[PATCH v2 1/2] arm64/arch_timer: include <linux/percpu.h>

[PATCH v2 1/2] arm64/arch_timer: include <linux/percpu.h>

[Puranjay Mohan]

arch_timer.h includes linux/smp.h to use DEFINE_PER_CPU() and it works
because smp.h includes percpu.h. The next commit will remove percpu.h
from smp.h and it will break this usage.

Explicitly include percpu.h and remove smp.h

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 arch/arm64/include/asm/arch_timer.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
index 934c658ee947..f5794d50f51d 100644
--- a/arch/arm64/include/asm/arch_timer.h
+++ b/arch/arm64/include/asm/arch_timer.h
@@ -15,7 +15,7 @@
 #include <linux/bug.h>
 #include <linux/init.h>
 #include <linux/jump_label.h>
-#include <linux/smp.h>
+#include <linux/percpu.h>
 #include <linux/types.h>
 
 #include <clocksource/arm_arch_timer.h>
-- 
2.40.1


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[PATCH v2 2/2] arm64: implement raw_smp_processor_id() using thread_info

[Puranjay Mohan]

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@kernel.org>
---
Changes in v1 -> v2:
v1: https://lore.kernel.org/all/20240501154236.10236-1-puranjay@kernel.org/
- 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


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Re: [PATCH v2 1/2] arm64/arch_timer: include <linux/percpu.h>

[Anshuman Khandual]

On 5/2/24 18:04, Puranjay Mohan wrote:
> arch_timer.h includes linux/smp.h to use DEFINE_PER_CPU() and it works
> because smp.h includes percpu.h. The next commit will remove percpu.h
> from smp.h and it will break this usage.
> 
> Explicitly include percpu.h and remove smp.h

But this particular change does not seem to be necessary for changing
raw_smp_processor_id() as current_thread_info()->cpu being done in the
later patch ? You might still leave header <asm/percpu.h> inclusion in
arch/arm64/include/asm/smp.h while dropping the per cpu cpu_number ?

> 
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> ---
>  arch/arm64/include/asm/arch_timer.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
> index 934c658ee947..f5794d50f51d 100644
> --- a/arch/arm64/include/asm/arch_timer.h
> +++ b/arch/arm64/include/asm/arch_timer.h
> @@ -15,7 +15,7 @@
>  #include <linux/bug.h>
>  #include <linux/init.h>
>  #include <linux/jump_label.h>
> -#include <linux/smp.h>
> +#include <linux/percpu.h>
>  #include <linux/types.h>
>  
>  #include <clocksource/arm_arch_timer.h>

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Re: [PATCH v2 2/2] arm64: implement raw_smp_processor_id() using thread_info

[Anshuman Khandual]

On 5/2/24 18:04, Puranjay Mohan wrote:
> 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 captures Mark's earlier suggestion on the commit message.


> 
> 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@kernel.org>
> ---
> Changes in v1 -> v2:
> v1: https://lore.kernel.org/all/20240501154236.10236-1-puranjay@kernel.org/
> - 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);
>  		}

set_smp_ipi_range() now looks similar to what we have on arm32 platform.

Besides the arch/arm64/include/asm/smp.h changes (not sure if is just a
clean up, then might be a stand alone patch on its own) this patch LGTM.

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Re: [PATCH v2 1/2] arm64/arch_timer: include <linux/percpu.h>

[Puranjay Mohan]

Anshuman Khandual <anshuman.khandual@arm.com> writes:

> On 5/2/24 18:04, Puranjay Mohan wrote:
>> arch_timer.h includes linux/smp.h to use DEFINE_PER_CPU() and it works
>> because smp.h includes percpu.h. The next commit will remove percpu.h
>> from smp.h and it will break this usage.
>> 
>> Explicitly include percpu.h and remove smp.h
>
> But this particular change does not seem to be necessary for changing
> raw_smp_processor_id() as current_thread_info()->cpu being done in the
> later patch ? You might still leave header <asm/percpu.h> inclusion in
> arch/arm64/include/asm/smp.h while dropping the per cpu cpu_number ?

commit 57c82954e77f ("arm64: make cpu number a percpu variable")
created this percpu variable and included <asm/percpu.h> in <asm/smp.h>

Now we are removing the percpu variable cpu_number from smp.h, so there
is no need to keep percpu.h in smp.h

I feel users of DECLARE_PER_CPU_[...], etc. should include percpu.h and
not smp.h as it makes reading the code more easier and can thwart build
issues in the future, when headers are changed. 

