linux-kernel - Re: [PATCH 5/7 v5] sched/fair: Add push task mechanism for EAS

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Message-ID: <xhsmh1putoxbz.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Wed, 19 Mar 2025 16:26:56 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, dietmar.eggemann@....com,
 rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
 lukasz.luba@....com, rafael.j.wysocki@...el.com, pierre.gondois@....com,
 linux-kernel@...r.kernel.org
Cc: qyousef@...alina.io, hongyan.xia2@....com, christian.loehle@....com,
 luis.machado@....com, qperret@...gle.com, Vincent Guittot
 <vincent.guittot@...aro.org>
Subject: Re: [PATCH 5/7 v5] sched/fair: Add push task mechanism for EAS

On 02/03/25 22:05, Vincent Guittot wrote:
> +static struct task_struct *pick_next_pushable_fair_task(struct rq *rq)
> +{
> +	struct task_struct *p;
> +
> +	if (!has_pushable_tasks(rq))
> +		return NULL;
> +
> +	p = plist_first_entry(&rq->cfs.pushable_tasks,
> +			      struct task_struct, pushable_tasks);
> +
> +	WARN_ON_ONCE(rq->cpu != task_cpu(p));
> +	WARN_ON_ONCE(task_current(rq, p));
> +	WARN_ON_ONCE(p->nr_cpus_allowed <= 1);
> +	WARN_ON_ONCE(!task_on_rq_queued(p));
> +
> +	/*
> +	 * Remove task from the pushable list as we try only once after that
> +	 * the task has been put back in enqueued list.
> +	 */
> +	plist_del(&p->pushable_tasks, &rq->cfs.pushable_tasks);
> +
> +	return p;

I've only had a look at this patch per the OSPM newidle balance discussion;
coupled with something like RT/DL's overload cpumask, this could be a
viable newidle_balance() replacement.

Unfortunately this means we now have a third copy of the push mechanism
along with RT and DL, so a third place to manually patch whenever a bug is
fixed in one of them [1].

We could perhaps have a skeleton of the pushable list handling in
{enqueue,dequeue)_task() and put_prev_task(), with class-specific conditions and
backing storage, (plist vs rbtree) handled via class callbacks.

Or even make the whole pushable enqueue/dequeue its own class callback,
which would simplify [2].

[1]: http://lore.kernel.org/r/20250304103001.0f89e953@gandalf.local.home
[2]: https://lore.kernel.org/lkml/20250312221147.1865364-7-jstultz@google.com/