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:
commit 57c82954e77f ("arm64: make cpu number a percpu variable")
Since then, thread_info::cpu was reintroduced, and core code was made to
use this in commits:
commit 001430c1910d ("arm64: add CPU field to struct thread_info")
commit bcf9033e5449 ("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.
[1] https://github.com/puranjaymohan/linux/commit/77d3fdd
Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>
---
Changes in v1 -> v2:
v1: https://lore.kernel.org/all/20240501154236.10236-1-puranjay@xxxxxxxxxx/
- Remove the percpu variable cpu_number
- Add more information to the commit message.
---
arch/arm64/include/asm/smp.h | 13 +------------
arch/arm64/kernel/smp.c | 9 ++-------
2 files changed, 3 insertions(+), 19 deletions(-)
diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
index efb13112b408..2510eec026f7 100644
--- a/arch/arm64/include/asm/smp.h
+++ b/arch/arm64/include/asm/smp.h
@@ -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.
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 4ced34f62dab..98d4e352c3d0 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -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
@@ -742,8 +739,6 @@ 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;
@@ -1021,12 +1016,12 @@ 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);
}
--
2.40.1
