Remove LLE read lock from IPv4 fast path.

LLE structure is mostly unchanged during its lifecycle.
To be more specific, there are 2 things relevant for fast path
  lookup code:
1) link-level address change. Since r286722, these updates are performed
  under AFDATA WLOCK.
2) Some sort of feedback indicating that this particular entry is used so
  we re-send arp request to perform reachability verification instead of
  expiring entry. The only signal that is needed from fast path is something
  like binary yes/no.

The latter is solved by the following changes:
1) introduce special r_skip_req field which is read lockless by fast path,
  but updated under (new) req_mutex mutex. If this field is non-zero, then
  fast path will acquire lock and set it back to 0.
2) introduce simple state machine: incomplete->reachable<->verify->deleted.
  Before that we implicitely had incomplete->reachable->deleted state machine,
  with V_arpt_keep between "reachable" and "deleted". Verification was performed
  in runtime 5 seconds before V_arpt_keep expire.
  This is changed to "change state to verify 5 seconds before V_arpt_keep,
  set r_skip_req to non-zero value and check it every second". If the value
  is zero - then send arp verification probe.
These changes do not introduce any signifficant control plane overhead:
  typically lle callout timer would fire 1 time more each V_arpt_keep (1200s)
  for used lles and up to arp_maxtries (5) for dead lles.

As a result, all packets towards "reachable" lle are handled by fast path without
acquiring lle read lock.

Additional "req_mutex" is needed because callout / arpresolve_slow() or eventhandler
  might keep LLE lock for signifficant amount of time, which might not be feasible
  for fast path locking (e.g. having rmlock as ether AFDATA or lltable own lock).

Differential Revision:	https://reviews.freebsd.org/D3688

1 files changed, 90 insertions, 50 deletions

diff --git a/sys/netinet/if_ether.c b/sys/netinet/if_ether.c
index dfba0b2..1dfa244 100644
--- a/sys/netinet/if_ether.c
+++ b/sys/netinet/if_ether.c
@@ -78,6 +78,14 @@ static struct timeval arp_lastlog;
 static int arp_curpps;
 static int arp_maxpps = 1;
 
+/* Simple ARP state machine */
+enum arp_llinfo_state {
+	ARP_LLINFO_INCOMPLETE = 0, /* No LLE data */
+	ARP_LLINFO_REACHABLE,	/* LLE is valid */
+	ARP_LLINFO_VERIFY,	/* LLE is valid, need refresh */
+	ARP_LLINFO_DELETED,	/* LLE is deleted */
+};
+
 SYSCTL_DECL(_net_link_ether);
 static SYSCTL_NODE(_net_link_ether, PF_INET, inet, CTLFLAG_RW, 0, "");
 static SYSCTL_NODE(_net_link_ether, PF_ARP, arp, CTLFLAG_RW, 0, "");
@@ -89,6 +97,7 @@ static VNET_DEFINE(int, arp_maxtries) = 5;
 static VNET_DEFINE(int, arp_proxyall) = 0;
 static VNET_DEFINE(int, arpt_down) = 20;	/* keep incomplete entries for
 						 * 20 seconds */
+static VNET_DEFINE(int, arpt_rexmit) = 1;	/* retransmit arp entries, sec*/
 VNET_PCPUSTAT_DEFINE(struct arpstat, arpstat);  /* ARP statistics, see if_arp.h */
 VNET_PCPUSTAT_SYSINIT(arpstat);
 
@@ -100,6 +109,7 @@ static VNET_DEFINE(int, arp_maxhold) = 1;
 
 #define	V_arpt_keep		VNET(arpt_keep)
 #define	V_arpt_down		VNET(arpt_down)
+#define	V_arpt_rexmit		VNET(arpt_rexmit)
 #define	V_arp_maxtries		VNET(arp_maxtries)
 #define	V_arp_proxyall		VNET(arp_proxyall)
 #define	V_arp_maxhold		VNET(arp_maxhold)
@@ -161,6 +171,7 @@ arptimer(void *arg)
 {
 	struct llentry *lle = (struct llentry *)arg;
 	struct ifnet *ifp;
+	int r_skip_req;
 
 	if (lle->la_flags & LLE_STATIC) {
 		return;
@@ -188,6 +199,55 @@ arptimer(void *arg)
 	ifp = lle->lle_tbl->llt_ifp;
 	CURVNET_SET(ifp->if_vnet);
 
