linux-kernel - Re: [PATCH v3 1/1] MIPS: Fix idle VS timer enqueue

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Message-ID: <CAAofZF7bZDJcDBqUanFxnnbxkiAeDkv60=NGXnT+5cbHfye2=g@mail.gmail.com>
Date: Thu, 27 Feb 2025 14:34:35 +0100
From: Marco Crivellari <marco.crivellari@...e.com>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>, Frederic Weisbecker <frederic@...nel.org>, 
	Anna-Maria Behnsen <anna-maria@...utronix.de>, Thomas Gleixner <tglx@...utronix.de>, 
	Peter Zijlstra <peterz@...radead.org>, "Maciej W . Rozycki" <macro@...am.me.uk>
Subject: Re: [PATCH v3 1/1] MIPS: Fix idle VS timer enqueue

Hi Huacai,

sorry, I completely forgot to check the comments!





On Thu, Feb 27, 2025 at 12:56 PM Huacai Chen <chenhuacai@...nel.org> wrote:
>
> Hi, Marco,
>
> On Thu, Feb 27, 2025 at 7:41 PM Marco Crivellari
> <marco.crivellari@...e.com> 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.
> >
> > CONFIG_CPU_MICROMIPS has been removed along with the nop instructions.
> > There, NOPs are 2 byte in size, so change the code with 3 _ssnop which are
> > always 4 byte and remove the ifdef. Added ehb to make sure the hazard
> > is always cleared.
> >
> > Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
> > ---
> >  arch/mips/kernel/genex.S | 40 ++++++++++++++++++++++------------------
> >  arch/mips/kernel/idle.c  |  1 -
> >  2 files changed, 22 insertions(+), 19 deletions(-)
> >
> > diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
> > index a572ce36a24f..7b66ab239049 100644
> > --- a/arch/mips/kernel/genex.S
> > +++ b/arch/mips/kernel/genex.S
> > @@ -104,25 +104,28 @@ 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
> > -#ifdef CONFIG_CPU_MICROMIPS
> > -       nop
> > -       nop
> > -       nop
> > -       nop
> > -#endif
> > +       /* start of idle interrupt region */
> > +       MFC0    t0, CP0_STATUS
> > +       /* Enable Interrput */
> > +       ori     t0, 0x1f
> > +       xori    t0, 0x1e
> > +       MTC0    t0, CP0_STATUS
> > +       _ssnop
> > +       _ssnop
> > +       _ssnop
> > +       _ehb
> >         .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) */
> Now you add a _ehb in this version, so "(the region size must be power
> of two)" should be removed.
>
> >  1:
> > @@ -136,9 +139,10 @@ 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
> > +       PTR_LA  k1, 1b
> > +       /* 32 byte idle interrupt region */
> And now the region is 36 bytes. :)
>
> Huacai
>
> > +       ori     k0, 0x1f
> > +       PTR_ADDIU       k0, 5
> >         bne     k0, k1, \handler
> >         MTC0    k0, CP0_EPC
> >         .set pop
> > 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@...e.com