Re: [PATCH v6 1/1] MIPS: Fix idle VS timer enqueue

[Maciej W. Rozycki]

On Fri, 28 Mar 2025, Maciej W. Rozycki wrote:

> just massaged a little.  FWIW I went ahead and chose to cook this piece up 
> myself since I realised how complex this issue actually is and that it 
> would take us forever to get all the individual aspects nailed over e-mail 
> communication.

 And yet after this many of internal iterations I did manage to miss one 
bit.  In the optimised version proposed we need to explicitly skip over 
the WAIT instruction like this:

r4k_wait_insn:
	wait
r4k_wait_exit:

and then:

	.set	noreorder
	bne	k0, k1, \handler
	 PTR_ADDIU	k0, r4k_wait_exit - r4k_wait_insn + 2
	.set	reorder

(and here we have a legitimate use for `.set noreorder' to avoid wasting a 
NOP for the branch delay slot due to a data antidependency on $k0; it's 
fine to clobber $k0 in the branch-taken case as by definition the register 
is dead at the exit from this macro).

 Updated patch follows.  I think we also need to replace "rollback" with 
another name as with new code we don't roll back anymore.

  Maciej

Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxx>
---
 arch/mips/include/asm/idle.h |    3 --
 arch/mips/kernel/genex.S     |   63 +++++++++++++++++++++++++------------------
 arch/mips/kernel/idle.c      |    7 ----
 3 files changed, 38 insertions(+), 35 deletions(-)

linux-mips-idle-vs-timer.diff
Index: linux-macro/arch/mips/include/asm/idle.h
===================================================================
--- linux-macro.orig/arch/mips/include/asm/idle.h
+++ linux-macro/arch/mips/include/asm/idle.h
@@ -6,8 +6,7 @@
 #include <linux/linkage.h>
 
 extern void (*cpu_wait)(void);
-extern void r4k_wait(void);
-extern asmlinkage void __r4k_wait(void);
+extern asmlinkage void r4k_wait(void);
 extern void r4k_wait_irqoff(void);
 
 static inline int using_rollback_handler(void)
Index: linux-macro/arch/mips/kernel/genex.S
===================================================================
--- linux-macro.orig/arch/mips/kernel/genex.S
+++ linux-macro/arch/mips/kernel/genex.S
@@ -104,42 +104,53 @@ NESTED(except_vec3_r4000, 0, sp)
 
 	__FINIT
 
-	.align	5	/* 32 byte rollback region */
-LEAF(__r4k_wait)
-	.set	push
-	.set	noreorder
-	/* start of rollback region */
-	LONG_L	t0, TI_FLAGS($28)
-	nop
-	andi	t0, _TIF_NEED_RESCHED
-	bnez	t0, 1f
-	 nop
-	nop
-	nop
-#ifdef CONFIG_CPU_MICROMIPS
-	nop
-	nop
-	nop
-	nop
-#endif
+	.section .cpuidle.text,"ax"
+	/* Align to 32 bytes for the maximum idle interrupt region size.  */
+	.align	5
+LEAF(r4k_wait)
+	/* Keep the ISA bit clear for calculations on local labels here.  */
+0:	.fill	0
+	/* Start of idle interrupt region.  */
+	local_irq_enable
+	/*
+	 * If an interrupt lands here, before going idle on the next
+	 * instruction, we must *NOT* go idle since the interrupt could
+	 * have set TIF_NEED_RESCHED or caused a timer to need resched.
+	 * Fall through -- see rollback_handler below -- and have the
+	 * idle loop take care of things.
+	 */
+1:	.fill	0
+	/* The R2 EI/EHB sequence takes 8 bytes, otherwise pad up.  */
+	.if	1b - 0b > 32
+	.error	"overlong idle interrupt region"
+	.elseif	1b - 0b > 8
+	.align	4
+	.endif
+2:	.fill	0
+	.equ	r4k_wait_idle_size, 2b - 0b
+	/* End of idle interrupt region; size has to be a power of 2.  */
 	.set	MIPS_ISA_ARCH_LEVEL_RAW
+r4k_wait_insn:
 	wait
-	/* end of rollback region (the region size must be power of two) */
-1:
+r4k_wait_exit:
+	.set	mips0
+	local_irq_disable
 	jr	ra
-	 nop
-	.set	pop
-	END(__r4k_wait)
+	END(r4k_wait)
+	.previous
 
 	.macro	BUILD_ROLLBACK_PROLOGUE handler
 	FEXPORT(rollback_\handler)
 	.set	push
 	.set	noat
 	MFC0	k0, CP0_EPC
-	PTR_LA	k1, __r4k_wait
-	ori	k0, 0x1f	/* 32 byte rollback region */
-	xori	k0, 0x1f
+	/* Subtract/add 2 to let the ISA bit propagate through the mask.  */
+	PTR_LA	k1, r4k_wait_insn - 2
+	ori	k0, r4k_wait_idle_size - 2
+	.set	noreorder
 	bne	k0, k1, \handler
+	 PTR_ADDIU	k0, r4k_wait_exit - r4k_wait_insn + 2
+	.set	reorder
 	MTC0	k0, CP0_EPC
 	.set pop
 	.endm
Index: linux-macro/arch/mips/kernel/idle.c
===================================================================
--- linux-macro.orig/arch/mips/kernel/idle.c
+++ linux-macro/arch/mips/kernel/idle.c
@@ -35,13 +35,6 @@ static void __cpuidle r3081_wait(void)
 	write_c0_conf(cfg | R30XX_CONF_HALT);
 }
 
-void __cpuidle r4k_wait(void)
-{
-	raw_local_irq_enable();
-	__r4k_wait();
-	raw_local_irq_disable();
-}
-
 /*
  * This variant is preferable as it allows testing need_resched and going to
  * sleep depending on the outcome atomically.  Unfortunately the "It is