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

[Charlie Jenkins <charlie@rivosinc.com>]

On Fri, Jan 24, 2025 at 08:19:18AM +0100, Alexandre Ghiti wrote:
> 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@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%.

It reduces the syscall path by 3 instructions, two for not needing to
move the fn and fn_args from:

move a0, s1 /* fn */
move a1, s0 /* fn_arg */

And one for not needing to do the conditional. This one is also saved on
kernel threads.

It's a very small improvement, but there is only something like 100
instructions along the direct syscall path so it ends up being a large
percentage. On hardware moving registers is very cheap and this branch
will be almost always be correctly predicted so the cost is close to
zero. I just figured that since I am making changes around here it would
be nice if it was optimal instead of being close to optimal.

- Charlie

> 
> Thanks,
> 
> Alex
>