Re: [PATCH v1] sched: idle: Consolidate the handling of two special cases

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

On Fri, 2026-03-13 at 13:25 +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> There are two special cases in the idle loop that are handled
> inconsistently even though they are analogous.
> 
> The first one is when a cpuidle driver is absent and the default CPU
> idle time power management implemented by the architecture code is used.
> In that case, the scheduler tick is stopped every time before invoking
> default_idle_call().
> 
> The second one is when a cpuidle driver is present, but there is only
> one idle state in its table.  In that case, the scheduler tick is never
> stopped.
> 
> Since each of these approaches leads to suboptimal choices in some
> cases, reconcile them with the help of one simple heuristic.  Namely,
> stop the tick if the CPU has been woken up by it in the previous
> iteration of the idle loop, or let it tick otherwise.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
> 
> Based on today's mainline.
> 
> ---
>  kernel/sched/idle.c |   30 +++++++++++++++++++++---------
>  1 file changed, 21 insertions(+), 9 deletions(-)
> 
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -161,6 +161,14 @@ static int call_cpuidle(struct cpuidle_d
>  	return cpuidle_enter(drv, dev, next_state);
>  }
>  
> +static void idle_call_stop_or_retain_tick(bool stop_tick)
> +{
> +	if (stop_tick || tick_nohz_tick_stopped())
> +		tick_nohz_idle_stop_tick();
> +	else
> +		tick_nohz_idle_retain_tick();
> +}
> +
>  /**
>   * cpuidle_idle_call - the main idle function
>   *
> @@ -170,7 +178,7 @@ static int call_cpuidle(struct cpuidle_d
>   * set, and it returns with polling set.  If it ever stops polling, it
>   * must clear the polling bit.
>   */
> -static void cpuidle_idle_call(void)
> +static void cpuidle_idle_call(bool stop_tick)
>  {
>  	struct cpuidle_device *dev = cpuidle_get_device();
>  	struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
> @@ -186,7 +194,7 @@ static void cpuidle_idle_call(void)
>  	}
>  
>  	if (cpuidle_not_available(drv, dev)) {
> -		tick_nohz_idle_stop_tick();
> +		idle_call_stop_or_retain_tick(stop_tick);
>  
>  		default_idle_call();
>  		goto exit_idle;
> @@ -222,17 +230,19 @@ static void cpuidle_idle_call(void)
>  		next_state = cpuidle_find_deepest_state(drv, dev, max_latency_ns);
>  		call_cpuidle(drv, dev, next_state);
>  	} else if (drv->state_count > 1) {
> -		bool stop_tick = true;
> +		/*
> +		 * stop_tick is expected to be true by default by cpuidle
> +		 * governors, which allows them to select idle states with
> +		 * target residency above the tick period length.
> +		 */
> +		stop_tick = true;
>  
>  		/*
>  		 * Ask the cpuidle framework to choose a convenient idle state.
>  		 */
>  		next_state = cpuidle_select(drv, dev, &stop_tick);
>  
> -		if (stop_tick || tick_nohz_tick_stopped())
> -			tick_nohz_idle_stop_tick();
> -		else
> -			tick_nohz_idle_retain_tick();
> +		idle_call_stop_or_retain_tick(stop_tick);
>  
>  		entered_state = call_cpuidle(drv, dev, next_state);
>  		/*
> @@ -240,7 +250,7 @@ static void cpuidle_idle_call(void)
>  		 */
>  		cpuidle_reflect(dev, entered_state);
>  	} else {
> -		tick_nohz_idle_retain_tick();
> +		idle_call_stop_or_retain_tick(stop_tick);
>  
>  		/*
>  		 * If there is only a single idle state (or none), there is
> @@ -268,6 +278,7 @@ exit_idle:
>  static void do_idle(void)
>  {
>  	int cpu = smp_processor_id();
> +	bool got_tick = false;
>  
>  	/*
>  	 * Check if we need to update blocked load
> @@ -338,8 +349,9 @@ static void do_idle(void)
>  			tick_nohz_idle_restart_tick();
>  			cpu_idle_poll();
>  		} else {
> -			cpuidle_idle_call();
> +			cpuidle_idle_call(got_tick);
>  		}
> +		got_tick = tick_nohz_idle_got_tick();
>  		arch_cpu_idle_exit();
>  	}
>  
> 

This unifies the special-case tick handling with a heuristic,
affecting only the no-driver and single-state paths while leaving the
common multi-state governor path behaviorally unchanged.

So,

Reviewed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>

Thanks,
Aboorva