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

[Waiman Long]

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@xxxxxxxxx>
> > > ---
> > > 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