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

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Date:   Sat, 22 Apr 2023 10:17:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        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 Fri, Apr 21, 2023 at 05:24:36PM +0200, Thomas Gleixner wrote:
> On Fri, Apr 21 2023 at 14:36, Frederic Weisbecker wrote:
> > I'm looking at the __arch_cpu_idle() implementation in Loongarch
> > and I'm wondering about the rollback code. I don't understand well
> > that code but with the help from PeterZ I might have seen something,
> > so tell me if I'm wrong: when an interrupt happens within
> > __arch_cpu_idle(), handle_vint() rolls back the return value to the
> > beginning of __arch_cpu_idle(), so that TIF_NEED_RESCHED is checked
> > again. Is that correct?
> >
> > Because if an interrupt fires while in __arch_cpu_idle(), that IRQ
> > might enqueue a new timer and that new timer needs to be reprogrammed
> > from the main idle loop and just checking TIF_NEED_RESCHED doesn't
> > tell about that information.
> 
> The check for TIF_NEED_RESCHED as loop termination condition is simply
> wrong. The architecture is not to supposed to loop in arch_cpu_idle().
> 
> That loop is from Linux 0.9 days. Seems MIPS::__r4k_wait() and
> loongarch, which copied that muck are still stuck in the 1990'ies.
> 
> It has to return when an interrupt brings it out of the "idle wait"
> instruction.

So I think the below is enough for these two...

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
diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
index b6de8e88c1bd..cd6aae441ad9 100644
--- a/arch/mips/kernel/genex.S
+++ b/arch/mips/kernel/genex.S
@@ -140,6 +140,7 @@ LEAF(__r4k_wait)
 	ori	k0, 0x1f	/* 32 byte rollback region */
 	xori	k0, 0x1f
 	bne	k0, k1, \handler
+	addiu	k0, 0x20
 	MTC0	k0, CP0_EPC
 	.set pop
 	.endm