linux-kernel - Re: [PATCH bpf-next v4 2/2] bpf, arm64: inline bpf_get_smp_processor

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Date: Tue, 30 Apr 2024 18:48:00 +0000
From: Puranjay Mohan <puranjay@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>, Will Deacon
 <will@...nel.org>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann
 <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, Martin KaFai
 Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu
 <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, John Fastabend
 <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, Stanislav
 Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa
 <jolsa@...nel.org>, Zi Shen Lim <zlim.lnx@...il.com>, Xu Kuohai
 <xukuohai@...wei.com>, Florent Revest <revest@...omium.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v4 2/2] bpf, arm64: inline
 bpf_get_smp_processor_id() helper

Puranjay Mohan <puranjay@...nel.org> writes:

> As ARM64 JIT now implements BPF_MOV64_PERCPU_REG instruction, inline
> bpf_get_smp_processor_id().
>
> ARM64 uses the per-cpu variable cpu_number to store the cpu id.

While implementing this on the RISC-V JIT[1], I realized that reading the
cpu_number from the per-cpu variable is not the best way to do this on
arm64.

arm64 now has the cpu number in the thread_info and reading that would
be more efficient.

Implementation in this patch is emitting:

; int cpu = bpf_get_smp_processor_id();
mov     x7, #0xffff8000ffffffff
movk    x7, #0x8207, lsl #16   
movk    x7, #0x2008            
mrs     x10, tpidr_el1         
add     x7, x7, x10            
ldr     w7, [x7]               

If we do this in the JIT like I did for RISC-V[1]
We can emit:

; int cpu = bpf_get_smp_processor_id();
mrs     x10, sp_el0
ldr     w7, [x10, #24]

This gives ~ 10% improvement on glob-arr-inc and hash-inc compared to
3-4 % which this patch is providing. 

I will send v5 using this approach.

[1] https://lore.kernel.org/all/20240430175834.33152-3-puranjay@kernel.org/

> Here is how the BPF and ARM64 JITed assembly changes after this commit:
>
>                                          BPF
>          		                =====
>               BEFORE                                       AFTER
>              --------                                     -------
>
> int cpu = bpf_get_smp_processor_id();           int cpu = bpf_get_smp_processor_id();
> (85) call bpf_get_smp_processor_id#229032       (18) r0 = 0xffff800082072008
>                                                 (bf) r0 = &(void __percpu *)(r0)
>                                                 (61) r0 = *(u32 *)(r0 +0)
>
> 				      ARM64 JIT
> 				     ===========
>
>               BEFORE                                       AFTER
>              --------                                     -------
>
> int cpu = bpf_get_smp_processor_id();           int cpu = bpf_get_smp_processor_id();
> mov     x10, #0xfffffffffffff4d0                mov     x7, #0xffff8000ffffffff
> movk    x10, #0x802b, lsl #16                   movk    x7, #0x8207, lsl #16
> movk    x10, #0x8000, lsl #32                   movk    x7, #0x2008
> blr     x10                                     mrs     x10, tpidr_el1
> add     x7, x0, #0x0                            add     x7, x7, x10
>                                                 ldr     w7, [x7]
>
> Performance improvement using benchmark[1]
>
>              BEFORE                                       AFTER
>             --------                                     -------
>
> glob-arr-inc   :   23.817 ± 0.019M/s      glob-arr-inc   :   24.631 ± 0.027M/s [+ 3.41%]
> arr-inc        :   23.253 ± 0.019M/s      arr-inc        :   23.742 ± 0.023M/s [+ 2.10%]
> hash-inc       :   12.258 ± 0.010M/s      hash-inc       :   12.625 ± 0.004M/s [+ 3.00%]
>
> [1] https://github.com/anakryiko/linux/commit/8dec900975ef


Thanks,
Puranjay