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

[Björn Töpel]

Puranjay Mohan <puranjay12@xxxxxxxxx> writes:

> 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
     ^
     k ;-)
> 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.

Just to clarify for readers; BPF programs are run with migrate disable,
which means that on RT we can be preempted, which means that per-cpu
operations are trickier (disabling interrupts/preemption).

However, this BPF instruction is about calculating the per-cpu address,
so the look up can be inlined.

It's not a per-cpu *operation*.

> 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
>
> [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);
> +#ifdef CONFIG_SMP
> +				/* Load current CPU number in T1 */
> +				emit_ld(RV_REG_T1, offsetof(struct thread_info, cpu), RV_REG_TP,
> +					ctx);
> +				/* << 3 because offsets are 8 bytes */
> +				emit_slli(RV_REG_T1, RV_REG_T1, 3, ctx);
> +				/* Load address of __per_cpu_offset array in T2 */
> +				emit_imm(RV_REG_T2, (u64)&__per_cpu_offset, ctx);

Did you try using emit_addr() here? I'd guess that'll be fewer
instructions, no?


Björn