Re: [PATCH v7 06/12] sched/core: Push current task from non preferred CPU

[Yury Norov <ynorov@nvidia.com>]

On Fri, Jul 10, 2026 at 03:26:42AM +0530, Shrikanth Hegde wrote:
> Actively push out task running on a non-preferred CPU. Since the task is
> running on the CPU, need to stop the cpu and push the task out.
> However, if the task is pinned only to non-preferred CPUs, it will continue
> running there. This will help in maintaining the userspace affinities
> unlike CPU hotplug or isolated cpusets.
> 
> Though code is similar to  __balance_push_cpu_stop and quite close to
> push_cpu_stop, it is being kept separate as it provides a cleaner
> implementation with CONFIG_PREFERRED_CPU.
> 
> Add push_task_work_done flag to protect work buffer.
> Works only with FAIR class.
> 
> For now, only current running task is pushed out. This keeps the code
> simpler. In future optimization maybe done to move all the queued
> task on the rq.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> ---
> v6->v7:
> - Moved is_migration_disabled
> - removed fair class check 
> 
>  kernel/sched/core.c  | 78 ++++++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/sched.h |  8 +++++
>  2 files changed, 86 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9e8eec4451b6..74c93a88bf84 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5774,6 +5774,9 @@ 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();
>  
> @@ -11292,3 +11295,78 @@ 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;
> +
> +	/* sanity checks and clear */
> +	if (cpu_preferred(rq->cpu) || is_migration_disabled(p)) {
> +		scoped_guard(rq_lock, 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);
> +	rq->push_task_work_done = false;
> +	update_rq_clock(rq);
> +
> +	context_unsafe_alias(rq);
> +
> +	if (task_rq(p) == rq && task_on_rq_queued(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 vCPU preemptions
> + * 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 non-preferred CPUs, it can't be moved out
> + */
> +void sched_push_current_non_preferred_cpu(struct rq *rq)
> +{
> +	struct task_struct *push_task = rq->curr;
> +
> +	/* Don't push the task if task's affinity doesn't allow */
> +	if (!task_can_sched_on_preferred(rq->cpu, push_task))
> +		return;

Shouldn't you protect it with the rq lock against races with affinity or
policy changes? The task_can_sched_on_preferred() checks p->sched_class
and p->cpus_ptr.

> +
> +	/* There is already a stopper thread. Don't race with it. */
> +	if (rq->push_task_work_done)
> +		return;
> +
> +	/* sched_tick runs with interrupts disabled. */
> +	get_task_struct(push_task);
> +
> +	scoped_guard(rq_lock, rq)
> +		rq->push_task_work_done = true;
> +
> +	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 6de6366f2faa..80c02e2c09eb 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1277,6 +1277,8 @@ struct rq {
>  
>  	struct list_head cfs_tasks;
>  
> +	bool			push_task_work_done;
> +
>  	struct sched_avg	avg_rt;
>  	struct sched_avg	avg_dl;
>  #ifdef CONFIG_HAVE_SCHED_AVG_IRQ
> @@ -4242,4 +4244,10 @@ static inline bool task_can_sched_on_preferred(int cpu, struct task_struct *p)
>  	return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
>  }
>  
> +#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 */
> -- 
> 2.47.3