Re: [patch 2/2] sched/idle: Make default_idle_call() NOHZ aware

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

On 3/10/26 03:54, Qais Yousef wrote:
> On 03/07/26 17:25, Rafael J. Wysocki wrote:
> 
>> In the meantime I realized that if the .select() governor
>> callback is skipped, its .reflect() callback should be skipped
>> either, so I've posted this:
>>
>> https://lkml.org/lkml/2026/3/7/569
>>
>> and here's a fixed version of the last patch on top of the above (for
>> completeness):
>>
>> ---
>>  kernel/sched/idle.c |   25 ++++++++++++++++---------
>>  1 file changed, 16 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 +230,14 @@ 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 = true;
> 
> Silly question, but wouldn't this benefit the normal path too to delay for one
> tick? This will only matter for the cases where the governor doesn't explicitly
> set stop_tick to either true or false - which I am not sure what they are :)
> 
Right now the governors will always set stop_tick explicitly (and overriding
that might confuse the governor-internal state).