[PATCH net-next v3 08/12] net: ethernet: cortina: Rotate free queue page allocation

[Linus Walleij <linusw@kernel.org>]

The free queue metadata bitmap currently searches for a free slot from
zero for every page allocation. Under sustained traffic this repeatedly
scans occupied low-numbered slots.

Remember the position after the most recently allocated slot and wrap the
bitmap search at its end. This keeps allocation cost distributed across
the metadata table.

Fixes: 4d5ae32f5e1e ("net: ethernet: Add a driver for Gemini gigabit ethernet")
Assisted-by: LLM
Signed-off-by: Linus Walleij <linusw@kernel.org>
---
 drivers/net/ethernet/cortina/gemini.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/cortina/gemini.c b/drivers/net/ethernet/cortina/gemini.c
index 2345d582cb59..87c537ae83e7 100644
--- a/drivers/net/ethernet/cortina/gemini.c
+++ b/drivers/net/ethernet/cortina/gemini.c
@@ -172,6 +172,7 @@ struct gemini_ethernet {
 	struct xarray	freeq_mappings;
 	unsigned int	num_freeq_pages;
 	unsigned long	*freeq_page_bitmap;
+	unsigned int	freeq_page_cursor;
 	spinlock_t	freeq_lock; /* Locks queue from reentrance */
 };
 
@@ -743,12 +744,20 @@ static int geth_freeq_alloc_slot(struct gemini_ethernet *geth)
 
 	lockdep_assert_held(&geth->freeq_lock);
 
-	slot = find_first_zero_bit(geth->freeq_page_bitmap,
-				   geth->num_freeq_pages);
-	if (slot == geth->num_freeq_pages)
-		return -ENOSPC;
+	slot = find_next_zero_bit(geth->freeq_page_bitmap,
+				  geth->num_freeq_pages,
+				  geth->freeq_page_cursor);
+	if (slot == geth->num_freeq_pages) {
+		slot = find_first_zero_bit(geth->freeq_page_bitmap,
+					   geth->freeq_page_cursor);
+		if (slot == geth->freeq_page_cursor)
+			return -ENOSPC;
+	}
 
 	__set_bit(slot, geth->freeq_page_bitmap);
+	geth->freeq_page_cursor = slot + 1;
+	if (geth->freeq_page_cursor == geth->num_freeq_pages)
+		geth->freeq_page_cursor = 0;
 
 	return slot;
 }
@@ -1110,6 +1119,7 @@ static int geth_setup_freeq(struct gemini_ethernet *geth)
 	if (!geth->freeq_page_bitmap)
 		goto err_freeq_pages;
 	geth->num_freeq_pages = page_slots;
+	geth->freeq_page_cursor = 0;
 
 	expected = len - (1 << fpp_order);
 	filled = geth_fill_freeq(geth);

-- 
2.55.0