linux-kernel - Re: [PATCH v2 3/3] sched/fair: do not scribble cache-hot CPU in select_idle

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Date: Mon, 19 Feb 2024 19:50:14 +0800
From: Hillf Danton <hdanton@...a.com>
To: Chen Yu <yu.c.chen@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Mel Gorman <mgorman@...e.de>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Chen Yu <yu.chen.surf@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] sched/fair: do not scribble cache-hot CPU in select_idle_cpu()

On Tue, 21 Nov 2023 15:40:14 +0800 Chen Yu <yu.c.chen@...el.com>
> Problem statement:
> When task p is woken up, the scheduler leverages select_idle_sibling()
> to find an idle CPU for it. p's previous CPU is usually a preference
> because it can improve cache locality. However in many cases, the
> previous CPU has already been taken by other wakees, thus p has to
> find another idle CPU.
> 
> Proposal:
> Introduce the SIS_CACHE. It considers the sleep time of the task for
> better task placement. Based on the task's short sleeping history,
> tag p's previous CPU as cache-hot. Later when p is woken up, it can
> choose its previous CPU in select_idle_sibling(). When other task is
> woken up, skip this cache-hot idle CPU when possible.
> 
> SIS_CACHE still prefers to choose an idle CPU during task wakeup,
> the idea is to optimize the idle CPU scan sequence.

Could you specify why the currently selected cpu fails to work in the
scenario described above?

	/*
	 * If the previous CPU is cache affine and idle, don't be stupid:
	 */
	if (prev != target && cpus_share_cache(prev, target) &&
	    (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
	    asym_fits_cpu(task_util, util_min, util_max, prev))
		return prev;