Mark Rutland <mark.rutland@xxxxxxx> writes:
> Hi Puranjay,
>
> On Wed, May 01, 2024 at 03:42:36PM +0000, Puranjay Mohan wrote:
>> ARM64 defines THREAD_INFO_IN_TASK which means the cpu id can be found
>> from current_thread_info()->cpu.
>
> Nice!
>
> This is something that we'd wanted to do, but there were some historical
> reasons that prevented that. I think it'd be worth describing that in the
> commit message, e.g.
>
> | Historically, arm64 implemented raw_smp_processor_id() as a read of
> | current_thread_info()->cpu. This changed when arm64 moved thread_info into
> | task struct, as at the time CONFIG_THREAD_INFO_IN_TASK made core code use
> | thread_struct::cpu for the cpu number, and due to header dependencies
> | prevented using this in raw_smp_processor_id(). As a workaround, we moved to
> | using a percpu variable in commit:
> |
> | 57c82954e77fa12c ("arm64: make cpu number a percpu variable")
> |
> | Since then, thread_info::cpu was reintroduced, and core code was made to use
> | this in commits:
> |
> | 001430c1910df65a ("arm64: add CPU field to struct thread_info")
> | bcf9033e5449bdca ("sched: move CPU field back into thread_info if THREAD_INFO_IN_TASK=y")
> |
> | Consequently it is possible to use current_thread_info()->cpu again.
>
>> Implement raw_smp_processor_id() using the above. This decreases the
>> number of emitted instructions like in the following example:
>>
>> Dump of assembler code for function bpf_get_smp_processor_id:
>> 0xffff8000802cd608 <+0>: nop
>> 0xffff8000802cd60c <+4>: nop
>> 0xffff8000802cd610 <+8>: adrp x0, 0xffff800082138000
>> 0xffff8000802cd614 <+12>: mrs x1, tpidr_el1
>> 0xffff8000802cd618 <+16>: add x0, x0, #0x8
>> 0xffff8000802cd61c <+20>: ldrsw x0, [x0, x1]
>> 0xffff8000802cd620 <+24>: ret
>>
>> After this patch:
>>
>> Dump of assembler code for function bpf_get_smp_processor_id:
>> 0xffff8000802c9130 <+0>: nop
>> 0xffff8000802c9134 <+4>: nop
>> 0xffff8000802c9138 <+8>: mrs x0, sp_el0
>> 0xffff8000802c913c <+12>: ldr w0, [x0, #24]
>> 0xffff8000802c9140 <+16>: ret
>>
>> A microbenchmark[1] was built to measure the performance improvement
>> provided by this change. It calls the following function given number of
>> times and finds the runtime overhead:
>>
>> static noinline int get_cpu_id(void)
>> {
>> return smp_processor_id();
>> }
>>
>> Run the benchmark like:
>> modprobe smp_processor_id nr_function_calls=1000000000
>>
>> +--------------------------+------------------------+
>> | | Number of Calls | Time taken |
>> +--------+-----------------+------------------------+
>> | Before | 1000000000 | 1602888401ns |
>> +--------+-----------------+------------------------+
>> | After | 1000000000 | 1206212658ns |
>> +--------+-----------------+------------------------+
>> | Difference (decrease) | 396675743ns (24.74%) |
>> +---------------------------------------------------+
>>
>> This improvement is in this very specific microbenchmark but it proves
>> the point.
>>
>> The percpu variable cpu_number is left as it is because it is used in
>> set_smp_ipi_range()
>>
>> [1] https://github.com/puranjaymohan/linux/commit/77d3fdd
>>
>> Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>
>> ---
>> arch/arm64/include/asm/smp.h | 8 ++------
>> 1 file changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
>> index efb13112b408..88fd2ab805ec 100644
>> --- a/arch/arm64/include/asm/smp.h
>> +++ b/arch/arm64/include/asm/smp.h
>> @@ -34,13 +34,9 @@
>> DECLARE_PER_CPU_READ_MOSTLY(int, cpu_number);
>>
>> /*
>> - * We don't use this_cpu_read(cpu_number) as that has implicit writes to
>> - * preempt_count, and associated (compiler) barriers, that we'd like to avoid
>> - * the expense of. If we're preemptible, the value can be stale at use anyway.
>> - * And we can't use this_cpu_ptr() either, as that winds up recursing back
>> - * here under CONFIG_DEBUG_PREEMPT=y.
>> + * This relies on THREAD_INFO_IN_TASK, but arm64 defines that unconditionally.
>> */
>> -#define raw_smp_processor_id() (*raw_cpu_ptr(&cpu_number))
>> +#define raw_smp_processor_id() (current_thread_info()->cpu)
>
> I think we can (and should) delete the comment entirely.
Sure,
I will add the information to the commit message and remove this comment
in the next version.
I think it would be useful to remove the cpu_number percpu variable as
well.
We can use &irq_stat in place of &cpu_number in set_smp_ipi_range() in
the calls to request_percpu_nmi/irq() as this is just a dummy value and
ipi_handler() doesn't use it.
There are no other users of cpu_number.
Thanks,
Puranjay
