Re: [RFC PATCH] sched/fair: Remove sched_idle_cpu() usages in select_task_rq_fair()

["Chen, Yu C" <yu.c.chen@intel.com>]

On 8/18/2025 9:24 PM, Christian Loehle wrote:
> On 8/18/25 13:47, Chengming Zhou wrote:
>> These sched_idle_cpu() considerations in select_task_rq_fair() is based
>> on an assumption that the wakee task can pick a cpu running sched_idle
>> task and preempt it to run, faster than picking an idle cpu to preempt
>> the idle task.
>>
>> This assumption is correct, but it also brings some problems:
>>
>> 1. work conservation: Often sched_idle tasks are also picking the cpu
>> which is already running sched_idle task, instead of utilizing a real
>> idle cpu, so work conservation is somewhat broken.
>>
>> 2. sched_idle group: This sched_idle_cpu() is just not correct with
>> sched_idle group running. Look a simple example below.
>>
>> 		root
>> 	/		\
>> 	kubepods	system
>> 	/	\
>> burstable	besteffort
>> 		(cpu.idle == 1)
>>
>> When a sched_idle cpu is just running tasks from besteffort group,
>> sched_idle_cpu() will return true in this case, but this cpu pick
>> is bad for wakee task from system group. Because the system group
>> has lower weight than kubepods, work conservation is somewhat
>> broken too.
>>
>> In a nutshell, sched_idle_cpu() should consider the wakee task group's
>> relationship with sched_idle tasks running on the cpu.
>>
>> Obviously, it's hard to do so. This patch chooses the simple approach
>> to remove all sched_idle_cpu() considerations in select_task_rq_fair()
>> to bring back work conservation in these cases.
> 
> OTOH sched_idle_cpu() CPUs are guaranteed to not be in an idle state and
> potentially already have DVFS on some higher level...
> 
Is it because the schedutil governor considers the utilization
of SCHED_IDLE, thus causing schedutil to request a higher
frequency?

The commit 3c29e651e16d ("sched/fair: Fall back to sched-idle
CPU if an idle CPU isn't found") mentions that choosing a CPU
running a SCHED_IDLE task can avoid waking a CPU from a deep
sleep state.

If this is the case, can we say that if an administrator sets
the cpufreq governor to "performance" and disables deep idle
states, an idle CPU would be more preferable than a CPU running
a SCHED_IDLE task? On the other hand, if
per_cpu(cpufreq_update_util_data, cpu) is NULL and only shallow
idle states are enabled in idle_get_state(), should we skip
SCHED_IDLE to achieve work conservation?

thanks,
Chenyu