Re: [PATCH bpf-next] riscv, bpf: add internal-only MOV instruction to resolve per-CPU addrs

[Björn Töpel]

Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:

> On Fri, Apr 5, 2024 at 5:44 AM Puranjay Mohan <puranjay12@xxxxxxxxx> wrote:
>>
>> 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.
>>
>> RISC-V uses generic per-cpu implementation where the offsets for CPUs
>> are kept in an array called __per_cpu_offset[cpu_number]. RISCV stores
>> the address of the task_struct in TP register. The first element in
>> tast_struct is struct thread_info, and we can get the cpu number by
>> reading from the TP register + offsetof(struct thread_info, cpu).
>>
>> Once we have the cpu number in a register we read the offset for that
>> cpu from address: &__per_cpu_offset + cpu_number << 3. Then we add this
>> offset to the destination register.
>>
>> To measure the improvement from this change, the benchmark in [1] was
>> used on Qemu:
>>
>> Before:
>> glob-arr-inc   :    1.127 ± 0.013M/s
>> arr-inc        :    1.121 ± 0.004M/s
>> hash-inc       :    0.681 ± 0.052M/s
>>
>> After:
>> glob-arr-inc   :    1.138 ± 0.011M/s
>> arr-inc        :    1.366 ± 0.006M/s
>> hash-inc       :    0.676 ± 0.001M/s
>
> TBH, I don't trust benchmarks done inside QEMU. Can you try running
> this on some real hardware?

I just ran it on a "VisionFive2" SBC:

BEFORE
======
glob-arr-inc   :   11.586 ± 0.021M/s 
arr-inc        :   10.892 ± 0.005M/s 
hash-inc       :    1.517 ± 0.001M/s 

AFTER
=====
glob-arr-inc   :   11.893 ± 0.017M/s  (+2.6%)
arr-inc        :   11.630 ± 0.020M/s  (+6.8%)
hash-inc       :    1.543 ± 0.002M/s  (+1.7%)

(It's early, and the coffee haven't kicked in, so I hope the
calculations are correct...)

>>
>> [1] https://github.com/anakryiko/linux/commit/8dec900975ef
>>
>> Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
>> ---
>>  arch/riscv/net/bpf_jit_comp64.c | 24 ++++++++++++++++++++++++
>>  1 file changed, 24 insertions(+)
>>
>> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
>> index 15e482f2c657..e95bd1d459a4 100644
>> --- a/arch/riscv/net/bpf_jit_comp64.c
>> +++ b/arch/riscv/net/bpf_jit_comp64.c
>> @@ -12,6 +12,7 @@
>>  #include <linux/stop_machine.h>
>>  #include <asm/patch.h>
>>  #include <asm/cfi.h>
>> +#include <asm/percpu.h>
>>  #include "bpf_jit.h"
>>
>>  #define RV_FENTRY_NINSNS 2
>> @@ -1089,6 +1090,24 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>>                         emit_or(RV_REG_T1, rd, RV_REG_T1, ctx);
>>                         emit_mv(rd, RV_REG_T1, ctx);
>>                         break;
>> +               } else if (insn_is_mov_percpu_addr(insn)) {
>> +                       if (rd != rs)
>> +                               emit_mv(rd, rs, ctx);
>
> Is this an unconditional move instruction? in x86-64, EMIT_mov checks
> whether source and destination registers are the same and doesn't emit
> anything if they match (which makes sense, right)?

Yeah, it is. Folding the check into the emit sounds like a good idea.


Björn