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

Andrew Jones
2025-03-13T12:57:35Z

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@rivosinc.com>
> ---
> 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@ventanamicro.com>