xref: /freebsd/sys/netinet/tcp_hostcache.c (revision 09000cc133d82fc614d9f61d064e3acf7fa8b875)

/*-
 * SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 2002 Andre Oppermann, Internet Business Solutions AG
 * Copyright (c) 2021 Gleb Smirnoff <glebius@FreeBSD.org>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote
 *    products derived from this software without specific prior written
 *    permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * The tcp_hostcache moves the tcp-specific cached metrics from the routing
 * table to a dedicated structure indexed by the remote IP address.  It keeps
 * information on the measured TCP parameters of past TCP sessions to allow
 * better initial start values to be used with later connections to/from the
 * same source.  Depending on the network parameters (delay, max MTU,
 * congestion window) between local and remote sites, this can lead to
 * significant speed-ups for new TCP connections after the first one.
 *
 * Due to the tcp_hostcache, all TCP-specific metrics information in the
 * routing table have been removed.  The inpcb no longer keeps a pointer to
 * the routing entry, and protocol-initiated route cloning has been removed
 * as well.  With these changes, the routing table has gone back to being
 * more lightwight and only carries information related to packet forwarding.
 *
 * tcp_hostcache is designed for multiple concurrent access in SMP
 * environments and high contention.  It is a straight hash.  Each bucket row
 * is protected by its own lock for modification.  Readers are protected by
 * SMR.  This puts certain restrictions on writers, e.g. a writer shall only
 * insert a fully populated entry into a row.  Writer can't reuse least used
 * entry if a hash is full.  Value updates for an entry shall be atomic.
 *
 * TCP stack(s) communication with tcp_hostcache() is done via KBI functions
 * tcp_hc_*() and the hc_metrics_lite structure.
 *
 * Since tcp_hostcache is only caching information, there are no fatal
 * consequences if we either can't allocate a new entry or have to drop
 * an existing entry, or return somewhat stale information.
 */

/*
 * Many thanks to jlemon for basic structure of tcp_syncache which is being
 * followed here.
 */

#include <sys/cdefs.h>
#include "opt_inet6.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/hash.h>
#include <sys/jail.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/malloc.h>
#include <sys/proc.h>
#include <sys/sbuf.h>
#include <sys/smr.h>
#include <sys/socket.h>
#include <sys/sysctl.h>

#include <net/vnet.h>

#include <netinet/in.h>
#include <netinet/in_pcb.h>
#include <netinet/tcp.h>
#include <netinet/tcp_var.h>

#include <vm/uma.h>

struct hc_head {
	CK_SLIST_HEAD(hc_qhead, hc_metrics) hch_bucket;
	u_int		hch_length;
	struct mtx	hch_mtx;
};

struct hc_metrics {
	/* housekeeping */
	CK_SLIST_ENTRY(hc_metrics) hc_q;
	struct		in_addr ip4;	/* IP address */
	struct		in6_addr ip6;	/* IP6 address */
	uint32_t	ip6_zoneid;	/* IPv6 scope zone id */
	/* endpoint specific values for tcp */
	uint32_t	hc_mtu;		/* MTU for this path */
	uint32_t	hc_ssthresh;	/* outbound gateway buffer limit */
	uint32_t	hc_rtt;		/* estimated round trip time */
	uint32_t	hc_rttvar;	/* estimated rtt variance */
	uint32_t	hc_cwnd;	/* congestion window */
	uint32_t	hc_sendpipe;	/* outbound delay-bandwidth product */
	uint32_t	hc_recvpipe;	/* inbound delay-bandwidth product */
	/* TCP hostcache internal data */
	int		hc_expire;	/* lifetime for object */
#ifdef	TCP_HC_COUNTERS
	u_long		hc_hits;	/* number of hits */
	u_long		hc_updates;	/* number of updates */
#endif
};

struct tcp_hostcache {
	struct hc_head	*hashbase;
	uma_zone_t	zone;
	smr_t		smr;
	u_int		hashsize;
	u_int		hashmask;
	u_int		hashsalt;
	u_int		bucket_limit;
	u_int		cache_count;
	u_int		cache_limit;
	int		expire;
	int		prune;
	int		purgeall;
};

/* Arbitrary values */
#define TCP_HOSTCACHE_HASHSIZE		512
#define TCP_HOSTCACHE_BUCKETLIMIT	30
#define TCP_HOSTCACHE_EXPIRE		60*60	/* one hour */
#define TCP_HOSTCACHE_PRUNE		5*60	/* every 5 minutes */

VNET_DEFINE_STATIC(struct tcp_hostcache, tcp_hostcache);
#define	V_tcp_hostcache		VNET(tcp_hostcache)

VNET_DEFINE_STATIC(struct callout, tcp_hc_callout);
#define	V_tcp_hc_callout	VNET(tcp_hc_callout)

static struct hc_metrics *tcp_hc_lookup(struct in_conninfo *);
static int sysctl_tcp_hc_list(SYSCTL_HANDLER_ARGS);
static int sysctl_tcp_hc_histo(SYSCTL_HANDLER_ARGS);
static int sysctl_tcp_hc_purgenow(SYSCTL_HANDLER_ARGS);
static void tcp_hc_purge_internal(int);
static void tcp_hc_purge(void *);

static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, hostcache,
    CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
    "TCP Host cache");

