[Linux-kernel-mentees][PATCH 2/2] net: reorder members of receive_queue in virtio_net for optimization

[Anant Thazhemadam]

Analysis of the structure receive_queue using pahole gives the
following stats.
	/* size: 1280, cachelines: 20, members: 11 */
        /* sum members: 1220, holes: 1, sum holes: 60 */
        /* paddings: 2, sum paddings: 44 */
        /* forced alignments: 2, forced holes: 1, sum forced holes: 60 */

Reordering the order in which the members of receive_queue are declared
helps in packing byte holes in the middle of receive_queue, and also
allows more members to be fully stored in a cacheline (of size 64bytes)
without overstepping over cachelines unnecessarily.

Analysis using pahole post-reordering of members gives us the following
stats.
	/* size: 1280, cachelines: 20, members: 11 */
        /* padding: 60 */
        /* paddings: 2, sum paddings: 44 */
        /* forced alignments: 2 */

Signed-off-by: Anant Thazhemadam <anant.thazhemadam@xxxxxxxxx>
---
The complete analysis done by pahole can be found below.

Before the change:
	struct receive_queue {
		struct virtqueue *         vq;                   /*     0     8 */
		struct napi_struct napi __attribute__((__aligned__(8))); /*     8   392 */

		/* XXX last struct has 4 bytes of padding */

		/* --- cacheline 6 boundary (384 bytes) was 16 bytes ago --- */
		struct bpf_prog *          xdp_prog;             /*   400     8 */
		struct virtnet_rq_stats stats;                   /*   408    64 */
		/* --- cacheline 7 boundary (448 bytes) was 24 bytes ago --- */
		struct page *              pages;                /*   472     8 */
		struct ewma_pkt_len mrg_avg_pkt_len;             /*   480     8 */
		struct page_frag   alloc_frag;                   /*   488    16 */
		struct scatterlist sg[19];                       /*   504   608 */
		/* --- cacheline 17 boundary (1088 bytes) was 24 bytes ago --- */
		unsigned int               min_buf_len;          /*  1112     4 */
		char                       name[40];             /*  1116    40 */

		/* XXX 60 bytes hole, try to pack */

		/* --- cacheline 19 boundary (1216 bytes) --- */
		struct xdp_rxq_info xdp_rxq __attribute__((__aligned__(64))); /*  1216    64 */

		/* XXX last struct has 40 bytes of padding */

		/* size: 1280, cachelines: 20, members: 11 */
		/* sum members: 1220, holes: 1, sum holes: 60 */
		/* paddings: 2, sum paddings: 44 */
		/* forced alignments: 2, forced holes: 1, sum forced holes: 60 */
	} __attribute__((__aligned__(64)));

After the change:
	struct receive_queue {
		struct virtqueue *         vq;                   /*     0     8 */
		struct napi_struct napi __attribute__((__aligned__(8))); /*     8   392 */

		/* XXX last struct has 4 bytes of padding */

		/* --- cacheline 6 boundary (384 bytes) was 16 bytes ago --- */
		char                       name[40];             /*   400    40 */
		struct bpf_prog *          xdp_prog;             /*   440     8 */
		/* --- cacheline 7 boundary (448 bytes) --- */
		struct virtnet_rq_stats stats;                   /*   448    64 */
		/* --- cacheline 8 boundary (512 bytes) --- */
		struct scatterlist sg[19];                       /*   512   608 */
		/* --- cacheline 17 boundary (1088 bytes) was 32 bytes ago --- */
		struct page_frag   alloc_frag;                   /*  1120    16 */
		struct page *              pages;                /*  1136     8 */
		struct ewma_pkt_len mrg_avg_pkt_len;             /*  1144     8 */
		/* --- cacheline 18 boundary (1152 bytes) --- */
		struct xdp_rxq_info xdp_rxq __attribute__((__aligned__(64))); /*  1152    64 */

		/* XXX last struct has 40 bytes of padding */

		/* --- cacheline 19 boundary (1216 bytes) --- */
		unsigned int               min_buf_len;          /*  1216     4 */

		/* size: 1280, cachelines: 20, members: 11 */
		/* padding: 60 */
		/* paddings: 2, sum paddings: 44 */
		/* forced alignments: 2 */
	} __attribute__((__aligned__(64)));

It can be observed that the holes have been eliminated. 
Also, more members of virtnet_info are accomodated within a cacheline (instead of 
unnecessarily crossing over the cacheline boundary).
There is a padding of 60 performed at the end since the min_buf_len is only of 
size 4, and xdp_rxq is of size 64. If declared anywhere else other than at the 
end, a 60 bytes hole would open up again.

 drivers/net/virtio_net.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index f7bd85001cf0..b52db0b4879a 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -137,29 +137,29 @@ struct receive_queue {
 
 	struct napi_struct napi;
 
+	/* Name of this receive queue: input.$index */
+	char name[40];
+
 	struct bpf_prog __rcu *xdp_prog;
 
 	struct virtnet_rq_stats stats;
 
+	/* RX: fragments + linear part + virtio header */
+	struct scatterlist sg[MAX_SKB_FRAGS + 2];
+
+	/* Page frag for packet buffer allocation. */
+	struct page_frag alloc_frag;
+
 	/* Chain pages by the private ptr. */
 	struct page *pages;
 
 	/* Average packet length for mergeable receive buffers. */
 	struct ewma_pkt_len mrg_avg_pkt_len;
 
-	/* Page frag for packet buffer allocation. */
-	struct page_frag alloc_frag;
-
-	/* RX: fragments + linear part + virtio header */
-	struct scatterlist sg[MAX_SKB_FRAGS + 2];
+	struct xdp_rxq_info xdp_rxq;
 
 	/* Min single buffer size for mergeable buffers case. */
 	unsigned int min_buf_len;
-
-	/* Name of this receive queue: input.$index */
-	char name[40];
-
-	struct xdp_rxq_info xdp_rxq;
 };
 
 /* Control VQ buffers: protected by the rtnl lock */
-- 
2.25.1