[PATCH] arm64: implement raw_smp_processor_id() using thread_info

[Puranjay Mohan]

ARM64 defines THREAD_INFO_IN_TASK which means the cpu id can be found
from current_thread_info()->cpu.

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 at kernel.org>
---
 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)
 
 /*
  * Logical CPU mapping.
-- 
2.40.1