cpuset: use Union-Find to optimize the merging of cpumasks

jira NONE_AUTOMATION
Rebuild_History Non-Buildable kernel-5.14.0-570.17.1.el9_6
commit-author Xavier <[email protected]>
commit 8a895c2

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.

	Signed-off-by: Xavier <[email protected]>
	Signed-off-by: Tejun Heo <[email protected]>
(cherry picked from commit 8a895c2)
	Signed-off-by: Jonathan Maple <[email protected]>

Author: PlaidCat

kernel/cgroup/cpuset.c

@@ -46,6 +46,7 @@
 #include <linux/cgroup.h>
 #include <linux/wait.h>
 #include <linux/workqueue.h>
+#include <linux/union_find.h>
 
 DEFINE_STATIC_KEY_FALSE(cpusets_pre_enable_key);
 DEFINE_STATIC_KEY_FALSE(cpusets_enabled_key);
@@ -175,9 +176,6 @@ struct cpuset {
 	 */
 	int attach_in_progress;
 
-	/* partition number for rebuild_sched_domains() */
-	int pn;
-
 	/* for custom sched domain */
 	int relax_domain_level;
 
@@ -211,6 +209,9 @@ struct cpuset {
 
 	/* Remote partition silbling list anchored at remote_children */
 	struct list_head remote_sibling;
+
+	/* Used to merge intersecting subsets for generate_sched_domains */
+	struct uf_node node;
 };
 
 /*
@@ -998,33 +999,31 @@ static inline int nr_cpusets(void)
  *	   were changed (added or removed.)
  *
  * Finding the best partition (set of domains):
- *	The triple nested loops below over i, j, k scan over the
- *	load balanced cpusets (using the array of cpuset pointers in
- *	csa[]) looking for pairs of cpusets that have overlapping
- *	cpus_allowed, but which don't have the same 'pn' partition
- *	number and gives them in the same partition number.  It keeps
- *	looping on the 'restart' label until it can no longer find
- *	any such pairs.
+ *	The double nested loops below over i, j scan over the load
+ *	balanced cpusets (using the array of cpuset pointers in csa[])
+ *	looking for pairs of cpusets that have overlapping cpus_allowed
+ *	and merging them using a union-find algorithm.
+ *
+ *	The union of the cpus_allowed masks from the set of all cpusets
+ *	having the same root then form the one element of the partition
+ *	(one sched domain) to be passed to partition_sched_domains().
  *
- *	The union of the cpus_allowed masks from the set of
- *	all cpusets having the same 'pn' value then form the one
- *	element of the partition (one sched domain) to be passed to
- *	partition_sched_domains().
  */
 static int generate_sched_domains(cpumask_var_t **domains,
 			struct sched_domain_attr **attributes)
 {
 	struct cpuset *cp;	/* top-down scan of cpusets */
 	struct cpuset **csa;	/* array of all cpuset ptrs */
 	int csn;		/* how many cpuset ptrs in csa so far */
-	int i, j, k;		/* indices for partition finding loops */
+	int i, j;		/* indices for partition finding loops */
 	cpumask_var_t *doms;	/* resulting partition; i.e. sched domains */
 	struct sched_domain_attr *dattr;  /* attributes for custom domains */
 	int ndoms = 0;		/* number of sched domains in result */
 	int nslot;		/* next empty doms[] struct cpumask slot */
 	struct cgroup_subsys_state *pos_css;
 	bool root_load_balance = is_sched_load_balance(&top_cpuset);
 	bool cgrpv2 = cgroup_subsys_on_dfl(cpuset_cgrp_subsys);
+	int nslot_update;
 
 	doms = NULL;
 	dattr = NULL;
@@ -1112,31 +1111,25 @@ static int generate_sched_domains(cpumask_var_t **domains,
 	if (root_load_balance && (csn == 1))
 		goto single_root_domain;
 
-	for (i = 0; i < csn; i++)
-		csa[i]->pn = i;
-	ndoms = csn;
-
-restart:
-	/* Find the best partition (set of sched domains) */
-	for (i = 0; i < csn; i++) {
-		struct cpuset *a = csa[i];
-		int apn = a->pn;
-
-		for (j = 0; j < csn; j++) {
-			struct cpuset *b = csa[j];
-			int bpn = b->pn;
-
-			if (apn != bpn && cpusets_overlap(a, b)) {
-				for (k = 0; k < csn; k++) {
-					struct cpuset *c = csa[k];
+	if (!cgrpv2) {
+		for (i = 0; i < csn; i++)
+			uf_node_init(&csa[i]->node);
 
-					if (c->pn == bpn)
-						c->pn = apn;
-				}
-				ndoms--;	/* one less element */
-				goto restart;
+		/* Merge overlapping cpusets */
+		for (i = 0; i < csn; i++) {
+			for (j = i + 1; j < csn; j++) {
+				if (cpusets_overlap(csa[i], csa[j]))
+					uf_union(&csa[i]->node, &csa[j]->node);
 			}
 		}
+
+		/* Count the total number of domains */
+		for (i = 0; i < csn; i++) {
+			if (uf_find(&csa[i]->node) == &csa[i]->node)
+				ndoms++;
+		}
+	} else {
+		ndoms = csn;
 	}
 
 	/*
@@ -1169,44 +1162,25 @@ static int generate_sched_domains(cpumask_var_t **domains,
 	}
 
 	for (nslot = 0, i = 0; i < csn; i++) {
-		struct cpuset *a = csa[i];
-		struct cpumask *dp;
-		int apn = a->pn;
-
-		if (apn < 0) {
-			/* Skip completed partitions */
-			continue;
-		}
-
-		dp = doms[nslot];
-
-		if (nslot == ndoms) {
-			static int warnings = 10;
-			if (warnings) {
-				pr_warn("rebuild_sched_domains confused: nslot %d, ndoms %d, csn %d, i %d, apn %d\n",
-					nslot, ndoms, csn, i, apn);
-				warnings--;
-			}
-			continue;
-		}
-
-		cpumask_clear(dp);
-		if (dattr)
-			*(dattr + nslot) = SD_ATTR_INIT;
+		nslot_update = 0;
 		for (j = i; j < csn; j++) {
-			struct cpuset *b = csa[j];
-
-			if (apn == b->pn) {
-				cpumask_or(dp, dp, b->effective_cpus);
+			if (uf_find(&csa[j]->node) == &csa[i]->node) {
+				struct cpumask *dp = doms[nslot];
+
+				if (i == j) {
+					nslot_update = 1;
+					cpumask_clear(dp);
+					if (dattr)
+						*(dattr + nslot) = SD_ATTR_INIT;
+				}
+				cpumask_or(dp, dp, csa[j]->effective_cpus);
 				cpumask_and(dp, dp, housekeeping_cpumask(HK_TYPE_DOMAIN));
 				if (dattr)
-					update_domain_attr_tree(dattr + nslot, b);
-
-				/* Done with this partition */
-				b->pn = -1;
+					update_domain_attr_tree(dattr + nslot, csa[j]);
 			}
 		}
-		nslot++;
+		if (nslot_update)
+			nslot++;
 	}
 	BUG_ON(nslot != ndoms);