linux-kernel - Re: [PATCH v2 5/7] sched: Implement shared runqueue in CFS

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Date:   Wed, 12 Jul 2023 11:30:23 +0530
From:   "Gautham R. Shenoy" <gautham.shenoy@....com>
To:     David Vernet <void@...ifault.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, kprateek.nayak@....com,
        aaron.lu@...el.com, clm@...a.com, tj@...nel.org,
        roman.gushchin@...ux.dev, kernel-team@...a.com
Subject: Re: [PATCH v2 5/7] sched: Implement shared runqueue in CFS

Hello David,

On Mon, Jul 10, 2023 at 03:03:40PM -0500, David Vernet wrote:

[..snip..]

> ---

> +
> +static struct task_struct *shared_runq_pop_task(struct rq *rq)
> +{
> +	unsigned long flags;
> +	struct task_struct *p;
> +	struct shared_runq *shared_runq;
> +
> +	shared_runq = rq_shared_runq(rq);
> +	if (list_empty(&shared_runq->list))
> +		return NULL;
> +
> +	spin_lock_irqsave(&shared_runq->lock, flags);
> +	p = list_first_entry_or_null(&shared_runq->list, struct task_struct,
> +				     shared_runq_node);


Apologies for the bikeshedding comment : Here you are attempting to
remove the task from the "head", while in shared_runq_push_task below,
you are adding a task to the tail. Which is the usual queue
semantics. Then why call them shared_runq_pop_task() and
shared_runq_push_task() ?

Can we name them __shared_runq_enqueue_task() and
__shared_runq_pick_next_task() instead ?

> +	if (p && is_cpu_allowed(p, cpu_of(rq)))
> +		list_del_init(&p->shared_runq_node);
> +	else
> +		p = NULL;
> +	spin_unlock_irqrestore(&shared_runq->lock, flags);
> +
> +	return p;
> +}
> +
> +static void shared_runq_push_task(struct rq *rq, struct task_struct *p)
> +{
> +	unsigned long flags;
> +	struct shared_runq *shared_runq;
> +
> +	shared_runq = rq_shared_runq(rq);
> +	spin_lock_irqsave(&shared_runq->lock, flags);
> +	list_add_tail(&p->shared_runq_node, &shared_runq->list);
> +	spin_unlock_irqrestore(&shared_runq->lock, flags);
> +}
> +
>  static void shared_runq_enqueue_task(struct rq *rq, struct task_struct *p,
>  				     int enq_flags)
> -{}
> +{
> +	bool task_migrated = enq_flags & ENQUEUE_MIGRATED;
> +	bool task_wakeup = enq_flags & ENQUEUE_WAKEUP;
> +
> +	/*
> +	 * Only enqueue the task in the shared runqueue if:
> +	 *
> +	 * - SWQUEUE is enabled
> +	 * - The task is on the wakeup path
> +	 * - The task wasn't purposefully migrated to the current rq by
> +	 *   select_task_rq()
> +	 * - The task isn't pinned to a specific CPU
> +	 */
> +	if (!task_wakeup || task_migrated || p->nr_cpus_allowed == 1)
> +		return;
> +
> +	shared_runq_push_task(rq, p);
> +}
>  
>  static int shared_runq_pick_next_task(struct rq *rq, struct rq_flags *rf)
>  {
> -	return 0;
> +	struct task_struct *p = NULL;
> +	struct rq *src_rq;
> +	struct rq_flags src_rf;
> +	int ret;
> +
> +	p = shared_runq_pop_task(rq);
> +	if (!p)
> +		return 0;
> +
> +	rq_unpin_lock(rq, rf);
> +	raw_spin_rq_unlock(rq);
> +
> +	src_rq = task_rq_lock(p, &src_rf);
> +
> +	if (task_on_rq_queued(p) && !task_on_cpu(rq, p)) {
> +		update_rq_clock(src_rq);
> +		src_rq = move_queued_task(src_rq, &src_rf, p, cpu_of(rq));
> +	}
> +
> +	if (src_rq->cpu != rq->cpu)
> +		ret = 1;
> +	else
> +		ret = -1;


So if src_rq->cpu != rq->cpu, then the task has _not_ been moved to
rq. But you return 1.

While in the else case, since src_rq->cpu == rq->cpu, the task has
been successfully moved to rq. But you are returning -1,

If newidle_balance() were to interpret this return value as the number
of tasks pulled, then, shouldn't it be the other way around ?

> +
> +	task_rq_unlock(src_rq, p, &src_rf);
> +
> +	raw_spin_rq_lock(rq);
> +	rq_repin_lock(rq, rf);
> +
> +	return ret;
>  }
>  

--
Thanks and Regards
gautham.