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Date: Fri, 3 May 2024 16:30:44 +0100
From: Mark Rutland <mark.rutland@arm.com>
To: Puranjay Mohan <puranjay@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Sumit Garg <sumit.garg@linaro.org>,
	Stephen Boyd <swboyd@chromium.org>,
	Douglas Anderson <dianders@chromium.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
	bpf@vger.kernel.org, puranjay12@gmail.com
Subject: Re: [PATCH v2 2/2] arm64: implement raw_smp_processor_id() using
 thread_info
Message-ID: <ZjUDJFbdMlnLho5M@FVFF77S0Q05N>
References: <20240502123449.2690-1-puranjay@kernel.org>
 <20240502123449.2690-2-puranjay@kernel.org>
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In-Reply-To: <20240502123449.2690-2-puranjay@kernel.org>

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
>