linux-kernel - Re: [PATCH 06/14] MIPS: entry: Remove unneeded need

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Date:   Thu, 14 Mar 2019 18:13:07 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Valentin Schneider <valentin.schneider@....com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>
Subject: Re: [PATCH 06/14] MIPS: entry: Remove unneeded need_resched() loop

Hi Valentin,

On Mon, Mar 11, 2019 at 10:47:44PM +0000, Valentin Schneider wrote:
> Since the enabling and disabling of IRQs within preempt_schedule_irq()
> is contained in a need_resched() loop, we don't need the outer arch
> code loop.
> 
> Do note that commit a18815abcdfd ("Use preempt_schedule_irq.")
> initially removed the existing loop, but commit cdaed73afb61 ("Fix
> preemption bug.") reintroduced it. It is however not clear what the
> issue was and why such a loop was reintroduced, so I'm probably
> missing something.

It looks to me like commit a18815abcdfd ("Use preempt_schedule_irq.")
forgot the branch to restore_all, so would have fallen through to
ret_from_fork() & done weird things.

Adding the branch to restore_all as you're doing here would have been a
better fix than commit cdaed73afb61 ("Fix preemption bug.").

> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> Cc: Ralf Baechle <ralf@...ux-mips.org>
> Cc: Paul Burton <paul.burton@...s.com>
> Cc: James Hogan <jhogan@...nel.org>
> Cc: linux-mips@...r.kernel.org
> ---
>  arch/mips/kernel/entry.S | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/mips/kernel/entry.S b/arch/mips/kernel/entry.S
> index d7de8adcfcc8..2240faeda62a 100644
> --- a/arch/mips/kernel/entry.S
> +++ b/arch/mips/kernel/entry.S
> @@ -58,7 +58,6 @@ resume_kernel:
>  	local_irq_disable
>  	lw	t0, TI_PRE_COUNT($28)
>  	bnez	t0, restore_all
> -need_resched:
>  	LONG_L	t0, TI_FLAGS($28)
>  	andi	t1, t0, _TIF_NEED_RESCHED
>  	beqz	t1, restore_all
> @@ -66,7 +65,7 @@ need_resched:
>  	andi	t0, 1
>  	beqz	t0, restore_all
>  	jal	preempt_schedule_irq
> -	b	need_resched
> +	j	restore_all

One nit - why change from branch to jump? It's not a big deal, but I'd
prefer we stick with the branch ("b") instruction for a few reasons:

- restore_all is nearby so there's no issue with it being out of range
  of a branch in any variation of the MIPS ISA.

- It's more consistent with the future of the MIPS architecture, eg.
  nanoMIPS in which branch instructions all use PC-relative immediate
  offsets & jump instructions are always of the "register" variety where
  the destination is specified by a register rather than an immediate
  encoded in the instruction (the assembler will fix it up & emit a
  branch anyway, but I generally prefer to invoke less magic in these
  areas...).

- A PC-relative branch won't generate an extra reloc in a relocatable
  kernel, whereas a jump will.

Even better would be if we take advantage of this being a tail call & do
this:

	PTR_LA	ra, restore_all
	j	preempt_schedule_irq

(Where I left the call to preempt_schedule_irq using a jump because it
may be further away.)

Though I don't mind if you wanna just s/j/b/ & leave the tail call
optimisation for someone else to do as a later change.

Thanks,
    Paul

>  #endif
>  
>  FEXPORT(ret_from_kernel_thread)
> -- 
> 2.20.1
>