linux-kernel - Re: [PATCH v2 5/6] cgroup/cpuset: Optimize domain counting using updated uf

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Message-ID: <ef7f2495-06fc-4409-8233-062d2e884271@redhat.com>
Date: Tue, 8 Oct 2024 10:02:23 -0400
From: Waiman Long <llong@...hat.com>
To: Kuan-Wei Chiu <visitorckw@...il.com>, xavier_qy@....com,
 lizefan.x@...edance.com, tj@...nel.org, hannes@...xchg.org,
 mkoutny@...e.com, akpm@...ux-foundation.org
Cc: jserv@...s.ncku.edu.tw, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org
Subject: Re: [PATCH v2 5/6] cgroup/cpuset: Optimize domain counting using
 updated uf_union()

On 10/7/24 11:28 AM, Kuan-Wei Chiu wrote:
> Improve the efficiency of calculating the total number of scheduling
> domains by using the updated uf_union function, which now returns a
> boolean to indicate if a merge occurred. Previously, an additional loop
> was needed to count root nodes for distinct groups. With this change,
> each successful merge reduces the domain count (ndoms) directly,
> eliminating the need for the final loop and enhancing performance.
>
> Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
> ---
> Note: Tested with test_cpuset_prs.sh
>
> Side note: I know this optimization provides limited efficiency
> improvements in this case, but since the union-find code is in the
> library and other users may need group counting in the future, and
> the required code change is minimal, I think it's still worthwhile.
>
>   kernel/cgroup/cpuset.c | 10 +++-------
>   1 file changed, 3 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index a4dd285cdf39..5e9301550d43 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -817,6 +817,8 @@ static int generate_sched_domains(cpumask_var_t **domains,
>   	if (root_load_balance && (csn == 1))
>   		goto single_root_domain;
>   
> +	ndoms = csn;
> +
>   	for (i = 0; i < csn; i++)
>   		uf_node_init(&csa[i]->node);
>   
> @@ -829,17 +831,11 @@ static int generate_sched_domains(cpumask_var_t **domains,
>   				 * partition root cpusets.
>   				 */
>   				WARN_ON_ONCE(cgrpv2);
> -				uf_union(&csa[i]->node, &csa[j]->node);
> +				ndoms -= uf_union(&csa[i]->node, &csa[j]->node);

You are taking the implicit assumption that a boolean true is casted to 
int 1. That is the usual practice, but it is not part of the C standard 
itself though it is for C++.  I will be more comfortable with the "if 
(cond) ndoms++" form. It will also be more clear.

Cheers,
Longman