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SubjectRe: [RFC PATCH for 4.21 03/16] sched: Implement push_task_to_cpu (v2)
Hi Mathieu,

> +int push_task_to_cpu(struct task_struct *p, unsigned int dest_cpu)
> +{

In your use case, is the task going to be current?
If yes, we should simply be using migrate_task_to.

> + struct rq_flags rf;
> + struct rq *rq;
> + int ret = 0;
> +
> + rq = task_rq_lock(p, &rf);
> + update_rq_clock(rq);
> +
> + if (!cpumask_test_cpu(dest_cpu, &p->cpus_allowed)) {
> + ret = -EINVAL;
> + goto out;
> + }

Ideally we should have checked cpus_allowed/cpu_active_mask before taking
the lock. This would help reduce the contention on the rqlock when the
passed parameter is not correct.

> +
> + if (!cpumask_test_cpu(dest_cpu, cpu_active_mask)) {
> + ret = -EBUSY;
> + goto out;
> + }
> +
> + if (task_cpu(p) == dest_cpu)
> + goto out;

Same as above.

> +
> + if (task_running(rq, p) || p->state == TASK_WAKING) {

Why are we using migration thread to move a task in TASK_WAKING state?

> + struct migration_arg arg = { p, dest_cpu };
> + /* Need help from migration thread: drop lock and wait. */
> + task_rq_unlock(rq, p, &rf);
> + stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
> + tlb_migrate_finish(p->mm);
> + return 0;

Why cant we use migrate_task_to instead?

> + } else if (task_on_rq_queued(p)) {
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 455fa330de04..27ad25780204 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1340,6 +1340,15 @@ static inline void __set_task_cpu(struct task_struct *p, unsigned int cpu)
> #endif
> }
>
> +#ifdef CONFIG_SMP
> +int push_task_to_cpu(struct task_struct *p, unsigned int dest_cpu);
> +#else
> +static inline int push_task_to_cpu(struct task_struct *p, unsigned int dest_cpu)
> +{
> + return 0;
> +}
> +#endif
> +

Your usecase is outside kernel/sched. So I am not sure if this is the right
place for the declaration.

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