Work around lack of TX IRQs in iflib for netmap

When poll() is called via netmap, txsync is initially called,
and if there are no available buffers to reclaim, it waits for the driver
to notify of new buffers. Since the TX IRQ is generally not used in iflib
drivers, this ends up causing a timeout.

Work around this by having the reclaim DELAY(1) if it's initially unable
to reclaim anything, then schedule the tx task, which will spin by
continuously rescheduling itself until some buffers are reclaimed. In
general, the delay is enough to allow some buffers to be reclaimed, so
spinning is minimized.

Reported by:	Johannes Lundberg <johalun0@gmail.com>
Reviewed by:	sbruno
Sponsored by:	Limelight Networks
Differential Revision:	https://reviews.freebsd.org/D15455

1 files changed, 33 insertions, 7 deletions

diff --git a/sys/net/iflib.c b/sys/net/iflib.c
index dfd77a75e68f..2dc2690699e9 100644
--- a/sys/net/iflib.c
+++ b/sys/net/iflib.c
@@ -960,10 +960,10 @@ iflib_netmap_txsync(struct netmap_kring *kring, int flags)
 	 */
 
 	nm_i = netmap_idx_n2k(kring, kring->nr_hwcur);
-	pkt_info_zero(&pi);
-	pi.ipi_segs = txq->ift_segs;
-	pi.ipi_qsidx = kring->ring_id;
 	if (nm_i != head) {	/* we have new packets to send */
+		pkt_info_zero(&pi);
+		pi.ipi_segs = txq->ift_segs;
+		pi.ipi_qsidx = kring->ring_id;
 		nic_i = netmap_idx_k2n(kring, nm_i);
 
 		__builtin_prefetch(&ring->slot[nm_i]);
@@ -1025,11 +1025,24 @@ iflib_netmap_txsync(struct netmap_kring *kring, int flags)
 
 	/*
 	 * Second part: reclaim buffers for completed transmissions.
+	 *
+	 * If there are unclaimed buffers, attempt to reclaim them.
+	 * If none are reclaimed, and TX IRQs are not in use, do an initial
+	 * minimal delay, then trigger the tx handler which will spin in the
+	 * group task queue.
 	 */
-	if (iflib_tx_credits_update(ctx, txq)) {
-		/* some tx completed, increment avail */
-		nic_i = txq->ift_cidx_processed;
-		kring->nr_hwtail = nm_prev(netmap_idx_n2k(kring, nic_i), lim);
+	if (kring->nr_hwtail != nm_prev(head, lim)) {
+		if (iflib_tx_credits_update(ctx, txq)) {
+			/* some tx completed, increment avail */
+			nic_i = txq->ift_cidx_processed;
+			kring->nr_hwtail = nm_prev(netmap_idx_n2k(kring, nic_i), lim);
+		}
+		else {
+			if (!(ctx->ifc_flags & IFC_NETMAP_TX_IRQ)) {
+				DELAY(1);
+				GROUPTASK_ENQUEUE(&ctx->ifc_txqs[txq->ift_id].ift_task);
+			}
+		}
 	}
 	return (0);
 }
@@ -3702,8 +3715,20 @@ _task_fn_tx(void *context)
 	if (!(if_getdrvflags(ctx->ifc_ifp) & IFF_DRV_RUNNING))
 		return;
 	if (if_getcapenable(ifp) & IFCAP_NETMAP) {
+		/*
+		 * If there are no available credits, and TX IRQs are not in use,
+		 * re-schedule the task immediately.
+		 */
 		if (ctx->isc_txd_credits_update(ctx->ifc_softc, txq->ift_id, false))
 			netmap_tx_irq(ifp, txq->ift_id);
+		else {
+			if (!(ctx->ifc_flags & IFC_NETMAP_TX_IRQ)) {
+				struct netmap_kring *kring = NA(ctx->ifc_ifp)->tx_rings[txq->ift_id];
+
+				if (kring->nr_hwtail != nm_prev(kring->rhead, kring->nkr_num_slots - 1))
+					GROUPTASK_ENQUEUE(&txq->ift_task);
+			}
+		}
 		IFDI_TX_QUEUE_INTR_ENABLE(ctx, txq->ift_id);
 		return;
 	}
@@ -5548,6 +5573,7 @@ iflib_irq_alloc_generic(if_ctx_t ctx, if_irq_t irq, int rid,
 		fn = _task_fn_tx;
 		intr_fast = iflib_fast_intr;
 		GROUPTASK_INIT(gtask, 0, fn, q);
+		ctx->ifc_flags |= IFC_NETMAP_TX_IRQ;
 		break;
 	case IFLIB_INTR_RX:
 		q = &ctx->ifc_rxqs[qid];