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:

  57c82954 ("arm64: make cpu number a percpu variable")

Since then, thread_info::cpu was reintroduced, and core code was made to use
this in commits:

  001430c1 ("arm64: add CPU field to struct thread_info")
  bcf9033e ("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.

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%) |
      +---------------------------------------------------+

Remove the percpu variable cpu_number as it is used only in
set_smp_ipi_range() as a dummy variable to be passed to ipi_handler().
Use irq_stat in place of cpu_number here like arm32.

[1] https://github.com/puranjaymohan/linux/commit/77d3fddSigned-off-by: Puranjay Mohan <puranjay@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20240503171847.68267-2-puranjay@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>