Re: [PATCH bpf-next v2 2/2] bpf: arm64: Optimize recursion detection by not using atomics

[Yonghong Song <yonghong.song@linux.dev>]

On 12/17/25 3:35 PM, Puranjay Mohan wrote:
> 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 by
> changing 'active' to a per-CPU array of four u8 counters, one per
> context: {NMI, hard-irq, soft-irq, normal}. The running context uses a
> non-atomic increment/decrement on its element.  After increment,
> recursion is detected by reading the array as a u32 and verifying that
> only the expected element changed; any change in another element
> indicates inter-context recursion, and a value > 1 in the same element
> indicates same-context recursion.
>
> For example, starting from {0,0,0,0}, a normal-context trigger changes
> the array to {0,0,0,1}.  If an NMI arrives on the same CPU and triggers
> the program, the array becomes {1,0,0,1}. When the NMI context checks
> the u32 against the expected mask for normal (0x00000001), it observes
> 0x01000001 and correctly reports recursion. Same-context recursion is
> detected analogously.
>
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>

LGTM with a few nits below.

Acked-by: Yonghong Song <yonghong.song@linux.dev>

> ---
>   include/linux/bpf.h | 33 ++++++++++++++++++++++++++++++---
>   kernel/bpf/core.c   |  3 ++-
>   2 files changed, 32 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 2da986136d26..5ca2a761d9a1 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -31,6 +31,7 @@
>   #include <linux/static_call.h>
>   #include <linux/memcontrol.h>
>   #include <linux/cfi.h>
> +#include <linux/unaligned.h>
>   #include <asm/rqspinlock.h>
>   
>   struct bpf_verifier_env;
> @@ -1746,6 +1747,8 @@ struct bpf_prog_aux {
>   	struct bpf_map __rcu *st_ops_assoc;
>   };
>   
> +#define BPF_NR_CONTEXTS        4       /* normal, softirq, hardirq, NMI */
> +
>   struct bpf_prog {
>   	u16			pages;		/* Number of allocated pages */
>   	u16			jited:1,	/* Is our filter JIT'ed? */
> @@ -1772,7 +1775,7 @@ struct bpf_prog {
>   		u8 tag[BPF_TAG_SIZE];
>   	};
>   	struct bpf_prog_stats __percpu *stats;
> -	int __percpu		*active;
> +	u8 __percpu		*active;	/* u8[BPF_NR_CONTEXTS] for rerecursion protection */
>   	unsigned int		(*bpf_func)(const void *ctx,
>   					    const struct bpf_insn *insn);
>   	struct bpf_prog_aux	*aux;		/* Auxiliary fields */
> @@ -2006,12 +2009,36 @@ struct bpf_struct_ops_common_value {
>   
>   static inline bool bpf_prog_get_recursion_context(struct bpf_prog *prog)
>   {
> -	return this_cpu_inc_return(*(prog->active)) == 1;
> +#ifdef CONFIG_ARM64
> +	u8 rctx = interrupt_context_level();
> +	u8 *active = this_cpu_ptr(prog->active);
> +	u32 val;
> +
> +	preempt_disable();
> +	active[rctx]++;
> +	val = get_unaligned_le32(active);

The 'active' already aligned with 8 (or 4 with my below suggestion).
The get_unaligned_le32() works, but maybe we could use le32_to_cpu()
instead. Maybe there is no performance difference between
get_unaligned_le32() and le32_to_cpu() so you pick get_unaligned_le32()?
It would be good to clarify in commit message if get_unaligned_le32()
is used.

> +	preempt_enable();
> +	if (val != BIT(rctx * 8))
> +		return false;
> +
> +	return true;
> +#else
> +	return this_cpu_inc_return(*(int __percpu *)(prog->active)) == 1;
> +#endif
>   }
>   
>   static inline void bpf_prog_put_recursion_context(struct bpf_prog *prog)
>   {
> -	this_cpu_dec(*(prog->active));
> +#ifdef CONFIG_ARM64
> +	u8 rctx = interrupt_context_level();
> +	u8 *active = this_cpu_ptr(prog->active);
> +
> +	preempt_disable();
> +	active[rctx]--;
> +	preempt_enable();
> +#else
> +	this_cpu_dec(*(int __percpu *)(prog->active));
> +#endif
>   }
>   
>   #if defined(CONFIG_BPF_JIT) && defined(CONFIG_BPF_SYSCALL)
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index c66316e32563..b5063acfcf92 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -112,7 +112,8 @@ struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flag
>   		vfree(fp);
>   		return NULL;
>   	}
> -	fp->active = alloc_percpu_gfp(int, bpf_memcg_flags(GFP_KERNEL | gfp_extra_flags));
> +	fp->active = __alloc_percpu_gfp(sizeof(u8[BPF_NR_CONTEXTS]), 8,
> +					bpf_memcg_flags(GFP_KERNEL | gfp_extra_flags));

Here, the alignment is 8. Can it be 4 since the above reads a 32bit value?

>   	if (!fp->active) {
>   		vfree(fp);
>   		kfree(aux);