Re: [PATCH v3 11/20] sched/core: Push current task from non preferred CPU

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Prateek.

On 6/4/26 12:28 PM, K Prateek Nayak wrote:
> Hello Shrikanth,
> 
> On 5/14/2026 8:51 PM, Shrikanth Hegde wrote:
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 86fa4bfaead0..508773e71929 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -5678,6 +5678,9 @@ void sched_tick(void)
>>   	unsigned long hw_pressure;
>>   	u64 resched_latency;
>>   
>> +	if (!cpu_preferred(cpu))
>> +		sched_push_current_non_preferred_cpu(rq);
>> +
> 
> Is there a reason why we don't do a bulk move of all the queued tasks at
> once?
> 
> Is it simply for the latency reason or could it lead to too many task
> movements if the CPU frequently transitions in and out of the preferred
> mask ?

It was simplicity of moving one at a time and latency associated with moving many.

> 
>>   	if (housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE))
>>   		arch_scale_freq_tick();
>>   
>> @@ -11263,3 +11266,87 @@ void sched_change_end(struct sched_change_ctx *ctx)
>>   		p->sched_class->prio_changed(rq, p, ctx->prio);
>>   	}
>>   }
>> +
>> +#ifdef CONFIG_PREFERRED_CPU
>> +/* npc - non 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;
> 
> I think we should do that sanity check for cpu_preferred() here instead
> of doing it back to back when sched_push_current_non_preferred_cpu() is
> called from the tick handler.
> 

Ok.

>> +
>> +	raw_spin_lock_irq(&p->pi_lock);
>> +	rq_lock(rq, &rf);
>> +	rq->push_task_work_done = 0;
>> +
>> +	update_rq_clock(rq);
>> +
>> +	if (task_rq(p) == rq && task_on_rq_queued(p)) {
>> +		cpu = select_fallback_rq(rq->cpu, 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.
>> + * 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/RT 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;
>> +	unsigned long flags;
>> +	struct rq_flags rf;
>> +
>> +	/* sanity check */
>> +	if (cpu_preferred(rq->cpu))
>> +		return;
>> +
>> +	/* Push only if it is FAIR/RT class */
>> +	if (push_task->sched_class != &fair_sched_class &&
>> +	    push_task->sched_class != &rt_sched_class)
>> +		return;
>> +
>> +	if (kthread_is_per_cpu(push_task) ||
>> +	    is_migration_disabled(push_task))
>> +		return;
>> +
>> +	/* Is there any preferred CPU in the affinity list */
>> +	if (!task_has_preferred_cpus(push_task))
>> +		return;
> 
> I think there is some value to teach the __migrate_enable() path about
> the preferred_cpu - that way, we don't end up with
> __set_cpus_allowed_ptr_locked() putting a task on non-preferred
> CPU by selecting it from:
> 
>      cpumask_any_and_distribute(cpu_valid_mask, ctx->new_mask);
> 
> and affine_move_task() can handle cases where the task does a
> migrate_enable() from a non-preferred CPU / changes affinity and can be
> pushed out readily.

Ok. will read more into this.

> 
> I'll let Peter comment since that bit already very complicated and this
> would be adding more to that already complicated machinery.
> 
>> +
>> +	/* There is already a stopper thread for this. Dont race with it */
>> +	if (rq->push_task_work_done == 1)
>> +		return;
>> +
>> +	local_irq_save(flags);
>> +
>> +	get_task_struct(push_task);
>> +
>> +	rq_lock(rq, &rf);
> 
> nit. the irqsave and rq_lock can use guards.

alright.

> 
>> +	rq->push_task_work_done = 1;
> 
> nit.
> 
> Apart from task_has_preferred_cpus(), all the checks above have no
> dependency on CONFIG_PREFERRED_CPU.

Code will bloat for other users. Not many may want to set CONFIG_PREFERRED_CPU=y.
That was the reason.

> 
> Can we set some local indicator in sched_tick() within the rq_lock() section
> to then schedule the stopper once we drop the rq_lock there? That way we
> don't have to grab the rq_lock thrice in the worst case scenario where we
> have to schedule the stopper.

I didn't get you. can you explain? how are we grabbing rq_lock thrice?

> 
>> +	rq_unlock(rq, &rf);
>> +
>> +	stop_one_cpu_nowait(rq->cpu, sched_non_preferred_cpu_push_stop,
>> +			    push_task, this_cpu_ptr(&npc_push_task_work));
>> +	local_irq_restore(flags);
>> +}
>> +#endif
>