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

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Date:   Mon, 24 Apr 2023 19:23:10 +0800
From:   maobibo <maobibo@...ngson.cn>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.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 2023/4/24 16:26, Frederic Weisbecker 写道:
> On Sun, Apr 23, 2023 at 09:52:49PM +0800, bibo, mao wrote:
>>
>>
>> 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.
> 
> Nop, TIF_NEED_RESCHED is set only if a task is ready to run after the interrupt,
> not if the interrupt only modified/added a timer.
Got it, thanks for your explanation, it is actually one issue in the LoongArch
ASM code __arch_cpu_idle().

Regards
Bibo, Mao

> 
>>> @@ -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?
> 
> Indeed we can remove the check to TIF_RESCHED!
> 
> Thanks!