Re: [PATCH 02/10] cpuidle: Fix CPUIDLE_FLAG_IRQ_ENABLE leaking timer reprogram

[Rafael J. Wysocki]

On Fri, Aug 11, 2023 at 7:01 PM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:
>
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>
> intel_idle_irq() re-enables IRQs very early. As a result, an interrupt
> may fire before mwait() is eventually called. If such an interrupt queues
> a timer, it may go unnoticed until mwait returns and the idle loop
> handles the tick re-evaluation. And monitoring TIF_NEED_RESCHED doesn't
> help because a local timer enqueue doesn't set that flag.
>
> The issue is mitigated by the fact that this idle handler is only invoked
> for shallow C-states when, presumably, the next tick is supposed to be
> close enough. There may still be rare cases though when the next tick
> is far away and the selected C-state is shallow, resulting in a timer
> getting ignored for a while.
>
> Fix this with using sti_mwait() whose IRQ-reenablement only triggers
> upon calling mwait(), dealing with the race while keeping the interrupt
> latency within acceptable bounds.
>
> Fixes: c227233ad64c (intel_idle: enable interrupts before C1 on Xeons)
> Not-yet-signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

Acked-by: Rafael J. Wysocki <rafael@xxxxxxxxxx>

> ---
>  arch/x86/include/asm/mwait.h | 11 +++++++++--
>  drivers/idle/intel_idle.c    | 19 +++++++------------
>  2 files changed, 16 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
> index 341ee4f1d91e..920426d691ce 100644
> --- a/arch/x86/include/asm/mwait.h
> +++ b/arch/x86/include/asm/mwait.h
> @@ -124,8 +124,15 @@ static __always_inline void mwait_idle_with_hints(unsigned long eax, unsigned lo
>                 }
>
>                 __monitor((void *)&current_thread_info()->flags, 0, 0);
> -               if (!need_resched())
> -                       __mwait(eax, ecx);
> +
> +               if (!need_resched()) {
> +                       if (ecx & 1) {
> +                               __mwait(eax, ecx);
> +                       } else {
> +                               __sti_mwait(eax, ecx);
> +                               raw_local_irq_disable();
> +                       }
> +               }
>         }
>         current_clr_polling();
>  }
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index 256c2d42e350..d676d32741da 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -131,11 +131,12 @@ static unsigned int mwait_substates __initdata;
>  #define MWAIT2flg(eax) ((eax & 0xFF) << 24)
>
>  static __always_inline int __intel_idle(struct cpuidle_device *dev,
> -                                       struct cpuidle_driver *drv, int index)
> +                                       struct cpuidle_driver *drv,
> +                                       int index, bool irqoff)
>  {
>         struct cpuidle_state *state = &drv->states[index];
>         unsigned long eax = flg2MWAIT(state->flags);
> -       unsigned long ecx = 1; /* break on interrupt flag */
> +       unsigned long ecx = 1*irqoff; /* break on interrupt flag */
>
>         mwait_idle_with_hints(eax, ecx);
>
> @@ -159,19 +160,13 @@ static __always_inline int __intel_idle(struct cpuidle_device *dev,
>  static __cpuidle int intel_idle(struct cpuidle_device *dev,
>                                 struct cpuidle_driver *drv, int index)
>  {
> -       return __intel_idle(dev, drv, index);
> +       return __intel_idle(dev, drv, index, true);
>  }
>
>  static __cpuidle int intel_idle_irq(struct cpuidle_device *dev,
>                                     struct cpuidle_driver *drv, int index)
>  {
> -       int ret;
> -
> -       raw_local_irq_enable();
> -       ret = __intel_idle(dev, drv, index);
> -       raw_local_irq_disable();
> -
> -       return ret;
> +       return __intel_idle(dev, drv, index, false);
>  }
>
>  static __cpuidle int intel_idle_ibrs(struct cpuidle_device *dev,
> @@ -184,7 +179,7 @@ static __cpuidle int intel_idle_ibrs(struct cpuidle_device *dev,
>         if (smt_active)
>                 native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
>
> -       ret = __intel_idle(dev, drv, index);
> +       ret = __intel_idle(dev, drv, index, true);
>
>         if (smt_active)
>                 native_wrmsrl(MSR_IA32_SPEC_CTRL, spec_ctrl);
> @@ -196,7 +191,7 @@ static __cpuidle int intel_idle_xstate(struct cpuidle_device *dev,
>                                        struct cpuidle_driver *drv, int index)
>  {
>         fpu_idle_fpregs();
> -       return __intel_idle(dev, drv, index);
> +       return __intel_idle(dev, drv, index, true);
>  }
>
>  /**
> --
> 2.34.1
>