+	switch (lle->ln_state) {
+	case ARP_LLINFO_REACHABLE:
+
+		/*
+		 * Expiration time is approaching.
+		 * Let's try to refresh entry if it is still
+		 * in use.
+		 *
+		 * Set r_skip_req to get feedback from
+		 * fast path. Change state and re-schedule
+		 * ourselves.
+		 */
+		LLE_REQ_LOCK(lle);
+		lle->r_skip_req = 1;
+		LLE_REQ_UNLOCK(lle);
+		lle->ln_state = ARP_LLINFO_VERIFY;
+		callout_schedule(&lle->lle_timer, hz * V_arpt_rexmit);
+		LLE_WUNLOCK(lle);
+		CURVNET_RESTORE();
+		return;
+	case ARP_LLINFO_VERIFY:
+		LLE_REQ_LOCK(lle);
+		r_skip_req = lle->r_skip_req;
+		LLE_REQ_UNLOCK(lle);
+
+		if (r_skip_req == 0 && lle->la_preempt > 0) {
+			/* Entry was used, issue refresh request */
+			struct in_addr dst;
+			dst = lle->r_l3addr.addr4;
+			lle->la_preempt--;
+			callout_schedule(&lle->lle_timer, hz * V_arpt_rexmit);
+			LLE_WUNLOCK(lle);
+			arprequest(ifp, NULL, &dst, NULL);
+			CURVNET_RESTORE();
+			return;
+		}
+		/* Nothing happened. Reschedule if not too late */
+		if (lle->la_expire > time_uptime) {
+			callout_schedule(&lle->lle_timer, hz * V_arpt_rexmit);
+			LLE_WUNLOCK(lle);
+			CURVNET_RESTORE();
+			return;
+		}
+		break;
+	case ARP_LLINFO_INCOMPLETE:
+	case ARP_LLINFO_DELETED:
+		break;
+	}
+
 	if ((lle->la_flags & LLE_DELETED) == 0) {
 		int evt;
 
@@ -353,26 +413,16 @@ arpresolve_full(struct ifnet *ifp, int is_gw, int create, struct mbuf *m,
 	if ((la->la_flags & LLE_VALID) &&
 	    ((la->la_flags & LLE_STATIC) || la->la_expire > time_uptime)) {
 		bcopy(&la->ll_addr, desten, ifp->if_addrlen);
-		renew = 0;
-		/*
-		 * If entry has an expiry time and it is approaching,
-		 * see if we need to send an ARP request within this
-		 * arpt_down interval.
-		 */
-		if (!(la->la_flags & LLE_STATIC) &&
-		    time_uptime + la->la_preempt > la->la_expire) {
-			renew = 1;
-			la->la_preempt--;
-		}
 
+		/* Check if we have feedback request from arptimer() */
+		if (la->r_skip_req != 0) {
+			LLE_REQ_LOCK(la);
+			la->r_skip_req = 0; /* Notify that entry was used */
+			LLE_REQ_UNLOCK(la);
+		}
 		if (pflags != NULL)
-			*pflags = la->la_flags;
-
+			*pflags = la->la_flags & (LLE_VALID|LLE_IFADDR);
 		LLE_WUNLOCK(la);
-
-		if (renew == 1)
-			arprequest(ifp, NULL, &SIN(dst)->sin_addr, NULL);
-
 		return (0);
 	}
 
@@ -440,7 +490,7 @@ arpresolve_full(struct ifnet *ifp, int is_gw, int create, struct mbuf *m,
  *    m is the mbuf. May be NULL if we don't have a packet.
  *    dst is the next hop,
  *    desten is the storage to put LL address.
- *    flags returns lle entry flags.
+ *    flags returns subset of lle flags: LLE_VALID | LLE_IFADDR
  *
  * On success, desten and flags are filled in and the function returns 0;
  * If the packet must be held pending resolution, we return EWOULDBLOCK
@@ -452,7 +502,6 @@ arpresolve(struct ifnet *ifp, int is_gw, struct mbuf *m,
 	const struct sockaddr *dst, u_char *desten, uint32_t *pflags)
 {
 	struct llentry *la = 0;
-	int renew;
 
 	if (pflags != NULL)
 		*pflags = 0;
@@ -472,40 +521,24 @@ arpresolve(struct ifnet *ifp, int is_gw, struct mbuf *m,
 	}
 
