From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2DFCA34B40C for ; Fri, 19 Dec 2025 18:45:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766169926; cv=none; b=j0bnKONGb1ZjFBHL3Bq9Xu4sS41I1liSWnITdslo7ArHhepVnzqtOH80u+e+md2kbgbDs1pM3jkrWBTcimWQmEwR7phbYvtKYRBj/xFWRHHJEOIPxkTSclv2gNRm6KdwrJ6m0G5Xi/t+R25IJ7IxtR7Limrpp4TKKA8rMQe+p2g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766169926; c=relaxed/simple; bh=MX7u6YCAf8ISSlBSjmO63NiEj72J0NgYtomqNWx848Q=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=aCxQdSEo7XDJ/cd81qwkpFc22C8hm9qY2LWo0wLTr8AsenS/hFa3QoftbkwiGJnsDA0X+smpgc+fOTk0p84yW6/R4kjCRkuIZGqmDe4+qvZ08BfRCcZrLUaS4nViQ4pdHmFvBxKffPdsOxvHiE8Wi8/9tFZZ1CJzgQVjp57IOzY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=nDW6/vgK; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="nDW6/vgK" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 73704C4CEF1; Fri, 19 Dec 2025 18:45:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1766169925; bh=MX7u6YCAf8ISSlBSjmO63NiEj72J0NgYtomqNWx848Q=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=nDW6/vgKTnJzLQNhfd/FgWJBM6F7bnYDZ7fWbB/Lq4456mxPv4v/tZKKPyhYmzhD0 mG32Jh7mT8vqC8XrthShEjEwQloJo1NKBd5qfUCZMGT2iFQt66cPH5eJtGWAdrSAIm yPMkS6Ogp6J7o06tRP/BrTj3MvWSRUMQSawtD5byKMOlyrLl0aAyCYG2iP6rzYoKDY MRMObERDYUbya6zAsXWX8HwNxRObnQtkCu3bDs8CtpsygQBSu96v6XdP7/KDFAj7/k lmsSfq/PWDa+wkxOGRUbPD0YKqZ8+VYFZpK9Kl7bzuqkEO/krJQwGO80ZDRkzA8YrL YcF7p3Wwb7gEg== From: Puranjay Mohan To: bpf@vger.kernel.org Cc: Puranjay Mohan , Puranjay Mohan , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau , Eduard Zingerman , Kumar Kartikeya Dwivedi , kernel-team@meta.com, Catalin Marinas , Will Deacon , Mark Rutland , linux-arm-kernel@lists.infradead.org, Yonghong Song Subject: [PATCH bpf-next v3 2/2] bpf: arm64: Optimize recursion detection by not using atomics Date: Fri, 19 Dec 2025 10:44:18 -0800 Message-ID: <20251219184422.2899902-3-puranjay@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20251219184422.2899902-1-puranjay@kernel.org> References: <20251219184422.2899902-1-puranjay@kernel.org> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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. Acked-by: Yonghong Song Signed-off-by: Puranjay Mohan --- include/linux/bpf.h | 32 +++++++++++++++++++++++++++++--- kernel/bpf/core.c | 3 ++- 2 files changed, 31 insertions(+), 4 deletions(-) diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 2da986136d26..da6a00dd313f 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -1746,6 +1746,8 @@ 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 +1774,7 @@ 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 recursion protection */ unsigned int (*bpf_func)(const void *ctx, const struct bpf_insn *insn); struct bpf_prog_aux *aux; /* Auxiliary fields */ @@ -2006,12 +2008,36 @@ 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 = le32_to_cpu(*(__le32 *)active); + 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..e0b8a8a5aaa9 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]), 4, + bpf_memcg_flags(GFP_KERNEL | gfp_extra_flags)); if (!fp->active) { vfree(fp); kfree(aux); -- 2.47.3