Re: [PATCH] cpuidle: Deny idle entry when CPU already have IPI interrupt pending

[Christian Loehle <christian.loehle@arm.com>]

On 3/16/26 07:37, Maulik Shah wrote:
> CPU can get IPI interrupt from another CPU while it is executing
> cpuidle_select() or about to execute same. The selection do not account
> for pending interrupts and may continue to enter selected idle state only
> to exit immediately.
> 
> Example trace collected when there is cross CPU IPI.
> 
>  [000] 154.892148: sched_waking: comm=sugov:4 pid=491 prio=-1 target_cpu=007
>  [000] 154.892148: ipi_raise: target_mask=00000000,00000080 (Function call interrupts)
>  [007] 154.892162: cpu_idle: state=2 cpu_id=7
>  [007] 154.892208: cpu_idle: state=4294967295 cpu_id=7
>  [007] 154.892211: irq_handler_entry: irq=2 name=IPI
>  [007] 154.892211: ipi_entry: (Function call interrupts)
>  [007] 154.892213: sched_wakeup: comm=sugov:4 pid=491 prio=-1 target_cpu=007
>  [007] 154.892214: ipi_exit: (Function call interrupts)
> 
> This impacts performance and the above count increments.
> 
> commit ccde6525183c ("smp: Introduce a helper function to check for pending
> IPIs") already introduced a helper function to check the pending IPIs and
> it is used in pmdomain governor to deny the cluster level idle state when
> there is a pending IPI on any of cluster CPUs.
> 
> This however does not stop CPU to enter CPU level idle state. Make use of
> same at CPUidle to deny the idle entry when there is already IPI pending.
> 
> With change observing glmark2 [1] off screen scores improving in the range
> of 25% to 30% on Qualcomm lemans-evk board which is arm64 based having two
> clusters each with 4 CPUs.
> 
> [1] https://github.com/glmark2/glmark2
> 
> Signed-off-by: Maulik Shah <maulik.shah@oss.qualcomm.com>
> ---
>  drivers/cpuidle/cpuidle.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index c7876e9e024f9076663063ad21cfc69343fdbbe7..c88c0cbf910d6c2c09697e6a3ac78c081868c2ad 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -224,6 +224,9 @@ noinstr int cpuidle_enter_state(struct cpuidle_device *dev,
>  	bool broadcast = !!(target_state->flags & CPUIDLE_FLAG_TIMER_STOP);
>  	ktime_t time_start, time_end;
>  
> +	if (cpus_peek_for_pending_ipi(drv->cpumask))
> +		return -EBUSY;
> +
>  	instrumentation_begin();
>  
>  	/*
> 
> ---
> base-commit: b84a0ebe421ca56995ff78b66307667b62b3a900
> change-id: 20260316-cpuidle_ipi-4c64036f9a48
> 
> Best regards,

So we already do a per-CPU IPI need_resched() check in the idle path.
Your patch uses drv->cpumask, which will contain all CPUs, preventing idle entry if
any CPU has an IPI pending?