linux-kernel - Re: [PATCH v2 04/23] sched/cache: Make LLC id continuous

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Message-ID: <20251209115846.GL3707891@noisy.programming.kicks-ass.net>
Date: Tue, 9 Dec 2025 12:58:46 +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 04/23] sched/cache: Make LLC id continuous

On Wed, Dec 03, 2025 at 03:07:23PM -0800, Tim Chen wrote:

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 710ed9943d27..0a3918269906 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1210,10 +1210,17 @@ __read_mostly unsigned int llc_imb_pct            = 20;
>  
>  static int llc_id(int cpu)
>  {
> +	int llc;
> +
>  	if (cpu < 0)
>  		return -1;
>  
> +	llc = per_cpu(sd_llc_id, cpu);
> +	/* avoid race with cpu hotplug */
> +	if (unlikely(llc >= max_llcs))
> +		return -1;
> +
> +	return llc;
>  }
>  
>  void mm_init_sched(struct mm_struct *mm, struct mm_sched __percpu *_pcpu_sched)

> @@ -668,6 +670,55 @@ DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity);
>  DEFINE_STATIC_KEY_FALSE(sched_asym_cpucapacity);
>  DEFINE_STATIC_KEY_FALSE(sched_cluster_active);
>  
> +/*
> + * Assign continuous llc id for the CPU, and return
> + * the assigned llc id.
> + */
> +static int update_llc_id(struct sched_domain *sd,
> +			 int cpu)
> +{
> +	int id = per_cpu(sd_llc_id, cpu), i;
> +
> +	if (id >= 0)
> +		return id;
> +
> +	if (sd) {
> +		/* Look for any assigned id and reuse it.*/
> +		for_each_cpu(i, sched_domain_span(sd)) {
> +			id = per_cpu(sd_llc_id, i);
> +
> +			if (id >= 0) {
> +				per_cpu(sd_llc_id, cpu) = id;
> +				return id;
> +			}
> +		}
> +	}
> +
> +	/*
> +	 * When 1. there is no id assigned to this LLC domain,
> +	 * or 2. the sd is NULL, we reach here.
> +	 * Consider the following scenario,
> +	 * CPU0~CPU95 are in the node0, CPU96~CPU191 are
> +	 * in the node1. During bootup, maxcpus=96 is
> +	 * appended.
> +	 * case 1: When running cpu_attach_domain(CPU24)
> +	 * during boot up, CPU24 is the first CPU in its
> +	 * non-NULL LLC domain. However,
> +	 * its corresponding llc id has not been assigned yet.
> +	 *
> +	 * case 2: After boot up, the CPU100 is brought up
> +	 * via sysfs manually. As a result, CPU100 has only a
> +	 * Numa domain attached, because CPU100 is the only CPU
> +	 * of a sched domain, all its bottom domains are degenerated.
> +	 * The LLC domain pointer sd is NULL for CPU100.
> +	 *
> +	 * For both cases, we want to increase the number of LLCs.
> +	 */
> +	per_cpu(sd_llc_id, cpu) = max_llcs++;
> +
> +	return per_cpu(sd_llc_id, cpu);
> +}

I'm not sure I follow. So partition_sched_domains() first calls
detach_destroy_domains() on the old set, and then build_sched_domains()
on the new set.

Do detach_destroy_domain() will do:

  cpu_attach_domain(NULL,..);

That is, it will explicitly attach the NULL sched_domain to a CPU. At
which point I feel update_llc_id() should be returning -1, no?

Then later, build_sched_domains() will set a !NULL sched_domain, at
which point update_llc_id() can set a real value.

This should then also get rid of that weird max_llcs check in llc_id(),
right?