Commit bf0baa5b authored by Puranjay Mohan's avatar Puranjay Mohan Committed by Catalin Marinas
Browse files

arm64: implement raw_smp_processor_id() using thread_info 

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/77d3fdd



Signed-off-by: default avatarPuranjay Mohan <puranjay@kernel.org>
Acked-by: default avatarMark Rutland <mark.rutland@arm.com>
Reviewed-by: default avatarStephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20240503171847.68267-2-puranjay@kernel.org


Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
parent 7647e2b1

arch/arm64/include/asm/smp.h

+1 −12
Original line number Diff line number Diff line
@@ -25,22 +25,11 @@ 


#ifndef __ASSEMBLY__



#include <asm/percpu.h>



#include <linux/threads.h>

#include <linux/cpumask.h>

#include <linux/thread_info.h>



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.

 */

#define raw_smp_processor_id() (*raw_cpu_ptr(&cpu_number))

#define raw_smp_processor_id() (current_thread_info()->cpu)



/*

 * Logical CPU mapping.

arch/arm64/kernel/smp.c

+2 −7
Original line number Diff line number Diff line
@@ -55,9 +55,6 @@ 


#include <trace/events/ipi.h>



DEFINE_PER_CPU_READ_MOSTLY(int, cpu_number);

EXPORT_PER_CPU_SYMBOL(cpu_number);



/*

 * as from 2.5, kernels no longer have an init_tasks structure

 * so we need some other way of telling a new secondary core
@@ -749,8 +746,6 @@ void __init smp_prepare_cpus(unsigned int max_cpus) 
	 */

	for_each_possible_cpu(cpu) {



		per_cpu(cpu_number, cpu) = cpu;



		if (cpu == smp_processor_id())

			continue;
@@ -1028,12 +1023,12 @@ void __init set_smp_ipi_range(int ipi_base, int n) 


		if (ipi_should_be_nmi(i)) {

			err = request_percpu_nmi(ipi_base + i, ipi_handler,

						 "IPI", &cpu_number);

						 "IPI", &irq_stat);

			WARN(err, "Could not request IPI %d as NMI, err=%d\n",

			     i, err);

		} else {

			err = request_percpu_irq(ipi_base + i, ipi_handler,

						 "IPI", &cpu_number);

						 "IPI", &irq_stat);

			WARN(err, "Could not request IPI %d as IRQ, err=%d\n",

			     i, err);

		}