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Date: Thu, 10 Apr 2025 12:00:22 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	linux-kernel@vger.kernel.org,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Valentin Schneider <vschneid@redhat.com>,
	"Gautham R. Shenoy" <gautham.shenoy@amd.com>,
	Swapnil Sapkal <swapnil.sapkal@amd.com>
Subject: Re: [RFC PATCH 1/5] sched/fair: Add push task framework
Message-ID: <20250410100022.GA30687@noisy.programming.kicks-ass.net>
References: <20250409111539.23791-1-kprateek.nayak@amd.com>
 <20250409111539.23791-2-kprateek.nayak@amd.com>
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In-Reply-To: <20250409111539.23791-2-kprateek.nayak@amd.com>

On Wed, Apr 09, 2025 at 11:15:35AM +0000, K Prateek Nayak wrote:

> +static void fair_add_pushable_task(struct rq *rq, struct task_struct *p)
> +{
> +	if (fair_push_task(p)) {
> +		plist_del(&p->pushable_tasks, &rq->cfs.pushable_tasks);
> +		plist_node_init(&p->pushable_tasks, p->prio);

I gotta aks, why do we care about ordering the push list on p->prio for
fair?

> +		plist_add(&p->pushable_tasks, &rq->cfs.pushable_tasks);
> +	}
> +}
> +
>  /*
>   * select_task_rq_fair: Select target runqueue for the waking task in domains
>   * that have the relevant SD flag set. In practice, this is SD_BALANCE_WAKE,

> @@ -8914,6 +8978,12 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
>  		put_prev_entity(cfs_rq, pse);
>  		set_next_entity(cfs_rq, se);
>  
> +		/*
> +		 * The previous task might be eligible for being pushed on
> +		 * another cpu if it is still active.
> +		 */
> +		fair_add_pushable_task(rq, prev);
> +
>  		__set_next_task_fair(rq, p, true);
>  	}
>  
> @@ -8986,6 +9056,13 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev, struct t
>  		cfs_rq = cfs_rq_of(se);
>  		put_prev_entity(cfs_rq, se);
>  	}
> +
> +	/*
> +	 * The previous task might be eligible for being pushed on another cpu
> +	 * if it is still active.
> +	 */
> +	fair_add_pushable_task(rq, prev);
> +
>  }

These two are tricky; while they will work with a push balance callback,
they will cause some pain with pulling from the push lists; a-la
newidle.

Notably, we must be sure to check ->on_cpu.

Perhaps later patches add this, let me continue reading...