From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Puranjay Mohan <puranjay@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
Martin KaFai Lau <martin.lau@linux.dev>,
Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
Yonghong Song <yonghong.song@linux.dev>,
John Fastabend <john.fastabend@gmail.com>,
KP Singh <kpsingh@kernel.org>,
Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
Jiri Olsa <jolsa@kernel.org>, Zi Shen Lim <zlim.lnx@gmail.com>,
Xu Kuohai <xukuohai@huawei.com>,
Florent Revest <revest@chromium.org>,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [PATCH bpf-next v3 1/2] arm64, bpf: add internal-only MOV instruction to resolve per-CPU addrs
Date: Fri, 26 Apr 2024 10:35:29 -0700 [thread overview]
Message-ID: <CAEf4BzZejgfw=GiX_LTWVupRzrKVaX5Ky6L3wziSoquEFUju2w@mail.gmail.com> (raw)
In-Reply-To: <mb61psez8vzbu.fsf@kernel.org>
On Fri, Apr 26, 2024 at 9:55 AM Puranjay Mohan <puranjay@kernel.org> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
>
> > On Fri, Apr 26, 2024 at 5:14 AM Puranjay Mohan <puranjay@kernel.org> wrote:
> >>
> >> From: Puranjay Mohan <puranjay12@gmail.com>
> >>
> >> Support an instruction for resolving absolute addresses of per-CPU
> >> data from their per-CPU offsets. This instruction is internal-only and
> >> users are not allowed to use them directly. They will only be used for
> >> internal inlining optimizations for now between BPF verifier and BPF
> >> JITs.
> >>
> >> Since commit 7158627686f0 ("arm64: percpu: implement optimised pcpu
> >> access using tpidr_el1"), the per-cpu offset for the CPU is stored in
> >> the tpidr_el1/2 register of that CPU.
> >>
> >> To support this BPF instruction in the ARM64 JIT, the following ARM64
> >> instructions are emitted:
> >>
> >> mov dst, src // Move src to dst, if src != dst
> >> mrs tmp, tpidr_el1/2 // Move per-cpu offset of the current cpu in tmp.
> >> add dst, dst, tmp // Add the per cpu offset to the dst.
> >>
> >> To measure the performance improvement provided by this change, the
> >> benchmark in [1] was used:
> >>
> >> Before:
> >> glob-arr-inc : 23.597 ± 0.012M/s
> >> arr-inc : 23.173 ± 0.019M/s
> >> hash-inc : 12.186 ± 0.028M/s
> >>
> >> After:
> >> glob-arr-inc : 23.819 ± 0.034M/s
> >> arr-inc : 23.285 ± 0.017M/s
> >
> > I still expected a better improvement (global-arr-inc's results
> > improved more than arr-inc, which is completely different from
> > x86-64), but it's still a good thing to support this for arm64, of
> > course.
> >
> > ack for generic parts I can understand:
> >
> > Acked-by: Andrii Nakryiko <andrii@kernel.org>
> >
>
> I will have to do more research to find why we don't see very high
> improvement.
>
> But this is what is happening here:
>
> This was the complete picture before inlining:
>
> int cpu = bpf_get_smp_processor_id();
> mov x10, #0xffffffffffffd4a8
> movk x10, #0x802c, lsl #16
> movk x10, #0x8000, lsl #32
> blr x10 ---------------------------------------> nop
> nop
> adrp x0, 0xffff800082128000
> mrs x1, tpidr_el1
> add x0, x0, #0x8
> ldrsw x0, [x0, x1]
> <----------------------------------------ret
> add x7, x0, #0x0
>
>
> Now we have:
>
> int cpu = bpf_get_smp_processor_id();
> mov x7, #0xffff8000ffffffff
> movk x7, #0x8212, lsl #16
> movk x7, #0x8008
> mrs x10, tpidr_el1
> add x7, x7, x10
> ldr w7, [x7]
>
>
> So, we have removed multiple instructions including a branch and a
> return. I was expecting to see more improvement. This benchmark is taken
> from a KVM based virtual machine, maybe if I do it on bare-metal I would
> see more improvement ?
I see, yeah, I think it might change significantly. I remember back
from times when I was benchmarking BPF ringbuf, I was getting
very-very different results from inside QEMU vs bare metal. And I
don't mean just in absolute numbers. QEMU/KVM seems to change a lot of
things when it comes to contentions, atomic instructions, etc, etc.
Anyways, for benchmarking, always try to do bare metal.
>
> Thanks,
> Puranjay
next prev parent reply other threads:[~2024-04-26 17:35 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-26 12:13 [PATCH bpf-next v3 0/2] bpf, arm64: Support per-cpu instruction Puranjay Mohan
2024-04-26 12:13 ` [PATCH bpf-next v3 1/2] arm64, bpf: add internal-only MOV instruction to resolve per-CPU addrs Puranjay Mohan
2024-04-26 16:19 ` Andrii Nakryiko
2024-04-26 16:55 ` Puranjay Mohan
2024-04-26 17:35 ` Andrii Nakryiko [this message]
2024-04-30 18:30 ` Puranjay Mohan
2024-04-26 12:13 ` [PATCH bpf-next v3 2/2] bpf, arm64: inline bpf_get_smp_processor_id() helper Puranjay Mohan
2024-04-26 16:26 ` Andrii Nakryiko
2024-04-26 17:06 ` Puranjay Mohan
2024-04-26 17:31 ` Andrii Nakryiko
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