[PATCH] udp_flow: Save 8 bytes in struct udp_flow on 64-bit architectures - dev

8 Apr 2025

Shuffle the fields just added by commits a7775e9550fa ("udp: support
traceroute in direction tap-socket") and 9725e7988837 ("udp_flow:
Don't discard packets that arrive between bind() and connect()").

On x86_64, as reported by pahole(1), before:

struct udp_flow {
        struct flow_common         f;                    /*     0    76 */
        /* --- cacheline 1 boundary (64 bytes) was 12 bytes ago --- */
        _Bool                      closed:1;             /*    76: 0  1 */

        /* XXX 7 bits hole, try to pack */

        _Bool                      flush0;               /*    77     1 */
        _Bool                      flush1:1;             /*    78: 0  1 */

        /* XXX 7 bits hole, try to pack */
        /* XXX 1 byte hole, try to pack */

        time_t                     ts;                   /*    80     8 */
        int                        s[2];                 /*    88     8 */
        uint8_t                    ttl[2];               /*    96     2 */

        /* size: 104, cachelines: 2, members: 7 */
        /* sum members: 95, holes: 1, sum holes: 1 */
        /* sum bitfield members: 2 bits, bit holes: 2, sum bit holes: 14 bits */
        /* padding: 6 */
        /* last cacheline: 40 bytes */
};

and after:

struct udp_flow {
        struct flow_common         f;                    /*     0    76 */
        /* --- cacheline 1 boundary (64 bytes) was 12 bytes ago --- */
        uint8_t                    ttl[2];               /*    76     2 */
        _Bool                      closed:1;             /*    78: 0  1 */
        _Bool                      flush0:1;             /*    78: 1  1 */
        _Bool                      flush1:1;             /*    78: 2  1 */

        /* XXX 5 bits hole, try to pack */
        /* XXX 1 byte hole, try to pack */

        time_t                     ts;                   /*    80     8 */
        int                        s[2];                 /*    88     8 */

        /* size: 96, cachelines: 2, members: 7 */
        /* sum members: 94, holes: 1, sum holes: 1 */
        /* sum bitfield members: 3 bits, bit holes: 1, sum bit holes: 5 bits */
        /* last cacheline: 32 bytes */
};

It doesn't matter much because anyway the typical storage for struct
udp_flow is given by union flow:

union flow {
        struct flow_common         f;                  /*     0    76 */
        struct flow_free_cluster   free;               /*     0    84 */
        struct tcp_tap_conn        tcp;                /*     0   120 */
        struct tcp_splice_conn     tcp_splice;         /*     0   120 */
        struct icmp_ping_flow      ping;               /*     0    96 */
        struct udp_flow            udp;                /*     0    96 */
};

but it still improves data locality somewhat, so let me fix this up
now that commits are fresh.

Signed-off-by: Stefano Brivio &lt;sbrivio(a)redhat.com&gt;
---
 udp_flow.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/udp_flow.h b/udp_flow.h
index 90d3b29..e289122 100644
--- a/udp_flow.h
+++ b/udp_flow.h
@@ -10,22 +10,25 @@
 /**
  * struct udp_flow - Descriptor for a flow of UDP packets
  * @f:		Generic flow information
+ * @ttl:	TTL or hop_limit for both sides
  * @closed:	Flow is already closed
  * @flush0:	@s[0] may have datagrams queued for other flows
  * @flush1:	@s[1] may have datagrams queued for other flows
  * @ts:		Activity timestamp
  * @s:		Socket fd (or -1) for each side of the flow
- * @ttl:	TTL or hop_limit for both sides
  */
 struct udp_flow {
 	/* Must be first element */
 	struct flow_common f;
 
-	bool closed :1;
-	bool flush0, flush1 :1;
+	uint8_t ttl[SIDES];
+
+	bool	closed	:1,
+		flush0	:1,
+		flush1	:1;
+
 	time_t ts;
 	int s[SIDES];
-	uint8_t ttl[SIDES];
 };
 
 struct udp_flow *udp_at_sidx(flow_sidx_t sidx);
-- 
2.43.0


    