 CPUs CPUs_utilized
25,977 cpu-migrations # 13.5 migrations/sec migrations_per_second
7,268,841 page-faults # 3783.0 faults/sec page_fault_per_second
18,662,357,052 branch-misses # 3.9 % branch_miss_rate (50.14%)
350,411,558,023 branches # 182.4 M/sec branch_frequency (66.85%)
1,435,774,261,319 cpu-cycles # 0.7 GHz cycles_frequency (66.95%)
1,714,154,229,503 instructions # 1.2 instructions insn_per_cycle (66.86%)
429,445,480,497 stalled-cycles-frontend # 0.30 frontend_cycles_idle (66.36%)
60.035899231 seconds time elapsed
Patched:
Performance counter stats for 'system wide':
6,662,371 context-switches # 3467.3 cs/sec cs_per_second
1,921,497.78 msec cpu-clock # 32.0 CPUs CPUs_utilized
23,524 cpu-migrations # 12.2 migrations/sec migrations_per_second
7,783,064 page-faults # 4050.5 faults/sec page_faults_per_second
18,181,655,163 branch-misses # 4.3 % branch_miss_rate (50.15%)
311,865,239,743 branches # 162.3 M/sec branch_frequency (66.86%)
1,334,859,779,821 cpu-cycles # 0.7 GHz cycles_frequency (66.96%)
1,555,086,465,845 instructions # 1.2 instructions insn_per_cycle (66.87%)
407,666,712,045 stalled-cycles-frontend # 0.31 frontend_cycles_idle (66.35%)
60.034702643 seconds time elapsed
Suggested-by: Eduard Zingerman <eddyz87@...il.com>
Suggested-by: Andrii Nakryiko <andrii@...nel.org>
Signed-off-by: Qiliang Yuan <realwujing@...il.com>
---
Hi Eduard and Andrii,
Thank you for the feedback on v1. I've optimized the ID mapping reset
logic to O(1) and updated the benchmark results as suggested.
v1 -> v2:
- Rename map_cnt to cnt (Andrii Nakryiko)
- Eliminate memset() by using cnt to bound search loop (Andrii Nakryiko)
- Remove unnecessary if() check in reset_idmap_scratch() (Eduard Zingerman)
- Use full name in Signed-off-by (Eduard Zingerman)
include/linux/bpf_verifier.h | 2 +-
kernel/bpf/verifier.c | 23 +++++++++++------------
2 files changed, 12 insertions(+), 13 deletions(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 562f7e63be29..8355b585cd18 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -692,7 +692,7 @@ struct bpf_id_pair {
struct bpf_idmap {
u32 tmp_id_gen;
- u32 map_cnt;
+ u32 cnt;
struct bpf_id_pair map[BPF_ID_MAP_SIZE];
};
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6220dde41107..c0e8604618de 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -18949,19 +18949,21 @@ static bool check_ids(u32 old_id, u32 cur_id, struct bpf_idmap *idmap)
if (old_id == 0) /* cur_id == 0 as well */
return true;
- for (i = 0; i < BPF_ID_MAP_SIZE; i++) {
- if (!map[i].old) {
- /* Reached an empty slot; haven't seen this id before */
- map[i].old = old_id;
- map[i].cur = cur_id;
- idmap->map_cnt = i + 1;
- return true;
- }
+ for (i = 0; i < idmap->cnt; i++) {
if (map[i].old == old_id)
return map[i].cur == cur_id;
if (map[i].cur == cur_id)
return false;
}
+
+ /* Reached the end of known mappings; haven't seen this id before */
+ if (idmap->cnt < BPF_ID_MAP_SIZE) {
+ map[idmap->cnt].old = old_id;
+ map[idmap->cnt].cur = cur_id;
+ idmap->cnt++;
+ return true;
+ }
+
/* We ran out of idmap slots, which should be impossible */
WARN_ON_ONCE(1);
return false;
@@ -19475,10 +19477,7 @@ static void reset_idmap_scratch(struct bpf_verifier_env *env)
struct bpf_idmap *idmap = &env->idmap_scratch;
idmap->tmp_id_gen = env->id_gen;
- if (idmap->map_cnt) {
- memset(idmap->map, 0, idmap->map_cnt * sizeof(struct bpf_id_pair));
- idmap->map_cnt = 0;
- }
+ idmap->cnt = 0;
}
static bool states_equal(struct bpf_verifier_env *env,
--
2.39.5
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