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

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Date:   Thu, 14 Mar 2019 18:38:33 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Paul Burton <paul.burton@...s.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 Paul,

On 14/03/2019 18:13, Paul Burton wrote:
[...]
> 
> 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.").
> 

I didn't notice the missing branch to restore_all in that first commit -
that makes (more) sense now.

[...]
>> @@ -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? 

No actual reason there, I most likely deleted the branch, looked around,
saw the "j restore_all" in @resume_userspace and went for that (shoddy
I know...)

> 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.
> 

Makes total sense, thanks for the detailed reasoning!

> 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.)
> 

Right, that's even better, I'll send a v2 with that.

> 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
>>