Re: [PATCH v3 05/17] riscv: misaligned: use on_each_cpu() for scalar misaligned access probing

[Andrew Jones]

On Mon, Mar 10, 2025 at 04:12:12PM +0100, Clément Léger wrote:
> schedule_on_each_cpu() was used without any good reason while documented
> as very slow. This call was in the boot path, so better use
> on_each_cpu() for scalar misaligned checking. Vector misaligned check
> still needs to use schedule_on_each_cpu() since it requires irqs to be
> enabled but that's less of a problem since this code is ran in a kthread.
> Add a comment to explicit that.
> 
> Signed-off-by: Clément Léger <cleger@xxxxxxxxxxxx>
> ---
>  arch/riscv/kernel/traps_misaligned.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
> index 90ac74191357..ffac424faa88 100644
> --- a/arch/riscv/kernel/traps_misaligned.c
> +++ b/arch/riscv/kernel/traps_misaligned.c
> @@ -616,6 +616,11 @@ bool check_vector_unaligned_access_emulated_all_cpus(void)
>  		return false;
>  	}
>  
> +	/*
> +	 * While being documented as very slow, schedule_on_each_cpu() is used
> +	 * since kernel_vector_begin() that is called inside the vector code
> +	 * expects irqs to be enabled or it will panic().

which expects

> +	 */
>  	schedule_on_each_cpu(check_vector_unaligned_access_emulated);
>  
>  	for_each_online_cpu(cpu)
> @@ -636,7 +641,7 @@ bool check_vector_unaligned_access_emulated_all_cpus(void)
>  
>  static bool unaligned_ctl __read_mostly;
>  
> -static void check_unaligned_access_emulated(struct work_struct *work __always_unused)
> +static void check_unaligned_access_emulated(void *arg __always_unused)
>  {
>  	int cpu = smp_processor_id();
>  	long *mas_ptr = per_cpu_ptr(&misaligned_access_speed, cpu);
> @@ -677,7 +682,7 @@ bool check_unaligned_access_emulated_all_cpus(void)
>  	 * accesses emulated since tasks requesting such control can run on any
>  	 * CPU.
>  	 */
> -	schedule_on_each_cpu(check_unaligned_access_emulated);
> +	on_each_cpu(check_unaligned_access_emulated, NULL, 1);
>  
>  	for_each_online_cpu(cpu)
>  		if (per_cpu(misaligned_access_speed, cpu)
> -- 
> 2.47.2
>

Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>