linux-kernel - Re: [RFC v2 6/9] sched/core: Push current task out if CPU is marked as avoid

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Message-ID: <d87c4b4f-959b-4726-9b4b-4ddeb7488b37@linux.ibm.com>
Date: Wed, 13 Aug 2025 00:10:23 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: yury.norov@...il.com
Cc: vschneid@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org,
        mingo@...hat.com, peterz@...radead.org, kprateek.nayak@....com,
        huschle@...ux.ibm.com, srikar@...ux.ibm.com,
        linux-kernel@...r.kernel.org, christophe.leroy@...roup.eu,
        linuxppc-dev@...ts.ozlabs.org, gregkh@...uxfoundation.org,
        maddy@...ux.ibm.com, tglx@...utronix.de, juri.lelli@...hat.com,
        vincent.guittot@...aro.org
Subject: Re: [RFC v2 6/9] sched/core: Push current task out if CPU is marked
 as avoid


Sorry for the delay in response to bloat-o-meter report. Since stop_one_cpu_nowait needs protection
against race, need to add a field in rq. So ifdef check of CONFIG_PARAVIRT makes sense.

> 
> Since the task is running, need to use the stopper class to push the
> task out. Use __balance_push_cpu_stop to achieve that.
> 
> This currently works only CFS and RT.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
> ---
>   kernel/sched/core.c  | 44 ++++++++++++++++++++++++++++++++++++++++++++
>   kernel/sched/sched.h |  1 +
>   2 files changed, 45 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 13e44d7a0b90..aea4232e3ec4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5577,6 +5577,10 @@ void sched_tick(void)
>   
>   	sched_clock_tick();
>   
> +	/* push the current task out if cpu is marked as avoid */
> +	if (cpu_avoid(cpu))
> +		push_current_task(rq);
> +
>   	rq_lock(rq, &rf);
>   	donor = rq->donor;
>   
> @@ -8028,6 +8032,43 @@ static void balance_hotplug_wait(void)
>   			   TASK_UNINTERRUPTIBLE);
>   }
>   
> +static DEFINE_PER_CPU(struct cpu_stop_work, push_task_work);
> +
> +/* A CPU is marked as Avoid when there is contention for underlying
> + * physical CPU and using this CPU will lead to hypervisor preemptions.
> + * It is better not to use this CPU.
> + *
> + * In case any task is scheduled on such CPU, move it out. In
> + * select_fallback_rq a non_avoid CPU will be chosen and henceforth
> + * task shouldn't come back to this CPU
> + */
> +void push_current_task(struct rq *rq)
> +{
> +	struct task_struct *push_task = rq->curr;
> +	unsigned long flags;
> +
> +	/* idle task can't be pused out */
> +	if (rq->curr == rq->idle || !cpu_avoid(rq->cpu))
> +		return;
> +
> +	/* Do for only SCHED_NORMAL AND RT for now */
> +	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;
> +
> +	local_irq_save(flags);
> +	get_task_struct(push_task);
> +	preempt_disable();
> +
> +	stop_one_cpu_nowait(rq->cpu, __balance_push_cpu_stop, push_task,
> +			    this_cpu_ptr(&push_task_work));

Doing a perf record occasionally caused the crash. This happens because stop_one_cpu_nowait
expects the callers to sync and push_task_work should be untouched until the stopper executes.

So, i had to do something similar to whats done in active_balance.
Add a field in rq and set/unset accordingly.

Using this field in __balance_push_cpu_stop is also hacky. I have to do something like below,
	if (rq->balance_callback != &balance_push_callback)
		rq->push_task_work_pending = 0;
or i have to copy __balance_push_cpu_stop and do the above.

After this, it makes sense to put all this under CONFIG_PARAVIRT.


(Also, i did explore using stop_one_cpu variant, got to it via scheduling a work and then execute it at
preemptible context. That occasionally ends up in deadlock. due to some issues at my end, haven't debugged that
further. a backup option for nowait)

> +	preempt_enable();
> +	local_irq_restore(flags);
> +}
>   #else /* !CONFIG_HOTPLUG_CPU: */
>   
>   static inline void balance_push(struct rq *rq)
> @@ -8042,6 +8083,9 @@ static inline void balance_hotplug_wait(void)
>   {
>   }
>   
> +void push_current_task(struct rq *rq)
> +{
> +}
>   #endif /* !CONFIG_HOTPLUG_CPU */
>   
>   void set_rq_online(struct rq *rq)
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 105190b18020..b9614873762e 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1709,6 +1709,7 @@ struct rq_flags {
>   };
>   
>   extern struct balance_callback balance_push_callback;
> +void push_current_task(struct rq *rq);
>   
>   #ifdef CONFIG_SCHED_CLASS_EXT
>   extern const struct sched_class ext_sched_class;

Hopefully i should be able to send out v3 soon addressing the comments.

Namewise, going to keep it cpu_paravirt_mask and cpu_paravirt(cpu).