Re: [PATCH v11 05/12] sched/core: Try to use a preferred CPU in is_cpu_allowed

Hi Dietmar, Vincent

On 8/28/26 4:08 PM, Vincent Guittot wrote:
> On Fri, 28 Aug 2026 at 09:51, Dietmar Eggemann <[email protected]> wrote:
>>
>> On 25.08.26 12:38, Shrikanth Hegde wrote:
>>> When possible, try to choose a preferred CPU.
>>>
>>> This is essential to maintain user affinities when preferred
>>> CPUs change. A task pinned on a non-preferred CPU should continue
>>> to run there, since this is a non-user triggered event.
>>>
>>> If a CPU is non-preferred and the task can run on other CPUs which are
>>> currently preferred, then choose a preferred CPU instead.
>>> This is decided by checking if cpus_ptr and cpu_preferred_mask
>>> intersect or not. If yes, then the task has other preferred CPUs.
>>>
>>> The push task mechanism uses a stopper thread which calls
>>> select_fallback_rq() and uses this mechanism to pick a preferred CPU.
>>>
>>> This takes care of the wakeup path for FAIR tasks too.
>>> is_cpu_allowed() is called to ensure wakeups happen on preferred CPUs.
>>> With that, additional checks in available_idle_cpu() are not necessary.
>>>
>>> Ignore the preferred CPU state if a task's affinity is changing and
>>> its new mask no longer includes the CPU it is currently running on.
>>> This ensures migration_cpu_stop() does not abort, preventing the task
>>> from being stranded outside its allowed affinity.
>>>
>>> Account for tasks with architecture-specific CPU masks
>>> (e.g., 32-bit tasks on arm64). For such tasks, explicitly check against
>>> the arch-allowed CPUs to determine if any of the preferred CPUs are
>>> actually valid.
>>>
>>> For the majority of cases, this would still keep select_fallback_rq()
>>> as O(N). cpumask_intersects(), which is O(N), is called only if
>>> !cpu_preferred. The task running there is expected to move out.
>>> Subsequently, it should run on a preferred CPU. This becomes O(N**2)
>>> only for tasks pinned solely to non-preferred CPUs. That is a rare case.
>>>
>>> Overhead is minimal when the CPU is preferred.
>>>
>>> Signed-off-by: Shrikanth Hegde <[email protected]>
>>> ---
>>>   kernel/sched/core.c | 41 +++++++++++++++++++++++++++++++++++++++--
>>>   1 file changed, 39 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>> index a45f7c308329..f71317fe281d 100644
>>> --- a/kernel/sched/core.c
>>> +++ b/kernel/sched/core.c
>>> @@ -2494,6 +2494,35 @@ static inline bool rq_has_pinned_tasks(struct rq *rq)
>>>        return rq->nr_pinned;
>>>   }
>>>
>>> +static inline bool task_can_sched_on_preferred(int cpu, struct task_struct *p)
>>> +{
>>> +     const struct cpumask *valid_mask;
>>> +     int i;
>>> +
>>> +     if (cpu_preferred(cpu))
>>> +             return false;
>>> +
>>> +     /* Only FAIR tasks honor preferred CPU state */
>>> +     if (unlikely(p->sched_class != &fair_sched_class))
>>> +             return false;
>>> +
>>> +     /* Ignore preferred state if task affinity is changing */
>>> +     if (unlikely(!cpumask_test_cpu(task_cpu(p), p->cpus_ptr)))
>>> +             return false;
>>> +
>>> +     valid_mask = task_cpu_possible_mask(p);
>>> +     if (likely(valid_mask == cpu_possible_mask))
>>> +             return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
>>> +
>>> +     /* Tasks with arch-specific CPU masks. e.g. 32-bit tasks on arm64. */
>>> +     for_each_cpu_and(i, p->cpus_ptr, cpu_preferred_mask) {
>>> +             if (cpumask_test_cpu(i, valid_mask))
>>> +                     return true;
>>> +     }
>>
>> Looking more into this, there might be a window in 64-32-bit execve()
>> for 32bit EL0 tasks on Arm64 (w/ allow_mismatched_32bit_el0 command line
>> option).
>>
>> The time before arch_setup_new_exec() calls
>> force_compatible_cpus_allowed_ptr() to restrict CPU affinity for those
>> tasks.
>>
>> Let me run more test on this ...
>>
>> Why not simply:
>>
>> - return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
>> + return cpumask_first_and_and(p->cpus_ptr, cpu_preferred_mask,
>> +                              task_cpu_possible_mask(p)) < nr_cpu_ids;
> 
> +1
> This is the best way to check that there is a valid cpu
> 
>>

Thanks Dietmar, Vincent for checking it further.

It is good to know that there might be a very narrow window.
The three-way cpumask check you suggested will handle that case safely.


>> IMHO, you want to know whether there is at least one CPU that belongs to
>> all three CPU masks?
>>
>> [...]

With that task_can_sched_on_preferred essentially becomes this.
I will put that in v12 and probably send it out after 7.3-rc1 lands.


static inline bool task_can_sched_on_preferred(int cpu, struct task_struct *p)
{
         if (cpu_preferred(cpu))
                 return false;

         /* Only FAIR tasks honor preferred CPU state */
         if (unlikely(p->sched_class != &fair_sched_class))
                 return false;

         /* Ignore preferred state if task affinity is changing */
         if (unlikely(!cpumask_test_cpu(task_cpu(p), p->cpus_ptr)))
                 return false;

         return cpumask_first_and_and(p->cpus_ptr, cpu_preferred_mask,
                              task_cpu_possible_mask(p)) < nr_cpu_ids;
}


Thanks again for checking and for the suggested code!