Thanks,
Puranjay

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Re: [PATCH v2 1/2] arm64/arch_timer: include <linux/percpu.h>

[Mark Rutland]

On Thu, May 02, 2024 at 12:34:48PM +0000, Puranjay Mohan wrote:
> arch_timer.h includes linux/smp.h to use DEFINE_PER_CPU() and it works
> because smp.h includes percpu.h. The next commit will remove percpu.h
> from smp.h and it will break this usage.
> 
> Explicitly include percpu.h and remove smp.h
> 
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>

Looks like this was a thinko in commit:

  6acc71ccac7187fc ("arm64: arch_timer: Allows a CPU-specific erratum to only affect a subset of CPUs")

... which, as you say, should have included <linux/percpu.h> rather than
<linux/smp.h>.

This is a cleanup regardless of the next patch.

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  arch/arm64/include/asm/arch_timer.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
> index 934c658ee947..f5794d50f51d 100644
> --- a/arch/arm64/include/asm/arch_timer.h
> +++ b/arch/arm64/include/asm/arch_timer.h
> @@ -15,7 +15,7 @@
>  #include <linux/bug.h>
>  #include <linux/init.h>
>  #include <linux/jump_label.h>
> -#include <linux/smp.h>
> +#include <linux/percpu.h>
>  #include <linux/types.h>
>  
>  #include <clocksource/arm_arch_timer.h>
> -- 
> 2.40.1
> 

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Re: [PATCH v2 1/2] arm64/arch_timer: include <linux/percpu.h>

[Mark Rutland]

On Fri, May 03, 2024 at 02:37:45PM +0530, Anshuman Khandual wrote:
> 
> 
> On 5/2/24 18:04, Puranjay Mohan wrote:
> > arch_timer.h includes linux/smp.h to use DEFINE_PER_CPU() and it works
> > because smp.h includes percpu.h. The next commit will remove percpu.h
> > from smp.h and it will break this usage.
> > 
> > Explicitly include percpu.h and remove smp.h
> 
> But this particular change does not seem to be necessary for changing
> raw_smp_processor_id() as current_thread_info()->cpu being done in the
> later patch ? You might still leave header <asm/percpu.h> inclusion in
> arch/arm64/include/asm/smp.h while dropping the per cpu cpu_number ?

Why would that be preferable?

The general rule is that if a file uses something explicitly, it should include
the relevant header directly rather than something that happens to transitively
include that header.

We made a mistake and included the wrong header in commit:

  6acc71ccac7187fc ("arm64: arch_timer: Allows a CPU-specific erratum to only affect a subset of CPUs")

... so we should fix that regardless of the next patch.

The point of the next patch is to effectively revert commit:

  57c82954e77fa12c ("arm64: make cpu number a percpu variable")

... and reverting that means we should stop including <asm/percpu.h> from
<asm/smp.h>; anything depending on that is already doing something wrong, and
leaving the include there only serves to paper over bugs.

Mark.

> 
> > 
> > Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> > ---
> >  arch/arm64/include/asm/arch_timer.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
> > index 934c658ee947..f5794d50f51d 100644
> > --- a/arch/arm64/include/asm/arch_timer.h
> > +++ b/arch/arm64/include/asm/arch_timer.h
> > @@ -15,7 +15,7 @@
> >  #include <linux/bug.h>
> >  #include <linux/init.h>
> >  #include <linux/jump_label.h>
> > -#include <linux/smp.h>
> > +#include <linux/percpu.h>
> >  #include <linux/types.h>
> >  
> >  #include <clocksource/arm_arch_timer.h>

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Re: [PATCH v2 2/2] arm64: implement raw_smp_processor_id() using thread_info

[Mark Rutland]

On Thu, May 02, 2024 at 12:34:49PM +0000, Puranjay Mohan wrote:
> 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.
 
