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

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Message-ID: <7451b309-6f7e-43d9-801b-224a0a5cb08d@redhat.com>
Date: Tue, 8 Oct 2024 16:00:12 -0400
From: Waiman Long <llong@...hat.com>
To: Kuan-Wei Chiu <visitorckw@...il.com>, Waiman Long <llong@...hat.com>
Cc: xavier_qy@....com, lizefan.x@...edance.com, tj@...nel.org,
 hannes@...xchg.org, mkoutny@...e.com, akpm@...ux-foundation.org,
 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 10/8/24 12:45 PM, Kuan-Wei Chiu wrote:
> On Tue, Oct 08, 2024 at 10:02:23AM -0400, Waiman Long wrote:
>> 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.
>>
> Thanks for your feedback. I appreciate your point regarding the implicit
> casting of boolean values. And changing the code to:
>
> if (uf_union(&csa[i]->node, &csa[j]->node))
>      --ndoms;
>
> would also enhance clarity and readability.
>
> Would you like me to resend v3? I'm asking because I suspect Tejun may
> want to see more user cases before considering such optimizations.

I agreed with Tejun that union-find performance is not that important 
for the cpuset use case which is also the only use case at the moment. I 
will support a v3 if you can find another use case where performance is 
more important.

Cheers,
Longman