Re: [PATCH v12 08/13] sched/core: Push current task from non preferred CPU

[Yury Norov <ynorov@nvidia.com>]

On Thu, Sep 03, 2026 at 12:02:35PM +0530, Shrikanth Hegde wrote:
> Actively push out the current running task on a non-preferred CPU. Since
> the task is currently running, a stopper thread must be queued to push the
> task out. However, if the task is pinned only to non-preferred CPUs,
> it will continue running there. This helps to maintain userspace
> affinities, unlike CPU hotplug or isolated cpusets.
> 
> Though the code is similar to __balance_push_cpu_stop and quite close to
> push_cpu_stop, it is kept separate as it provides a cleaner
> implementation specifically for CONFIG_PREFERRED_CPU.
> 
> Add the push_task_work_done flag to protect the work buffer.
> 
> For now, only the currently running task is pushed out. This keeps the code
> simpler. In the future, an optimization may be added to move all queued
> tasks on the runqueue.
> 
> This works only for the FAIR scheduling class.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
>  kernel/sched/core.c  | 81 ++++++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/sched.h |  8 +++++
>  2 files changed, 89 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b4ef2e92d786..35e7eedad104 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5808,6 +5808,9 @@ void sched_tick(void)
>  	unsigned long hw_pressure;
>  	u64 resched_latency;
>  
> +	if (!cpu_preferred(cpu))
> +		sched_push_current_non_preferred_cpu(rq);
> +
>  	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE))
>  		arch_scale_freq_tick();
>  
> @@ -11202,3 +11205,81 @@ void sched_change_end(struct sched_change_ctx *ctx)
>  		p->sched_class->prio_changed(rq, p, ctx->prio);
>  	}
>  }
> +
> +#ifdef CONFIG_PREFERRED_CPU
> +static DEFINE_PER_CPU(struct cpu_stop_work, npc_push_task_work);
> +
> +static int sched_non_preferred_cpu_push_stop(void *arg)
> +{
> +	struct task_struct *p = arg;
> +	struct rq *rq = this_rq();
> +	struct rq_flags rf;
> +	int cpu;
> +
> +	if (cpu_preferred(rq->cpu)) {
> +		scoped_guard(rq_lock_irqsave, rq)
> +			rq->push_task_work_done = false;
> +		put_task_struct(p);
> +		return 0;
> +	}
> +
> +	raw_spin_lock_irq(&p->pi_lock);
> +
> +	/* This could take rq lock. So call it before rq lock is taken */
> +	cpu = select_fallback_rq(rq->cpu, p);
> +	rq_lock(rq, &rf);

If select_fallback_rq() grabs the lock, then when it releases the
lock, there's a window for race between the other process and the
subsequent rq_lock(). Or I misunderstand it?

> +	rq->push_task_work_done = false;
> +	update_rq_clock(rq);
> +
> +	context_unsafe_alias(rq);
> +
> +	if (task_rq(p) == rq && task_on_rq_queued(p) &&
> +	    !is_migration_disabled(p))
> +		rq = __migrate_task(rq, &rf, p, cpu);
> +
> +	rq_unlock(rq, &rf);
> +	raw_spin_unlock_irq(&p->pi_lock);
> +	put_task_struct(p);
> +
> +	return 0;
> +}
> +
> +/*
> + * Push the current task running on non-preferred CPU(npc).
> + * Using this non preferred CPU will lead to more contention
> + * in the host. So it is better not to use this CPU.
> + *
> + * Since task is running, call a stopper to push the task out. This is
> + * similar to how task moves during hotplug. In select_fallback_rq a
> + * preferred CPU will be chosen and henceforth task shouldn't come back to
> + * this CPU again.
> + *
> + * Works for FAIR class only.
> + *
> + * If task is affined only on non-preferred CPUs, no point in moving it out.
> + */
> +void sched_push_current_non_preferred_cpu(struct rq *rq)
> +{
> +	struct task_struct *push_task = rq->curr;
> +
> +	scoped_guard(rq_lock, rq) {
> +		/* Push the task if its explicit affinity allows */
> +		if (!task_can_sched_on_preferred(rq->cpu, push_task))
> +			return;
> +
> +		/* There is already a stopper thread. Don't race with it. */
> +		if (rq->push_task_work_done)
> +			return;
> +
> +		if (is_migration_disabled(push_task))
> +			return;
> +
> +		rq->push_task_work_done = true;
> +	}
> +
> +	/* sched_tick runs with interrupts disabled. */
> +	get_task_struct(push_task);
> +	stop_one_cpu_nowait(rq->cpu, sched_non_preferred_cpu_push_stop,
> +			    push_task, this_cpu_ptr(&npc_push_task_work));
> +}
> +#endif
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 6c3ad70e58b8..678e44134acf 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1298,6 +1298,8 @@ struct rq {
>  
>  	struct list_head cfs_tasks;
>  
> +	bool			push_task_work_done;
> +

It should be protected with CONFIG_PREFERRED_CPU. Also, the name
doesn't look correct. You set the variable to 'true' even before
calling the stopper. Maybe need_push_to_npc, or similar?

Why did you place it between cfs_tasks and avg_rt? If no specific
reason, maybe place it next to CONFIG_PARAVIRT-guarded fields.

What about pahole?

>  	struct sched_avg	avg_rt;
>  	struct sched_avg	avg_dl;
>  #ifdef CONFIG_HAVE_SCHED_AVG_IRQ
> @@ -4280,4 +4282,10 @@ DEFINE_CLASS_IS_UNCONDITIONAL(sched_change)
>  
>  #include "ext/ext.h"
>  
> +#ifdef CONFIG_PREFERRED_CPU
> +void sched_push_current_non_preferred_cpu(struct rq *rq);
> +#else	/* !CONFIG_PREFERRED_CPU */
> +static inline void sched_push_current_non_preferred_cpu(struct rq *rq) { }
> +#endif
> +
>  #endif /* _KERNEL_SCHED_SCHED_H */
> -- 
> 2.52.0