rge: make rx_process_limit a sysctl tunable

The number of packets processed per interrupt was hardcoded to 16.
Add a per-interface sysctl dev.rge.%d.rx_process_limit tunable so
users can adjust this value at runtime.

Signed-off-by: Christos Longros <chris.longros@gmail.com>

Reviewed by:	ziaee, adrian
Differential Revision:	https://reviews.freebsd.org/D56014

share/man/man4/rge.4

@@ -147,6 +147,11 @@ tunables:
 .Bl -tag -width "xxxxxx"
 .It Va dev.rge.%d.debug
 Configure runtime debug output.  This is a 32 bit bitmask.
+.It Va dev.rge.%d.rx_process_limit
+Maximum number of RX packets to process per interrupt.
+The default value is 16.
+Increasing this value may improve throughput on high-speed links at the
+cost of increased interrupt latency.
 .El
 .Sh DIAGNOSTICS
 .Bl -diag

sys/dev/rge/if_rge.c

@@ -2099,9 +2099,10 @@ rge_rxeof(struct rge_queues *q, struct mbufq *mq)
 	uint32_t rxstat, extsts;
 	int i, mlen, rx = 0;
 	int cons, prod;
-	int maxpkt = 16; /* XXX TODO: make this a tunable */
+	int maxpkt;
 	bool check_hwcsum;
 
+	maxpkt = sc->sc_rx_process_limit;
 	check_hwcsum = ((if_getcapenable(sc->sc_ifp) & IFCAP_RXCSUM) != 0);
 
 	RGE_ASSERT_LOCKED(sc);

sys/dev/rge/if_rge_sysctl.c

@@ -232,6 +232,11 @@ rge_sysctl_attach(struct rge_softc *sc)
 	    "debug", CTLFLAG_RW, &sc->sc_debug, 0,
 	    "control debugging printfs");
 
+	sc->sc_rx_process_limit = 16;
+	SYSCTL_ADD_INT(ctx, SYSCTL_CHILDREN(tree), OID_AUTO,
+	    "rx_process_limit", CTLFLAG_RW, &sc->sc_rx_process_limit, 0,
+	    "max number of RX packets to process per interrupt");
+
 	/* Stats */
 	rge_sysctl_drv_stats_attach(sc);
 	rge_sysctl_mac_stats_attach(sc);

sys/dev/rge/if_rgevar.h

@@ -204,6 +204,8 @@ struct rge_softc {
 
 	uint32_t		sc_debug;
 
+	int			sc_rx_process_limit;
+
 	struct rge_drv_stats	sc_drv_stats;
 
 	struct rge_mac_stats	sc_mac_stats;