Re: [PATCH v2 5/6] cgroup/cpuset: Optimize domain counting using updated uf_union()

[Kuan-Wei Chiu <visitorckw@gmail.com>]

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@gmail.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.

Regards,
Kuan-Wei