net: xsk: introduce XDP_MAX_TX_SKB_BUDGET setsockopt

This patch provides a setsockopt method to let applications leverage to adjust how many descs to be handled at most in one send syscall. It mitigates the situation where the default value (32) that is too small leads to higher frequency of triggering send syscall. Considering the prosperity/complexity the applications have, there is no absolutely ideal suggestion fitting all cases. So keep 32 as its default value like before. The patch does the following things: - Add XDP_MAX_TX_SKB_BUDGET socket option. - Set max_tx_budget to 32 by default in the initialization phase as a per-socket granular control. - Set the range of max_tx_budget as [32, xs->tx->nentries]. The idea behind this comes out of real workloads in production. We use a user-level stack with xsk support to accelerate sending packets and minimize triggering syscalls. When the packets are aggregated, it's not hard to hit the upper bound (namely, 32). The moment user-space stack fetches the -EAGAIN error number passed from sendto(), it will loop to try again until all the expected descs from tx ring are sent out to the driver. Enlarging the XDP_MAX_TX_SKB_BUDGET value contributes to less frequency of sendto() and higher throughput/PPS. Here is what I did in production, along with some numbers as follows: For one application I saw lately, I suggested using 128 as max_tx_budget because I saw two limitations without changing any default configuration: 1) XDP_MAX_TX_SKB_BUDGET, 2) socket sndbuf which is 212992 decided by net.core.wmem_default. As to XDP_MAX_TX_SKB_BUDGET, the scenario behind this was I counted how many descs are transmitted to the driver at one time of sendto() based on [1] patch and then I calculated the possibility of hitting the upper bound. Finally I chose 128 as a suitable value because 1) it covers most of the cases, 2) a higher number would not bring evident results. After twisting the parameters, a stable improvement of around 4% for both PPS and throughput and less resources consumption were found to be observed by strace -c -p xxx: 1) %time was decreased by 7.8% 2) error counter was decreased from 18367 to 572 [1]: https://lore.kernel.org/all/20250619093641.70700-1-kerneljasonxing@gmail.com/

Jason Xing committed 1 year ago

Commit 45e359b1 parent c65d3425 files · +31 −2Browse files

Unified · Split

5 files

Documentation/networking/af_xdp.rst +9 −0
include/net/xdp_sock.h +1 −0
include/uapi/linux/if_xdp.h +1 −0
net/xdp/xsk.c +19 −2
tools/include/uapi/linux/if_xdp.h +1 −0

Documentation/networking/af_xdp.rst+9−0

@@ -438,6 +438,15 @@ is created by a privileged process and passed to a non-privileged one. · 437 unmodified lines
438	Once the option is set, kernel will refuse attempts to bind that socket	438	Once the option is set, kernel will refuse attempts to bind that socket
439	to a different interface. Updating the value requires CAP_NET_RAW.	439	to a different interface. Updating the value requires CAP_NET_RAW.
440		440
		441	XDP_MAX_TX_SKB_BUDGET setsockopt
		442	--------------------------------
		443
		444	This setsockopt sets the maximum number of descriptors that can be handled
		445	and passed to the driver at one send syscall. It is applied in the copy
		446	mode to allow application to tune the per-socket maximum iteration for
		447	better throughput and less frequency of send syscall.
		448	Allowed range is [32, xs->tx->nentries].
		449
441	XDP_STATISTICS getsockopt	450	XDP_STATISTICS getsockopt
442	-------------------------	451	-------------------------
443		452

include/net/xdp_sock.h+1−0

@@ -84,6 +84,7 @@ struct xdp_sock { · 83 unmodified lines
84	struct list_head map_list;	84	struct list_head map_list;
85	/* Protects map_list */	85	/* Protects map_list */
86	spinlock_t map_list_lock;	86	spinlock_t map_list_lock;
		87	u32 max_tx_budget;
87	/* Protects multiple processes in the control path */	88	/* Protects multiple processes in the control path */
88	struct mutex mutex;	89	struct mutex mutex;
89	struct xsk_queue fq_tmp; / Only as tmp storage before bind */	90	struct xsk_queue fq_tmp; / Only as tmp storage before bind */

include/uapi/linux/if_xdp.h+1−0

@@ -79,6 +79,7 @@ struct xdp_mmap_offsets { · 78 unmodified lines
79	#define XDP_UMEM_COMPLETION_RING 6	79	#define XDP_UMEM_COMPLETION_RING 6
80	#define XDP_STATISTICS 7	80	#define XDP_STATISTICS 7
81	#define XDP_OPTIONS 8	81	#define XDP_OPTIONS 8
		82	#define XDP_MAX_TX_SKB_BUDGET 9
82		83
83	struct xdp_umem_reg {	84	struct xdp_umem_reg {
84	__u64 addr; /* Start of packet data area */	85	__u64 addr; /* Start of packet data area */

net/xdp/xsk.c+19−2

 #include "xsk.h"
 #define TX_BATCH_SIZE 32
-#define MAX_PER_SOCKET_BUDGET (TX_BATCH_SIZE)
+#define MAX_PER_SOCKET_BUDGET 32
 void xsk_set_rx_need_wakeup(struct xsk_buff_pool *pool)
 {
 static int __xsk_generic_xmit(struct sock *sk)
 {
 	struct xdp_sock *xs = xdp_sk(sk);
-	u32 max_batch = TX_BATCH_SIZE;
 	bool sent_frame = false;
 	struct xdp_desc desc;
 	struct sk_buff *skb;
+	u32 max_batch;
 	int err = 0;
 	mutex_lock(&xs->mutex);
 	if (xs->queue_id >= xs->dev->real_num_tx_queues)
 		goto out;
+	max_batch = READ_ONCE(xs->max_tx_budget);
 	while (xskq_cons_peek_desc(xs->tx, &desc, xs->pool)) {
 		if (max_batch-- == 0) {
 			err = -EAGAIN;
 		mutex_unlock(&xs->mutex);
 		return err;
 	}
+	case XDP_MAX_TX_SKB_BUDGET:
+	{
+		unsigned int budget;
+		if (optlen != sizeof(budget))
+			return -EINVAL;
+		if (copy_from_sockptr(&budget, optval, sizeof(budget)))
+			return -EFAULT;
+		if (!xs->tx ||
+		    budget < TX_BATCH_SIZE || budget > xs->tx->nentries)
+			return -EACCES;
+		WRITE_ONCE(xs->max_tx_budget, budget);
+		return 0;
+	}
 	default:
 		break;
 	}
 	xs = xdp_sk(sk);
 	xs->state = XSK_READY;
+	xs->max_tx_budget = TX_BATCH_SIZE;
 	mutex_init(&xs->mutex);
 	INIT_LIST_HEAD(&xs->map_list);

tools/include/uapi/linux/if_xdp.h+1−0

@@ -79,6 +79,7 @@ struct xdp_mmap_offsets { · 78 unmodified lines
79	#define XDP_UMEM_COMPLETION_RING 6	79	#define XDP_UMEM_COMPLETION_RING 6
80	#define XDP_STATISTICS 7	80	#define XDP_STATISTICS 7
81	#define XDP_OPTIONS 8	81	#define XDP_OPTIONS 8
		82	#define XDP_MAX_TX_SKB_BUDGET 9
82		83
83	struct xdp_umem_reg {	84	struct xdp_umem_reg {
84	__u64 addr; /* Start of packet data area */	85	__u64 addr; /* Start of packet data area */

net: xsk: introduce XDP_MAX_TX_SKB_BUDGET setsockopt

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