linux-kernel - Re: [PATCH v3] cpuset: use Union-Find to optimize the merging of cpumasks

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Date: Mon, 10 Jun 2024 19:18:50 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Xavier <ghostxavier@...a.com>
Cc: longman@...hat.com, lizefan.x@...edance.com, tj@...nel.org, 
	hannes@...xchg.org, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] cpuset: use Union-Find to optimize the merging of
 cpumasks

Hello.

On Mon, Jun 03, 2024 at 08:31:01PM GMT, Xavier <ghostxavier@...a.com> wrote:
> The process of constructing scheduling domains involves multiple loops
> and repeated evaluations, leading to numerous redundant and ineffective
> assessments that impact code efficiency.
> 
> Here, we use Union-Find to optimize the merging of cpumasks. By employing
> path compression and union by rank, we effectively reduce the number of
> lookups and merge comparisons.

Nice that you found such an application. (As Waiman wrote, the
efficiency is not so important here and it may not be dencreased but I
still think it makes the code more understandable by using standard data
structures.)

Have you looked whether there are other instances of U-F in the kernel?
(My quick search didn't show any.) Still, I think it'd be a good idea to
decouple this into two commits -- 1) implementation of the new U-F (into
lib/), 2) application within cpuset.

> +/*define a union find node struct*/
> +struct uf_node {
> +	int parent;

I think this would be better as `struct uf_node *`.

> +	int rank;
> +};

`unsigned int` if rank cannot be negative?

> +	/* Each node is initially its own parent */
> +	for (i = 0; i < csn; i++) {
> +		nodes[i].parent = i;
> +		nodes[i].rank = 0;
> +	}

With the suggestion above, nodes could start with parent = NULL and
self-parent be corrected during the first find_root -- thus whole array
could be simply init'd to zeroes with kzalloc.

My 0.02€,
Michal

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