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

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Date:   Sat, 22 Apr 2023 17:04:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     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

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 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
 	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();
 }