Re: [PATCH 1/3] ARM: Pass cpu number to secondary_start_kernel

Ard Biesheuvel
2021-09-05T20:25:51Z

Hi Keith,

This looks like a worthwhile cleanup to me regardless of the following patches.

On Sat, 4 Sept 2021 at 08:09, Keith Packard <keithp@keithp.com> wrote:
>
> Instead of pulling the CPU value out of the thread_info struct, pass
> it as an argument. When first initializing secondary processors, this
> is done by stashing the value in the secondary_data struct. When
> restarting idle processors, the previous CPU value is passed.
>
> Because the cpu is now known at the top of secondary_start_kernel, the
> per_cpu_offset value can now be set at this point, instead of in
> cpu_init where it was also incorrectly setting the per_cpu_offset for
> the boot processor before that value had been computed.
>
> Signed-off-by: Keith Packard <keithpac@amazon.com>

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

> ---
> arch/arm/include/asm/smp.h | 3 ++-
> arch/arm/kernel/head-nommu.S | 1 +
> arch/arm/kernel/head.S | 1 +
> arch/arm/kernel/setup.c | 6 ------
> arch/arm/kernel/smp.c | 14 +++++++++-----
> 5 files changed, 13 insertions(+), 12 deletions(-)
>
> diff --git a/arch/arm/include/asm/smp.h b/arch/arm/include/asm/smp.h
> index 5d508f5d56c4..86a7fd721556 100644
> --- a/arch/arm/include/asm/smp.h
> +++ b/arch/arm/include/asm/smp.h
> @@ -48,7 +48,7 @@ extern void set_smp_ipi_range(int ipi_base, int nr_ipi);
> * Called from platform specific assembly code, this is the
> * secondary CPU entry point.
> */
> -asmlinkage void secondary_start_kernel(void);
> +asmlinkage void secondary_start_kernel(unsigned int cpu);
>
>
> /*
> @@ -61,6 +61,7 @@ struct secondary_data {
> };
> unsigned long swapper_pg_dir;
> void *stack;
> + unsigned int cpu;
> };
> extern struct secondary_data secondary_data;
> extern void secondary_startup(void);
> diff --git a/arch/arm/kernel/head-nommu.S b/arch/arm/kernel/head-nommu.S
> index 0fc814bbc34b..5aa8ef42717f 100644
> --- a/arch/arm/kernel/head-nommu.S
> +++ b/arch/arm/kernel/head-nommu.S
> @@ -114,6 +114,7 @@ ENTRY(secondary_startup)
> add r12, r12, r10
> ret r12
> 1: bl __after_proc_init
> + ldr r0, [r7, #16] @ set up cpu number
> ldr sp, [r7, #12] @ set up the stack pointer
> mov fp, #0
> b secondary_start_kernel
> diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
> index 7f62c5eccdf3..0e541af738e2 100644
> --- a/arch/arm/kernel/head.S
> +++ b/arch/arm/kernel/head.S
> @@ -394,6 +394,7 @@ ENDPROC(secondary_startup_arm)
>
> ENTRY(__secondary_switched)
> ldr_l r7, secondary_data + 12 @ get secondary_data.stack
> + ldr_l r0, secondary_data + 16 @ get secondary_data.cpu
> mov sp, r7
> mov fp, #0
> b secondary_start_kernel
> diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> index 73ca7797b92f..ca0201635fac 100644
> --- a/arch/arm/kernel/setup.c
> +++ b/arch/arm/kernel/setup.c
> @@ -532,12 +532,6 @@ void notrace cpu_init(void)
> BUG();
> }
>
> - /*
> - * This only works on resume and secondary cores. For booting on the
> - * boot cpu, smp_prepare_boot_cpu is called after percpu area setup.
> - */
> - set_my_cpu_offset(per_cpu_offset(cpu));
> -
> cpu_proc_init();
>
> /*
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 74679240a9d8..55cb1689a4b3 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -153,6 +153,7 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
> secondary_data.pgdir = virt_to_phys(idmap_pgd);
> secondary_data.swapper_pg_dir = get_arch_pgd(swapper_pg_dir);
> #endif
> + secondary_data.cpu = cpu;
> sync_cache_w(&secondary_data);
>
> /*
> @@ -373,11 +374,14 @@ void arch_cpu_idle_dead(void)
> * cpu initialisation. There's some initialisation which needs
> * to be repeated to undo the effects of taking the CPU offline.
> */
> - __asm__("mov sp, %0\n"
> + __asm__("mov r0, %1\n"
> + " mov sp, %0\n"
> " mov fp, #0\n"
> " b secondary_start_kernel"
> :
> - : "r" (task_stack_page(current) + THREAD_SIZE - 8));
> + : "r" (task_stack_page(current) + THREAD_SIZE - 8),
> + "r" (cpu)
> + : "r0");
> }
> #endif /* CONFIG_HOTPLUG_CPU */
>
> @@ -400,10 +404,11 @@ static void smp_store_cpu_info(unsigned int cpuid)
> * This is the secondary CPU boot entry. We're using this CPUs
> * idle thread stack, but a set of temporary page tables.
> */
> -asmlinkage void secondary_start_kernel(void)
> +asmlinkage void secondary_start_kernel(unsigned int cpu)
> {
> struct mm_struct *mm = &init_mm;
> - unsigned int cpu;
> +
> + set_my_cpu_offset(per_cpu_offset(cpu));
>
> secondary_biglittle_init();
>
> @@ -420,7 +425,6 @@ asmlinkage void secondary_start_kernel(void)
> * All kernel threads share the same mm context; grab a
> * reference and switch to it.
> */
> - cpu = smp_processor_id();
> mmgrab(mm);
> current->active_mm = mm;
> cpumask_set_cpu(cpu, mm_cpumask(mm));
> --
> 2.33.0
>