VNET_DEFINE(int, tcp_use_hostcache) = 1;
#define V_tcp_use_hostcache  VNET(tcp_use_hostcache)
SYSCTL_INT(_net_inet_tcp_hostcache, OID_AUTO, enable, CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(tcp_use_hostcache), 0,
    "Enable the TCP hostcache");

SYSCTL_UINT(_net_inet_tcp_hostcache, OID_AUTO, cachelimit, CTLFLAG_VNET | CTLFLAG_RDTUN,
    &VNET_NAME(tcp_hostcache.cache_limit), 0,
    "Overall entry limit for hostcache");

SYSCTL_UINT(_net_inet_tcp_hostcache, OID_AUTO, hashsize, CTLFLAG_VNET | CTLFLAG_RDTUN,
    &VNET_NAME(tcp_hostcache.hashsize), 0,
    "Size of TCP hostcache hashtable");

SYSCTL_UINT(_net_inet_tcp_hostcache, OID_AUTO, bucketlimit,
    CTLFLAG_VNET | CTLFLAG_RDTUN, &VNET_NAME(tcp_hostcache.bucket_limit), 0,
    "Per-bucket hash limit for hostcache");

SYSCTL_UINT(_net_inet_tcp_hostcache, OID_AUTO, count, CTLFLAG_VNET | CTLFLAG_RD,
    &VNET_NAME(tcp_hostcache.cache_count), 0,
    "Current number of entries in hostcache");

SYSCTL_INT(_net_inet_tcp_hostcache, OID_AUTO, expire, CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(tcp_hostcache.expire), 0,
    "Expire time of TCP hostcache entries");

SYSCTL_INT(_net_inet_tcp_hostcache, OID_AUTO, prune, CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(tcp_hostcache.prune), 0,
    "Time between purge runs");

SYSCTL_INT(_net_inet_tcp_hostcache, OID_AUTO, purge, CTLFLAG_VNET | CTLFLAG_RW,
    &VNET_NAME(tcp_hostcache.purgeall), 0,
    "Expire all entries on next purge run");

SYSCTL_PROC(_net_inet_tcp_hostcache, OID_AUTO, list,
    CTLTYPE_STRING | CTLFLAG_RD | CTLFLAG_SKIP | CTLFLAG_MPSAFE,
    0, 0, sysctl_tcp_hc_list, "A",
    "List of all hostcache entries");

SYSCTL_PROC(_net_inet_tcp_hostcache, OID_AUTO, histo,
    CTLTYPE_STRING | CTLFLAG_RD | CTLFLAG_SKIP | CTLFLAG_MPSAFE,
    0, 0, sysctl_tcp_hc_histo, "A",
    "Print a histogram of hostcache hashbucket utilization");

SYSCTL_PROC(_net_inet_tcp_hostcache, OID_AUTO, purgenow,
    CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_MPSAFE,
    NULL, 0, sysctl_tcp_hc_purgenow, "I",
    "Immediately purge all entries");

static MALLOC_DEFINE(M_HOSTCACHE, "hostcache", "TCP hostcache");

/* Use jenkins_hash32(), as in other parts of the tcp stack */
#define	HOSTCACHE_HASH(inc)						\
	((inc)->inc_flags & INC_ISIPV6) ?				\
		(jenkins_hash32((inc)->inc6_faddr.s6_addr32, 4,		\
		V_tcp_hostcache.hashsalt) & V_tcp_hostcache.hashmask)	\
	:								\
		(jenkins_hash32(&(inc)->inc_faddr.s_addr, 1,		\
		V_tcp_hostcache.hashsalt) & V_tcp_hostcache.hashmask)

#define THC_LOCK(h)		mtx_lock(&(h)->hch_mtx)
#define THC_UNLOCK(h)		mtx_unlock(&(h)->hch_mtx)

void
tcp_hc_init(void)
{
	u_int cache_limit;
	int i;

	/*
	 * Initialize hostcache structures.
	 */
	atomic_store_int(&V_tcp_hostcache.cache_count, 0);
	V_tcp_hostcache.hashsize = TCP_HOSTCACHE_HASHSIZE;
	V_tcp_hostcache.bucket_limit = TCP_HOSTCACHE_BUCKETLIMIT;
	V_tcp_hostcache.expire = TCP_HOSTCACHE_EXPIRE;
	V_tcp_hostcache.prune = TCP_HOSTCACHE_PRUNE;
	V_tcp_hostcache.hashsalt = arc4random();

