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

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Date:   Sun, 23 Apr 2023 21:52:49 +0800
From:   "bibo, mao" <maobibo@...ngson.cn>
To:     Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>
Cc:     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 2023/4/22 23:04, Peter Zijlstra 写道:
> On Sat, Apr 22, 2023 at 04:21:45PM +0200, Frederic Weisbecker wrote:
>> On Sat, Apr 22, 2023 at 10:17:00AM +0200, Peter Zijlstra wrote:
>>> diff --git a/arch/loongarch/kernel/genex.S b/arch/loongarch/kernel/genex.S
>>> index 44ff1ff64260..5a102ff80de0 100644
>>> --- a/arch/loongarch/kernel/genex.S
>>> +++ b/arch/loongarch/kernel/genex.S
>>> @@ -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
>>>   	LONG_S	t0, sp, PT_ERA
>>>   1:	move	a0, sp
>>>   	move	a1, sp
>>
>> But the interrupts are enabled in C from arch_cpu_idle(), which
>> only then calls the ASM __arch_cpu_idle(). So if the interrupt happens
>> somewhere in between the call, the rollback (or fast-forward now)
>> doesn't apply.
I do not know much details about scheduler and timer, if the interrupt 
happens between the call, will flag _TIF_NEED_RESCHED be set? If it is 
set, the rollback will still apply.


>>
>> I guess interrupts need to be re-enabled from ASM in the beginning
>> of __arch_cpu_idle() so that it's part of the fast-forward region.
> 
> Right; something like so I suppose, but at this point I'm really just
> guessing... Loongarch person will have to do.
> 
> diff --git a/arch/loongarch/kernel/genex.S b/arch/loongarch/kernel/genex.S
> index 44ff1ff64260..4814ac5334ef 100644
> --- a/arch/loongarch/kernel/genex.S
> +++ b/arch/loongarch/kernel/genex.S
> @@ -19,13 +19,13 @@
>   	.align	5
>   SYM_FUNC_START(__arch_cpu_idle)
>   	/* start of rollback region */
> +	move	t0, CSR_CRMD_IE
> +	csrxchg	t0, t0, LOONGARCH_CSR_CRMD
>   	LONG_L	t0, tp, TI_FLAGS
>   	nop
>   	andi	t0, t0, _TIF_NEED_RESCHED
>   	bnez	t0, 1f
>   	nop
> -	nop
> -	nop
>   	idle	0
>   	/* end of rollback region */
>   1:	jr	ra
> @@ -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

Regards
Bibo, Mao
>   	LONG_S	t0, sp, PT_ERA
>   1:	move	a0, sp
>   	move	a1, 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();
>   }