Re: [PATCH bpf-next v3 1/2] arm64, bpf: add internal-only MOV instruction to resolve per-CPU addrs

[Puranjay Mohan]

Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:

> On Fri, Apr 26, 2024 at 9:55 AM Puranjay Mohan <puranjay@xxxxxxxxxx> wrote:
>>
>> Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:
>>
>> > On Fri, Apr 26, 2024 at 5:14 AM Puranjay Mohan <puranjay@xxxxxxxxxx> wrote:
>> >>
>> >> From: Puranjay Mohan <puranjay12@xxxxxxxxx>
>> >>
>> >> 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@xxxxxxxxxx>
>> >
>>
>> 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.
>

I found the solution to this. I am seeing much better performance when
implementing this inlining in the JIT through another method, similar to
what I did for riscv see[1]

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

Will do the same for ARM64 in V5 of this series.

Thanks,
Puranjay