linux-kernel - Re: [PATCH v2 07/23] sched/cache: Introduce per runqueue task LLC preference counter

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Message-ID: <20251210125114.GS3707891@noisy.programming.kicks-ass.net>
Date: Wed, 10 Dec 2025 13:51:14 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Ingo Molnar <mingo@...hat.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	Hillf Danton <hdanton@...a.com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Jianyong Wu <jianyong.wu@...look.com>,
	Yangyu Chen <cyy@...self.name>,
	Tingyin Duan <tingyin.duan@...il.com>,
	Vern Hao <vernhao@...cent.com>, Vern Hao <haoxing990@...il.com>,
	Len Brown <len.brown@...el.com>, Aubrey Li <aubrey.li@...el.com>,
	Zhao Liu <zhao1.liu@...el.com>, Chen Yu <yu.chen.surf@...il.com>,
	Chen Yu <yu.c.chen@...el.com>,
	Adam Li <adamli@...amperecomputing.com>,
	Aaron Lu <ziqianlu@...edance.com>, Tim Chen <tim.c.chen@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 07/23] sched/cache: Introduce per runqueue task LLC
 preference counter

On Wed, Dec 03, 2025 at 03:07:26PM -0800, Tim Chen wrote:

> +static int resize_llc_pref(void)
> +{
> +	unsigned int *__percpu *tmp_llc_pref;
> +	int i, ret = 0;
> +
> +	if (new_max_llcs <= max_llcs)
> +		return 0;
> +
> +	/*
> +	 * Allocate temp percpu pointer for old llc_pref,
> +	 * which will be released after switching to the
> +	 * new buffer.
> +	 */
> +	tmp_llc_pref = alloc_percpu_noprof(unsigned int *);
> +	if (!tmp_llc_pref)
> +		return -ENOMEM;
> +
> +	for_each_present_cpu(i)
> +		*per_cpu_ptr(tmp_llc_pref, i) = NULL;
> +
> +	/*
> +	 * Resize the per rq nr_pref_llc buffer and
> +	 * switch to this new buffer.
> +	 */
> +	for_each_present_cpu(i) {
> +		struct rq_flags rf;
> +		unsigned int *new;
> +		struct rq *rq;
> +
> +		rq = cpu_rq(i);
> +		new = alloc_new_pref_llcs(rq->nr_pref_llc, per_cpu_ptr(tmp_llc_pref, i));
> +		if (!new) {
> +			ret = -ENOMEM;
> +
> +			goto release_old;
> +		}
> +
> +		/*
> +		 * Locking rq ensures that rq->nr_pref_llc values
> +		 * don't change with new task enqueue/dequeue
> +		 * when we repopulate the newly enlarged array.
> +		 */
> +		rq_lock_irqsave(rq, &rf);
> +		populate_new_pref_llcs(rq->nr_pref_llc, new);
> +		rq->nr_pref_llc = new;
> +		rq_unlock_irqrestore(rq, &rf);
> +	}
> +
> +release_old:
> +	/*
> +	 * Load balance is done under rcu_lock.
> +	 * Wait for load balance before and during resizing to
> +	 * be done. They may refer to old nr_pref_llc[]
> +	 * that hasn't been resized.
> +	 */
> +	synchronize_rcu();
> +	for_each_present_cpu(i)
> +		kfree(*per_cpu_ptr(tmp_llc_pref, i));
> +
> +	free_percpu(tmp_llc_pref);
> +
> +	/* succeed and update */
> +	if (!ret)
> +		max_llcs = new_max_llcs;
> +
> +	return ret;
> +}

Would it perhaps be easier to stick this thing in rq->sd rather than in
rq->nr_pref_llc. That way it automagically switches with the 'new'
domain. And then, with a bit of care, a singe load-balance pass should
see a consistent view (there should not be reloads of rq->sd -- which
will be a bit of an audit I suppose).