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

[Puranjay Mohan <puranjay12@gmail.com>]

Björn Töpel <bjorn@kernel.org> writes:

> Puranjay Mohan <puranjay12@gmail.com> 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 ;-)

I need to start using some kind of spell-check in vim :D.

>> 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*.

Will add this information to the commit message.

>> 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@gmail.com>
>> ---
>>  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?

Yes, I should have used that, the address would always be in the range
of auipc+addi right? I will try and see. 

Thanks,
Puranjay