[PATCH v2] sched_ext: Skip per-CPU data allocation for built-in DSQs

[Qiurong Fang]

From: fangqiurong <fangqiurong@xxxxxxxxxx>

Deferred reenqueue tracking is only supported for user DSQs:
schedule_dsq_reenq() serves the local DSQ through
sch->pcpu->deferred_reenq_local and rejects all other built-in DSQ
ids. Every DSQ still allocates nr_cpu_ids * sizeof(struct scx_dsq_pcpu)
bytes of per-CPU memory for it.

Skip the allocation for built-in DSQ ids. While at it, rename ->pcpu to
->pcpu_user so that the per-CPU data reads as user-DSQ only.

Signed-off-by: fangqiurong <fangqiurong@xxxxxxxxxx>
---
v2: Rename ->pcpu to ->pcpu_user as suggested by Tejun. Describe the
    memory waste as quadratic as pointed out by Zhan Xusheng. Drop the
    Fixes: tag as this is a plain memory saving with no correctness
    impact.
---
 include/linux/sched/ext.h |  2 +-
 kernel/sched/ext/ext.c    | 20 ++++++++++++++------
 2 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 582d7cd4a983..3c793d51c000 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -91,7 +91,7 @@ struct scx_dispatch_q {
 	struct rhash_head	hash_node;
 	struct llist_node	free_node;
 	struct scx_sched	*sched;
-	struct scx_dsq_pcpu __percpu *pcpu;
+	struct scx_dsq_pcpu __percpu *pcpu_user;
 	struct rcu_head		rcu;
 };
 
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index c539d15cda63..efb492876cd6 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -1127,7 +1127,8 @@ void schedule_dsq_reenq(struct scx_sched *sch, struct scx_dispatch_q *dsq,
 	} else if (!(dsq->id & SCX_DSQ_FLAG_BUILTIN)) {
 		rq = this_rq();
 
-		struct scx_dsq_pcpu *dsq_pcpu = per_cpu_ptr(dsq->pcpu, cpu_of(rq));
+		struct scx_dsq_pcpu *dsq_pcpu =
+			per_cpu_ptr(dsq->pcpu_user, cpu_of(rq));
 		struct scx_deferred_reenq_user *dru = &dsq_pcpu->deferred_reenq_user;
 
 		/*
@@ -5048,12 +5049,16 @@ s32 scx_init_dsq(struct scx_dispatch_q *dsq, u64 dsq_id, struct scx_sched *sch)
 	dsq->id = dsq_id;
 	dsq->sched = sch;
 
-	dsq->pcpu = alloc_percpu(struct scx_dsq_pcpu);
-	if (!dsq->pcpu)
+	/* Deferred reenqueue tracking is only supported for user DSQs */
+	if (dsq_id & SCX_DSQ_FLAG_BUILTIN)
+		return 0;
+
+	dsq->pcpu_user = alloc_percpu(struct scx_dsq_pcpu);
+	if (!dsq->pcpu_user)
 		return -ENOMEM;
 
 	for_each_possible_cpu(cpu) {
-		struct scx_dsq_pcpu *pcpu = per_cpu_ptr(dsq->pcpu, cpu);
+		struct scx_dsq_pcpu *pcpu = per_cpu_ptr(dsq->pcpu_user, cpu);
 
 		pcpu->dsq = dsq;
 		INIT_LIST_HEAD(&pcpu->deferred_reenq_user.node);
@@ -5066,8 +5071,11 @@ static void exit_dsq(struct scx_dispatch_q *dsq)
 {
 	s32 cpu;
 
+	if (!dsq->pcpu_user)
+		return;
+
 	for_each_possible_cpu(cpu) {
-		struct scx_dsq_pcpu *pcpu = per_cpu_ptr(dsq->pcpu, cpu);
+		struct scx_dsq_pcpu *pcpu = per_cpu_ptr(dsq->pcpu_user, cpu);
 		struct scx_deferred_reenq_user *dru = &pcpu->deferred_reenq_user;
 		struct rq *rq = cpu_rq(cpu);
 
@@ -5081,7 +5089,7 @@ static void exit_dsq(struct scx_dispatch_q *dsq)
 		}
 	}
 
-	free_percpu(dsq->pcpu);
+	free_percpu(dsq->pcpu_user);
 }
 
 static void free_dsq_rcufn(struct rcu_head *rcu)
-- 
2.43.0