linux-kernel - Re: Loongson (and other $ARCHs?) idle VS timer enqueue

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Date:   Tue, 25 Apr 2023 21:25:26 +0800
From:   maobibo <maobibo@...ngson.cn>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Loongson (and other $ARCHs?) idle VS timer enqueue



在 2023/4/25 19:49, Peter Zijlstra 写道:
> On Sun, Apr 23, 2023 at 09:52:49PM +0800, bibo, mao wrote:
> 
>>> @@ -40,6 +40,7 @@ SYM_FUNC_START(handle_vint)
>>>   	ori	t0, t0, 0x1f
>>>   	xori	t0, t0, 0x1f
>>>   	bne	t0, t1, 1f
>>> +	addi.d	t0, t0, 0x20
>> It is more reasonable with this patch, this will jump out of idle function
>> directly after interrupt returns. If so, can we remove checking
>> _TIF_NEED_RESCHED in idle ASM function?
>>
>>> +	move	t0, CSR_CRMD_IE
>>> +	csrxchg	t0, t0, LOONGARCH_CSR_CRMD
>> -   	LONG_L	t0, tp, TI_FLAGS
>> +	nop
>>>   	nop
>> -	andi	t0, t0, _TIF_NEED_RESCHED
>> -	bnez	t0, 1f
>> +	nop
>> +	nop
>>>   	nop
>>> -	nop
>>> -	nop
>>>   	idle	0
> 
> Would not something like the below be a more compact form?
> That is; afaict there is no reason to keep it 32 bytes, we can easily go
> 16 and drop 4 nops.
> 
> Additionally, instead of truncating to the start, increase to the end by
> doing:
> 
> 	ip |= 0xf;
> 	ip++;
> 
> Also; I added a wee comment.
Excellent, you are so smart, I test the patch, it works.

> 
> 
> diff --git a/arch/loongarch/kernel/genex.S b/arch/loongarch/kernel/genex.S
> index 44ff1ff64260..3c8a6bab98fe 100644
> --- a/arch/loongarch/kernel/genex.S
> +++ b/arch/loongarch/kernel/genex.S
> @@ -18,27 +18,31 @@
>  
>  	.align	5
>  SYM_FUNC_START(__arch_cpu_idle)
> -	/* start of rollback region */
> -	LONG_L	t0, tp, TI_FLAGS
> -	nop
> -	andi	t0, t0, _TIF_NEED_RESCHED
> -	bnez	t0, 1f
> -	nop
> -	nop
> -	nop
> +	/* start of idle interrupt region */
> +	move	t0, CSR_CRMD_IE
addi.d  t0, zero, CSR_CRMD_IE can be used here, move is used for reg to reg

> +	csrxchg	t0, t0, LOONGARCH_CSR_CRMD
> +	/*
> +	 * If an interrupt lands here; between enabling interrupts above and
> +	 * going idle on the next instruction, we must *NOT* go idle since the
> +	 * interrupt could have set TIF_NEED_RESCHED or caused an timer to need
> +	 * reprogramming. Fall through -- see handle_vint() below -- and have
> +	 * the idle loop take care of things.
> +	 */
>  	idle	0
> -	/* end of rollback region */
> -1:	jr	ra
> +	nop
> +	/* end of idle interrupt region */
> +SYM_INNER_LBEL(__arch_cpu_idle_exit, SYM_L_LOCAL)
typo SYM_INNER_LABEL

otherwise LGTM

Tested-by: Bibo, Mao <maobibo@...ngson.cn>

> +	jr	ra
>  SYM_FUNC_END(__arch_cpu_idle)
>  
>  SYM_FUNC_START(handle_vint)
>  	BACKUP_T0T1
>  	SAVE_ALL
> -	la_abs	t1, __arch_cpu_idle
> +	la_abs	t1, __arch_cpu_idle_exit
>  	LONG_L	t0, sp, PT_ERA
> -	/* 32 byte rollback region */
> -	ori	t0, t0, 0x1f
> -	xori	t0, t0, 0x1f
> +	/* 16 byte idle interrupt region */
> +	ori	t0, t0, 0x0f
> +	addi.d	t0, t0, 1
>  	bne	t0, t1, 1f
>  	LONG_S	t0, sp, PT_ERA
>  1:	move	a0, sp
> diff --git a/arch/loongarch/kernel/idle.c b/arch/loongarch/kernel/idle.c
> index 0b5dd2faeb90..5ba72d229920 100644
> --- a/arch/loongarch/kernel/idle.c
> +++ b/arch/loongarch/kernel/idle.c
> @@ -11,7 +11,6 @@
>  
>  void __cpuidle arch_cpu_idle(void)
>  {
> -	raw_local_irq_enable();
>  	__arch_cpu_idle(); /* idle instruction needs irq enabled */
>  	raw_local_irq_disable();
>  }