Re: [PATCH 2/4] x86: Fix CPUIDLE_FLAG_IRQ_ENABLE leaking timer reprogram

Peter Zijlstra
2023-11-15T15:53:38Z

On Wed, Nov 15, 2023 at 10:13:23AM -0500, Frederic Weisbecker wrote:
> From: Peter Zijlstra <peterz@infradead.org>
>
> 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@infradead.org>

Feel free to change to normal SOB, I'm assuming it actually compiles and
works by now :-)

> Acked-by: Rafael J. Wysocki <rafael@kernel.org>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
> 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 dcda0afecfc5..3e01a6b23e75 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)
> __update_spec_ctrl(0);
>
> - ret = __intel_idle(dev, drv, index);
> + ret = __intel_idle(dev, drv, index, true);
>
> if (smt_active)
> __update_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);
> }
>
> /**
> --
> 2.42.1
>