 	IF_AFDATA_RLOCK(ifp);
-	la = lla_lookup(LLTABLE(ifp), 0, dst);
-	IF_AFDATA_RUNLOCK(ifp);
-
-	if (la == NULL)
-		return (arpresolve_full(ifp, is_gw, 1, m, dst, desten, pflags));
-
-	if ((la->la_flags & LLE_VALID) &&
-	    ((la->la_flags & LLE_STATIC) || la->la_expire > time_uptime)) {
+	la = lla_lookup(LLTABLE(ifp), LLE_UNLOCKED, dst);
+	if (la != NULL && (la->r_flags & RLLE_VALID) != 0) {
+		/* Entry found, let's copy lle info */
 		bcopy(&la->ll_addr, desten, ifp->if_addrlen);
-		renew = 0;
-		/*
-		 * If entry has an expiry time and it is approaching,
-		 * see if we need to send an ARP request within this
-		 * arpt_down interval.
-		 */
-		if (!(la->la_flags & LLE_STATIC) &&
-		    time_uptime + la->la_preempt > la->la_expire) {
-			renew = 1;
-			la->la_preempt--;
-		}
-
 		if (pflags != NULL)
-			*pflags = la->la_flags;
-
-		LLE_RUNLOCK(la);
-
-		if (renew == 1)
-			arprequest(ifp, NULL, &SIN(dst)->sin_addr, NULL);
-
+			*pflags = LLE_VALID | (la->r_flags & RLLE_IFADDR);
+		/* Check if we have feedback request from arptimer() */
+		if (la->r_skip_req != 0) {
+			LLE_REQ_LOCK(la);
+			la->r_skip_req = 0; /* Notify that entry was used */
+			LLE_REQ_UNLOCK(la);
+		}
+		IF_AFDATA_RUNLOCK(ifp);
 		return (0);
 	}
-	LLE_RUNLOCK(la);
+	IF_AFDATA_RUNLOCK(ifp);
 
-	return (arpresolve_full(ifp, is_gw, 0, m, dst, desten, pflags));
+	return (arpresolve_full(ifp, is_gw, 1, m, dst, desten, pflags));
 }
 
 /*
@@ -1042,6 +1075,9 @@ arp_check_update_lle(struct arphdr *ah, struct in_addr isaddr, struct ifnet *ifp
 
 		IF_AFDATA_WUNLOCK(ifp);
 		LLE_REMREF(la);
+
+		/* Clear fast path feedback request if set */
+		la->r_skip_req = 0;
 	}
 
 	arp_mark_lle_reachable(la);
@@ -1073,17 +1109,21 @@ arp_check_update_lle(struct arphdr *ah, struct in_addr isaddr, struct ifnet *ifp
 static void
 arp_mark_lle_reachable(struct llentry *la)
 {
-	int canceled;
+	int canceled, wtime;
 
 	LLE_WLOCK_ASSERT(la);
 
+	la->ln_state = ARP_LLINFO_REACHABLE;
 	EVENTHANDLER_INVOKE(lle_event, la, LLENTRY_RESOLVED);
 
 	if (!(la->la_flags & LLE_STATIC)) {
 		LLE_ADDREF(la);
 		la->la_expire = time_uptime + V_arpt_keep;
+		wtime = V_arpt_keep - V_arp_maxtries * V_arpt_rexmit;
+		if (wtime < 0)
+			wtime = V_arpt_keep;
 		canceled = callout_reset(&la->lle_timer,
-		    hz * V_arpt_keep, arptimer, la);
+		    hz * wtime, arptimer, la);
 		if (canceled)
 			LLE_REMREF(la);
 	}