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

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Message-ID: <ZEY9UvvuTXYx3QEA@lothringen>
Date:   Mon, 24 Apr 2023 10:26:58 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "bibo, mao" <maobibo@...ngson.cn>
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

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.

> > @@ -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!