[PATCH v2 2/4] riscv: entry: Split ret_from_fork() into user and kernel

[Alexandre Ghiti]

Hi Charlie,

On 23/01/2025 20:14, Charlie Jenkins wrote:
> This function was unified into a single function in commit ab9164dae273
> ("riscv: entry: Consolidate ret_from_kernel_thread into ret_from_fork").
> However that imposed a performance degradation. Partially reverting this
> commit to have ret_from_fork() split again results in a 1% increase on
> the number of times fork is able to be called per second.
>
> Signed-off-by: Charlie Jenkins <charlie at rivosinc.com>
> ---
>   arch/riscv/include/asm/asm-prototypes.h |  3 ++-
>   arch/riscv/kernel/entry.S               | 13 ++++++++++---
>   arch/riscv/kernel/process.c             | 17 +++++++++++------
>   3 files changed, 23 insertions(+), 10 deletions(-)
>
> diff --git a/arch/riscv/include/asm/asm-prototypes.h b/arch/riscv/include/asm/asm-prototypes.h
> index 733ff609778797001006c33bba9e3cc5b1f15387..bfc8ea5f9319b19449ec59493b45b926df888832 100644
> --- a/arch/riscv/include/asm/asm-prototypes.h
> +++ b/arch/riscv/include/asm/asm-prototypes.h
> @@ -52,7 +52,8 @@ DECLARE_DO_ERROR_INFO(do_trap_ecall_s);
>   DECLARE_DO_ERROR_INFO(do_trap_ecall_m);
>   DECLARE_DO_ERROR_INFO(do_trap_break);
>   
> -asmlinkage void ret_from_fork(void *fn_arg, int (*fn)(void *), struct pt_regs *regs);
> +asmlinkage void ret_from_fork_kernel(void *fn_arg, int (*fn)(void *), struct pt_regs *regs);
> +asmlinkage void ret_from_fork_user(struct pt_regs *regs);
>   asmlinkage void handle_bad_stack(struct pt_regs *regs);
>   asmlinkage void do_page_fault(struct pt_regs *regs);
>   asmlinkage void do_irq(struct pt_regs *regs);
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 9225c322279aa90e737b1d7144db084319cf8103..9386ef7444267f0b9bf8a0550f4e31deaeb85881 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -319,14 +319,21 @@ SYM_CODE_END(handle_kernel_stack_overflow)
>   ASM_NOKPROBE(handle_kernel_stack_overflow)
>   #endif
>   
> -SYM_CODE_START(ret_from_fork_asm)
> +SYM_CODE_START(ret_from_fork_kernel_asm)
>   	call schedule_tail
>   	move a0, s1 /* fn */
>   	move a1, s0 /* fn_arg */
>   	move a2, sp /* pt_regs */
> -	call ret_from_fork
> +	call ret_from_fork_kernel
>   	j ret_from_exception
> -SYM_CODE_END(ret_from_fork_asm)
> +SYM_CODE_END(ret_from_fork_kernel_asm)
> +
> +SYM_CODE_START(ret_from_fork_user_asm)
> +	call schedule_tail
> +	move a0, sp /* pt_regs */
> +	call ret_from_fork_user
> +	j ret_from_exception
> +SYM_CODE_END(ret_from_fork_user_asm)
>   
>   #ifdef CONFIG_IRQ_STACKS
>   /*
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> index 0d07e6d8f6b57beba438dbba5e8c74a014582bee..5f15236cb526bd9fe61636ed372b4b76c94df946 100644
> --- a/arch/riscv/kernel/process.c
> +++ b/arch/riscv/kernel/process.c
> @@ -38,7 +38,8 @@ unsigned long __stack_chk_guard __read_mostly;
>   EXPORT_SYMBOL(__stack_chk_guard);
>   #endif
>   
> -extern asmlinkage void ret_from_fork_asm(void);
> +extern asmlinkage void ret_from_fork_kernel_asm(void);
> +extern asmlinkage void ret_from_fork_user_asm(void);
>   
>   void noinstr arch_cpu_idle(void)
>   {
> @@ -208,14 +209,18 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
>   	return 0;
>   }
>   
> -asmlinkage void ret_from_fork(void *fn_arg, int (*fn)(void *), struct pt_regs *regs)
> +asmlinkage void ret_from_fork_kernel(void *fn_arg, int (*fn)(void *), struct pt_regs *regs)
>   {
> -	if (unlikely(fn))
> -		fn(fn_arg);
> +	fn(fn_arg);
>   
>   	syscall_exit_to_user_mode(regs);
>   }
>   
> +asmlinkage void ret_from_fork_user(struct pt_regs *regs)
> +{
> +	syscall_exit_to_user_mode(regs);
> +}
> +
>   int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>   {
>   	unsigned long clone_flags = args->flags;
> @@ -238,6 +243,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>   
>   		p->thread.s[0] = (unsigned long)args->fn;
>   		p->thread.s[1] = (unsigned long)args->fn_arg;
> +		p->thread.ra = (unsigned long)ret_from_fork_kernel_asm;
>   	} else {
>   		*childregs = *(current_pt_regs());
>   		/* Turn off status.VS */
> @@ -247,12 +253,11 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>   		if (clone_flags & CLONE_SETTLS)
>   			childregs->tp = tls;
>   		childregs->a0 = 0; /* Return value of fork() */
> -		p->thread.s[0] = 0;
> +		p->thread.ra = (unsigned long)ret_from_fork_user_asm;
>   	}
>   	p->thread.riscv_v_flags = 0;
>   	if (has_vector())
>   		riscv_v_thread_alloc(p);
> -	p->thread.ra = (unsigned long)ret_from_fork_asm;
>   	p->thread.sp = (unsigned long)childregs; /* kernel sp */
>   	return 0;
>   }
>

Can you benchmark this change on some HW? I'm not sure we would indeed 
gain this 1%.

Thanks,

Alex