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Alexei Starovoitov
c2f2f005a1
Puranjay Mohan says:
====================
bpf: Optimize recursion detection on arm64
V2: https://lore.kernel.org/all/20251217233608.2374187-1-puranjay@kernel.org/
Changes in v2->v3:
- Added acked by Yonghong
- Patch 2:
- Change alignment of active from 8 to 4
- Use le32_to_cpu in place of get_unaligned_le32()
V1: https://lore.kernel.org/all/20251217162830.2597286-1-puranjay@kernel.org/
Changes in V1->V2:
- Patch 2:
- Put preempt_enable()/disable() around RMW accesses to mitigate
race conditions. Because on CONFIG_PREEMPT_RCU and sleepable
bpf programs, preemption can cause no bpf prog to execute in
case of recursion.
BPF programs detect recursion using a per-CPU 'active' flag in struct
bpf_prog. The trampoline currently sets/clears this flag with atomic
operations.
On some arm64 platforms (e.g., Neoverse V2 with LSE), per-CPU atomic
operations are relatively slow. Unlike x86_64 - where per-CPU updates
can avoid cross-core atomicity, arm64 LSE atomics are always atomic
across all cores, which is unnecessary overhead for strictly per-CPU
state.
This patch removes atomics from the recursion detection path on arm64.
It was discovered in [1] that per-CPU atomics that don't return a value
were extremely slow on some arm64 platforms, Catalin added a fix in
commit 535fdfc5a2 ("arm64: Use load LSE atomics for the non-return
per-CPU atomic operations") to solve this issue, but it seems to have
caused a regression on the fentry benchmark.
Using the fentry benchmark from the bpf selftests shows the following:
./tools/testing/selftests/bpf/bench trig-fentry
+---------------------------------------------+------------------------+
| Configuration | Total Operations (M/s) |
+---------------------------------------------+------------------------+
| bpf-next/master with Catalin’s fix reverted | 51.770 |
|---------------------------------------------|------------------------|
| bpf-next/master | 43.271 |
| bpf-next/master with this change | 43.271 |
+---------------------------------------------+------------------------+
All benchmarks were run on a KVM based vm with Neoverse-V2 and 8 cpus.
This patch yields a 25% improvement in this benchmark compared to
bpf-next. Notably, reverting Catalin's fix also results in a performance
gain for this benchmark, which is interesting but expected.
For completeness, this benchmark was also run with the change enabled on
x86-64, which resulted in a 30% regression in the fentry benchmark. So,
it is only enabled on arm64.
P.S. - Here is more data with other program types:
+-----------------+-----------+-----------+----------+
| Metric | Before | After | % Diff |
+-----------------+-----------+-----------+----------+
| fentry | 43.149 | 53.948 | +25.03% |
| fentry.s | 41.831 | 50.937 | +21.76% |
| rawtp | 50.834 | 58.731 | +15.53% |
| fexit | 31.118 | 34.360 | +10.42% |
| tp | 39.536 | 41.632 | +5.30% |
| syscall-count | 8.053 | 8.305 | +3.13% |
| fmodret | 33.940 | 34.769 | +2.44% |
| kprobe | 9.970 | 9.998 | +0.28% |
| usermode-count | 224.886 | 224.839 | -0.02% |
| kernel-count | 154.229 | 153.043 | -0.77% |
+-----------------+-----------+-----------+----------+
[1] https://lore.kernel.org/all/e7d539ed-ced0-4b96-8ecd-048a5b803b85@paulmck-laptop/
====================
Link: https://patch.msgid.link/20251219184422.2899902-1-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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