[PATCH bpf-next v2 0/2] bpf: Optimize recursion detection on arm64

[Puranjay Mohan <puranjay@kernel.org>]

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 prog to execute.

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 535fdfc5a228 ("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 |         51.862         |
 |---------------------------------------------|------------------------|
 | bpf-next/master                             |         43.067         |
 | bpf-next/master with this change            |         53.856         |
 +---------------------------------------------+------------------------+

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.

[1] https://lore.kernel.org/all/e7d539ed-ced0-4b96-8ecd-048a5b803b85@paulmck-laptop/

Puranjay Mohan (2):
  bpf: move recursion detection logic to helpers
  bpf: arm64: Optimize recursion detection by not using atomics

 include/linux/bpf.h      | 39 ++++++++++++++++++++++++++++++++++++++-
 kernel/bpf/core.c        |  3 ++-
 kernel/bpf/trampoline.c  |  8 ++++----
 kernel/trace/bpf_trace.c |  4 ++--
 4 files changed, 46 insertions(+), 8 deletions(-)


base-commit: ec439c38013550420aecc15988ae6acb670838c1
-- 
2.47.3