Minor nits:

* There's no need to say "commit" before each of these when the previous line
  ends with "commit:" or "commits:"

* It'd be better for these to each be single lines, even if they go over the
  usual line limit.

* I'd deliberately indented those commit lines with double-spaces to
  distinguish them from regular text in the commit message.

... so if you could use that text as-is (minus the "| " prefix on each line)
from:

  https://lore.kernel.org/linux-arm-kernel/ZjJwos7KpvzhoK_f@FVFF77S0Q05N.cambridge.arm.com/

... that'd be preferable.

> 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@kernel.org>
> ---
> Changes in v1 -> v2:
> v1: https://lore.kernel.org/all/20240501154236.10236-1-puranjay@kernel.org/
> - 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);

I was going to say that it might be worth having a dummy percpu variable
specifically for these, but given this is what 32-bit arm does, it makse sense
to do the same thing.

This looks good to me, so:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

>  			WARN(err, "Could not request IPI %d as IRQ, err=%d\n",
>  			     i, err);
>  		}
> -- 
> 2.40.1
> 

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Re: [PATCH v2 1/2] arm64/arch_timer: include <linux/percpu.h>

[Anshuman Khandual]

On 5/3/24 15:14, Puranjay Mohan wrote:
> Anshuman Khandual <anshuman.khandual@arm.com> writes:
> 
>> On 5/2/24 18:04, Puranjay Mohan wrote:
>>> arch_timer.h includes linux/smp.h to use DEFINE_PER_CPU() and it works
>>> because smp.h includes percpu.h. The next commit will remove percpu.h
>>> from smp.h and it will break this usage.
>>>
>>> Explicitly include percpu.h and remove smp.h
>>
>> But this particular change does not seem to be necessary for changing
>> raw_smp_processor_id() as current_thread_info()->cpu being done in the
>> later patch ? You might still leave header <asm/percpu.h> inclusion in
>> arch/arm64/include/asm/smp.h while dropping the per cpu cpu_number ?
> 
> commit 57c82954e77f ("arm64: make cpu number a percpu variable")
> created this percpu variable and included <asm/percpu.h> in <asm/smp.h>
> 
> Now we are removing the percpu variable cpu_number from smp.h, so there
> is no need to keep percpu.h in smp.h

Fair enough.

> 
> I feel users of DECLARE_PER_CPU_[...], etc. should include percpu.h and
> not smp.h as it makes reading the code more easier and can thwart build
> issues in the future, when headers are changed. 
Right, makes sense, hope there is no more such cases using smp.h to pull
in DECLARE_PER_CPU_[...]. A quick build on defconfig is successful after
this patch.

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Re: [PATCH v2 1/2] arm64/arch_timer: include <linux/percpu.h>

[Anshuman Khandual]

On 5/2/24 18:04, Puranjay Mohan wrote:
> arch_timer.h includes linux/smp.h to use DEFINE_PER_CPU() and it works
> because smp.h includes percpu.h. The next commit will remove percpu.h
> from smp.h and it will break this usage.
> 
> Explicitly include percpu.h and remove smp.h
> 
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

> ---
>  arch/arm64/include/asm/arch_timer.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
> index 934c658ee947..f5794d50f51d 100644
> --- a/arch/arm64/include/asm/arch_timer.h
> +++ b/arch/arm64/include/asm/arch_timer.h
> @@ -15,7 +15,7 @@
>  #include <linux/bug.h>
>  #include <linux/init.h>
>  #include <linux/jump_label.h>
> -#include <linux/smp.h>
> +#include <linux/percpu.h>
>  #include <linux/types.h>
>  
>  #include <clocksource/arm_arch_timer.h>

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Re: [PATCH v2 2/2] arm64: implement raw_smp_processor_id() using thread_info

[Anshuman Khandual]

On 5/2/24 18:04, Puranjay Mohan wrote:
> 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@kernel.org>

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

> ---
> Changes in v1 -> v2:
> v1: https://lore.kernel.org/all/20240501154236.10236-1-puranjay@kernel.org/
> - 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);
>  		}

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