Re: [PATCH] MIPS: Fix idle VS timer enqueue

[Marco Crivellari]

Hi, thanks for the replies!

Huacai: I used the '1' label when I compiled the first time, but I
noticed this warning, so I decided to use the label to avoid this:

"WARNING: modpost: vmlinux: section mismatch in reference:
rollback_except_vec_vi+0x4 (section: .text) -> except_vec4 (section:
.init.text)"

By the way, I will use the 1 label like you suggested.

> For MIPS the rollback region is not 3 instructions, and so you cannot
> use 0xc below. I think there is no chance for the wait instruction
> after this patch.

About this, does it look better if i don't change this part of the
code (except for PTR_LA  k1, 1b) ?
eg.

@@ -136,7 +144,7 @@ LEAF(__r4k_wait)
       .set    push
       .set    noat
       MFC0    k0, CP0_EPC
-       PTR_LA  k1, __r4k_wait
+       PTR_LA  k1, 1b
       ori     k0, 0x1f        /* 32 byte rollback region */
       xori    k0, 0x1f
       bne     k0, k1, \handler

So the real change would be:

-       /* start of rollback region */
-       LONG_L  t0, TI_FLAGS($28)
+       /* start of idle interrupt region */
+       MFC0    k0, CP0_STATUS
       nop
-       andi    t0, _TIF_NEED_RESCHED
-       bnez    t0, 1f
-        nop
+       /* Enable Interrupt */
+       ori     k0, 0x1f
+       xori    k0, 0x1e
+       MTC0    k0, CP0_STATUS
       nop
       nop

Maybe what I'm going to say is silly...
But considering the region should be a power of 2, maybe can also be
changed with:
ori   k0, 0x0f
addi  k0, 1

Thanks for your time!

On Fri, Feb 14, 2025 at 3:47 PM Huacai Chen <chenhuacai@xxxxxxxxxx> wrote:
>
> Hi, Marco,
>
> On Fri, Feb 14, 2025 at 6:51 PM Marco Crivellari
> <marco.crivellari@xxxxxxxx> wrote:
> >
> > MIPS re-enables interrupts on its idle routine and performs
> > a TIF_NEED_RESCHED check afterwards before putting the CPU to sleep.
> >
> > The IRQs firing between the check and the 'wait' instruction may set the
> > TIF_NEED_RESCHED flag. In order to deal with this possible race, IRQs
> > interrupting __r4k_wait() rollback their return address to the
> > beginning of __r4k_wait() so that TIF_NEED_RESCHED is checked
> > again before going back to sleep.
> >
> > However idle IRQs can also queue timers that may require a tick
> > reprogramming through a new generic idle loop iteration but those timers
> > would go unnoticed here because __r4k_wait() only checks
> > TIF_NEED_RESCHED. It doesn't check for pending timers.
> >
> > Fix this with fast-forwarding idle IRQs return address to the end of the
> > idle routine instead of the beginning, so that the generic idle loop
> > handles both TIF_NEED_RESCHED and pending timers.
> >
> > Signed-off-by: Marco Crivellari <marco.crivellari@xxxxxxxx>
> > ---
> >  arch/mips/kernel/genex.S | 36 ++++++++++++++++++++----------------
> >  arch/mips/kernel/idle.c  |  1 -
> >  2 files changed, 20 insertions(+), 17 deletions(-)
> >
> > diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
> > index a572ce36a24f..a78d5132c940 100644
> > --- a/arch/mips/kernel/genex.S
> > +++ b/arch/mips/kernel/genex.S
> > @@ -104,18 +104,16 @@ handle_vcei:
> >
> >         __FINIT
> >
> > -       .align  5       /* 32 byte rollback region */
> > +       .align  5
> >  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
> > +       /* start of idle interrupt region */
> > +       MFC0    k0, CP0_STATUS
> > +       /* Enable Interrupt */
> > +       ori     k0, 0x1f
> > +       xori    k0, 0x1e
> > +       MTC0    k0, CP0_STATUS
> >  #ifdef CONFIG_CPU_MICROMIPS
> >         nop
> >         nop
> > @@ -123,11 +121,17 @@ LEAF(__r4k_wait)
> >         nop
> >  #endif
> >         .set    MIPS_ISA_ARCH_LEVEL_RAW
> > +       /*
> > +        * If an interrupt lands here, between enabling interrupts above and
> > +        * 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.
> > +        */
> >         wait
> > -       /* end of rollback region (the region size must be power of two) */
> > -1:
> > +       /* end of idle interrupt region (the region size must be power of two) */
> > +SYM_INNER_LABEL(__r4k_wait_exit, SYM_L_LOCAL)
> You can also use label 1 as the LoongArch version.
>
> >         jr      ra
> > -        nop
> >         .set    pop
> >         END(__r4k_wait)
> >
> > @@ -136,10 +140,10 @@ LEAF(__r4k_wait)
> >         .set    push
> >         .set    noat
> >         MFC0    k0, CP0_EPC
> > -       PTR_LA  k1, __r4k_wait
> > -       ori     k0, 0x1f        /* 32 byte rollback region */
> > -       xori    k0, 0x1f
> > -       bne     k0, k1, \handler
> > +       PTR_LA  k1, __r4k_wait_exit
> > +       /* 3 instructions rollback region */
> For MIPS the rollback region is not 3 instructions, and so you cannot
> use 0xc below. I think there is no chance for the wait instruction
> after this patch.
>
> Huacai
>
> > +       ori     k0, k0, 0x0c
> > +       bne     k0, k1, \handler
> >         MTC0    k0, CP0_EPC
> >         .set pop
> >         .endm
> > diff --git a/arch/mips/kernel/idle.c b/arch/mips/kernel/idle.c
> > index 5abc8b7340f8..1f74c0589f7e 100644
> > --- a/arch/mips/kernel/idle.c
> > +++ b/arch/mips/kernel/idle.c
> > @@ -37,7 +37,6 @@ static void __cpuidle r3081_wait(void)
> >
> >  void __cpuidle r4k_wait(void)
> >  {
> > -       raw_local_irq_enable();
> >         __r4k_wait();
> >         raw_local_irq_disable();
> >  }
> > --
> > 2.48.1
> >
> >



-- 

Marco Crivellari

L3 Support Engineer, Technology & Product




marco.crivellari@xxxxxxxx