Re: [PATCH v9 04/11] sched/core: Try to use a preferred CPU in is_cpu_allowed

Hi Mete, thanks for taking a look.

On 8/10/26 5:28 PM, Mete Durlu wrote:
> Hi,
> 
> ...
>> For majority of the cases this would still keep select_fallback_rq
>> as O(N). task_has_preferred_cpus which is O(N) is called only if
>> !cpu_preferred. Then task running there is expected to move out.
>> So subsequent it should run on preferred CPU. This becomes O(N**2)
>> only for tasks pinned only non preferred CPUs. That is rare case.
>>
>> Signed-off-by: Shrikanth Hegde <[email protected]>
>> ---
>>   kernel/sched/core.c  | 12 ++++++++++--
>>   kernel/sched/sched.h | 12 ++++++++++++
>>   2 files changed, 22 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index a45f7c308329..9e8eec4451b6 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2509,8 +2509,12 @@ static inline bool is_cpu_allowed(struct 
>> task_struct *p, int cpu)
>>           return cpu_online(cpu);
>>       /* Non kernel threads are not allowed during either online or 
>> offline. */
>> -    if (!(p->flags & PF_KTHREAD))
>> +    if (!(p->flags & PF_KTHREAD)) {
>> +        /* Try to use preferred CPU if task's affinity allows */
>> +        if (task_can_sched_on_preferred(cpu, p))
>> +            return false;
>>           return cpu_active(cpu);
>> +    }
>>       /* KTHREAD_IS_PER_CPU is always allowed. */
>>       if (kthread_is_per_cpu(p))
>> @@ -2520,7 +2524,11 @@ static inline bool is_cpu_allowed(struct 
>> task_struct *p, int cpu)
>>       if (cpu_dying(cpu))
>>           return false;
>> -    /* But are allowed during online. */
>> +    /* Try to keep unbound kthreads on a preferred CPU if possible. */
>> +    if (task_can_sched_on_preferred(cpu, p))
>> +        return false;
>> +
>> +    /* Otherwise, they are allowed to run on online CPU. */
>>       return cpu_online(cpu);
>>   }
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 26ae13c86b69..6de6366f2faa 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -4230,4 +4230,16 @@ DEFINE_CLASS_IS_UNCONDITIONAL(sched_change)
>>   #include "ext/ext.h"
>> +static inline bool task_can_sched_on_preferred(int cpu, struct 
>> task_struct *p)
> 
> 
> Sorry, I glanced to the earlier versions but I couldn't find
> the answer to my question. Trivial one but still.
> 
> FWICT, task_can_sched_on_preferred() is only being used in
> core.c, is there a reason why it is defined in
> kernel/sched/sched.h and not in core.c ?
> 

Will move it out of sched.h. Thanks for pointing it out.

> Thanks.
> -Mete

Also, thanks for checking that s390 change.