sched_ext: Compact struct bpf_iter_scx_dsq_kern

struct scx_iter_scx_dsq is defined as 6 u64's and scx_dsq_iter_kern was
using 5 of them. We want to add two more u64 fields but it's better if we do
so while staying within scx_iter_scx_dsq to maintain binary compatibility.

The way scx_iter_scx_dsq_kern is laid out is rather inefficient - the node
field takes up three u64's but only one bit of the last u64 is used. Turn
the bool into u32 flags and only use the lower 16 bits freeing up 48 bits -
16 bits for flags, 32 bits for a u32 - for use by struct
bpf_iter_scx_dsq_kern.

This allows moving the dsq_seq and flags fields of bpf_iter_scx_dsq_kern
into the cursor field reducing the struct size by a full u64.

No behavior changes intended.

Signed-off-by: Tejun Heo <tj@kernel.org>

Author: htejun

include/linux/sched/ext.h

@@ -119,9 +119,17 @@ enum scx_kf_mask {
 	__SCX_KF_TERMINAL	= SCX_KF_ENQUEUE | SCX_KF_SELECT_CPU | SCX_KF_REST,
 };
 
+enum scx_dsq_lnode_flags {
+	SCX_DSQ_LNODE_ITER_CURSOR = 1 << 0,
+
+	/* high 16 bits can be for iter cursor flags */
+	__SCX_DSQ_LNODE_PRIV_SHIFT = 16,
+};
+
 struct scx_dsq_list_node {
 	struct list_head	node;
-	bool			is_bpf_iter_cursor;
+	u32			flags;
+	u32			priv;		/* can be used by iter cursor */
 };
 
 /*

kernel/sched/ext.c

@@ -1191,7 +1191,7 @@ static struct task_struct *nldsq_next_task(struct scx_dispatch_q *dsq,
 
 		dsq_lnode = container_of(list_node, struct scx_dsq_list_node,
 					 node);
-	} while (dsq_lnode->is_bpf_iter_cursor);
+	} while (dsq_lnode->flags & SCX_DSQ_LNODE_ITER_CURSOR);
 
 	return container_of(dsq_lnode, struct task_struct, scx.dsq_list);
 }
@@ -1209,16 +1209,15 @@ static struct task_struct *nldsq_next_task(struct scx_dispatch_q *dsq,
  */
 enum scx_dsq_iter_flags {
 	/* iterate in the reverse dispatch order */
-	SCX_DSQ_ITER_REV		= 1U << 0,
+	SCX_DSQ_ITER_REV		= 1U << 16,
 
-	__SCX_DSQ_ITER_ALL_FLAGS	= SCX_DSQ_ITER_REV,
+	__SCX_DSQ_ITER_USER_FLAGS	= SCX_DSQ_ITER_REV,
+	__SCX_DSQ_ITER_ALL_FLAGS	= __SCX_DSQ_ITER_USER_FLAGS,
 };
 
 struct bpf_iter_scx_dsq_kern {
 	struct scx_dsq_list_node	cursor;
 	struct scx_dispatch_q		*dsq;
-	u32				dsq_seq;
-	u32				flags;
 } __attribute__((aligned(8)));
 
 struct bpf_iter_scx_dsq {
@@ -1256,6 +1255,9 @@ static void scx_task_iter_init(struct scx_task_iter *iter)
 {
 	lockdep_assert_held(&scx_tasks_lock);
 
+	BUILD_BUG_ON(__SCX_DSQ_ITER_ALL_FLAGS &
+		     ((1U << __SCX_DSQ_LNODE_PRIV_SHIFT) - 1));
+
 	iter->cursor = (struct sched_ext_entity){ .flags = SCX_TASK_CURSOR };
 	list_add(&iter->cursor.tasks_node, &scx_tasks);
 	iter->locked = NULL;
@@ -6285,17 +6287,16 @@ __bpf_kfunc int bpf_iter_scx_dsq_new(struct bpf_iter_scx_dsq *it, u64 dsq_id,
 	BUILD_BUG_ON(__alignof__(struct bpf_iter_scx_dsq_kern) !=
 		     __alignof__(struct bpf_iter_scx_dsq));
 
-	if (flags & ~__SCX_DSQ_ITER_ALL_FLAGS)
+	if (flags & ~__SCX_DSQ_ITER_USER_FLAGS)
 		return -EINVAL;
 
 	kit->dsq = find_non_local_dsq(dsq_id);
 	if (!kit->dsq)
 		return -ENOENT;
 
 	INIT_LIST_HEAD(&kit->cursor.node);
-	kit->cursor.is_bpf_iter_cursor = true;
-	kit->dsq_seq = READ_ONCE(kit->dsq->seq);
-	kit->flags = flags;
+	kit->cursor.flags |= SCX_DSQ_LNODE_ITER_CURSOR | flags;
+	kit->cursor.priv = READ_ONCE(kit->dsq->seq);
 
 	return 0;
 }
@@ -6309,7 +6310,7 @@ __bpf_kfunc int bpf_iter_scx_dsq_new(struct bpf_iter_scx_dsq *it, u64 dsq_id,
 __bpf_kfunc struct task_struct *bpf_iter_scx_dsq_next(struct bpf_iter_scx_dsq *it)
 {
 	struct bpf_iter_scx_dsq_kern *kit = (void *)it;
-	bool rev = kit->flags & SCX_DSQ_ITER_REV;
+	bool rev = kit->cursor.flags & SCX_DSQ_ITER_REV;
 	struct task_struct *p;
 	unsigned long flags;
 
@@ -6330,7 +6331,7 @@ __bpf_kfunc struct task_struct *bpf_iter_scx_dsq_next(struct bpf_iter_scx_dsq *i
 	 */
 	do {
 		p = nldsq_next_task(kit->dsq, p, rev);
-	} while (p && unlikely(u32_before(kit->dsq_seq, p->scx.dsq_seq)));
+	} while (p && unlikely(u32_before(kit->cursor.priv, p->scx.dsq_seq)));
 
 	if (p) {
 		if (rev)