	TUNABLE_INT_FETCH("net.inet.tcp.hostcache.hashsize",
	    &V_tcp_hostcache.hashsize);
	if (!powerof2(V_tcp_hostcache.hashsize)) {
		printf("WARNING: hostcache hash size is not a power of 2.\n");
		V_tcp_hostcache.hashsize = TCP_HOSTCACHE_HASHSIZE; /* default */
	}
	V_tcp_hostcache.hashmask = V_tcp_hostcache.hashsize - 1;

	TUNABLE_INT_FETCH("net.inet.tcp.hostcache.bucketlimit",
	    &V_tcp_hostcache.bucket_limit);

	cache_limit = V_tcp_hostcache.hashsize * V_tcp_hostcache.bucket_limit;
	V_tcp_hostcache.cache_limit = cache_limit;
	TUNABLE_INT_FETCH("net.inet.tcp.hostcache.cachelimit",
	    &V_tcp_hostcache.cache_limit);
	if (V_tcp_hostcache.cache_limit > cache_limit)
		V_tcp_hostcache.cache_limit = cache_limit;

	/*
	 * Allocate the hash table.
	 */
	V_tcp_hostcache.hashbase = (struct hc_head *)
	    malloc(V_tcp_hostcache.hashsize * sizeof(struct hc_head),
		   M_HOSTCACHE, M_WAITOK | M_ZERO);

	/*
	 * Initialize the hash buckets.
	 */
	for (i = 0; i < V_tcp_hostcache.hashsize; i++) {
		CK_SLIST_INIT(&V_tcp_hostcache.hashbase[i].hch_bucket);
		V_tcp_hostcache.hashbase[i].hch_length = 0;
		mtx_init(&V_tcp_hostcache.hashbase[i].hch_mtx, "tcp_hc_entry",
			  NULL, MTX_DEF);
	}

	/*
	 * Allocate the hostcache entries.
	 */
	V_tcp_hostcache.zone =
	    uma_zcreate("hostcache", sizeof(struct hc_metrics),
	    NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, UMA_ZONE_SMR);
	uma_zone_set_max(V_tcp_hostcache.zone, V_tcp_hostcache.cache_limit);
	V_tcp_hostcache.smr = uma_zone_get_smr(V_tcp_hostcache.zone);

	/*
	 * Set up periodic cache cleanup.
	 */
	callout_init(&V_tcp_hc_callout, 1);
	callout_reset(&V_tcp_hc_callout, V_tcp_hostcache.prune * hz,
	    tcp_hc_purge, curvnet);
}

#ifdef VIMAGE
void
tcp_hc_destroy(void)
{
	int i;

	callout_drain(&V_tcp_hc_callout);

	/* Purge all hc entries. */
	tcp_hc_purge_internal(1);

	/* Free the uma zone and the allocated hash table. */
	uma_zdestroy(V_tcp_hostcache.zone);

	for (i = 0; i < V_tcp_hostcache.hashsize; i++)
		mtx_destroy(&V_tcp_hostcache.hashbase[i].hch_mtx);
	free(V_tcp_hostcache.hashbase, M_HOSTCACHE);
}
#endif

/*
 * Internal function: compare cache entry to a connection.
 */
static bool
tcp_hc_cmp(struct hc_metrics *hc_entry, struct in_conninfo *inc)
{

	if (inc->inc_flags & INC_ISIPV6) {
		/* XXX: check ip6_zoneid */
		if (memcmp(&inc->inc6_faddr, &hc_entry->ip6,
		    sizeof(inc->inc6_faddr)) == 0)
			return (true);
	} else {
		if (memcmp(&inc->inc_faddr, &hc_entry->ip4,
		    sizeof(inc->inc_faddr)) == 0)
			return (true);
	}

	return (false);
}

/*
 * Internal function: look up an entry in the hostcache for read.
 * On success returns in SMR section.
 */
static struct hc_metrics *
tcp_hc_lookup(struct in_conninfo *inc)
{
	struct hc_head *hc_head;
	struct hc_metrics *hc_entry;

	KASSERT(inc != NULL, ("%s: NULL in_conninfo", __func__));

	hc_head = &V_tcp_hostcache.hashbase[HOSTCACHE_HASH(inc)];

	/*
	 * Iterate through entries in bucket row looking for a match.
	 */
	smr_enter(V_tcp_hostcache.smr);
	CK_SLIST_FOREACH(hc_entry, &hc_head->hch_bucket, hc_q)
		if (tcp_hc_cmp(hc_entry, inc))
			break;

	if (hc_entry != NULL) {
		if (atomic_load_int(&hc_entry->hc_expire) !=
		    V_tcp_hostcache.expire)
			atomic_store_int(&hc_entry->hc_expire,
			    V_tcp_hostcache.expire);
#ifdef	TCP_HC_COUNTERS
		hc_entry->hc_hits++;
#endif
	} else
		smr_exit(V_tcp_hostcache.smr);

	return (hc_entry);
}

/*
 * External function: look up an entry in the hostcache and fill out the
 * supplied TCP metrics structure.  Fills in NULL when no entry was found or
 * a value is not set.
 */
void
tcp_hc_get(struct in_conninfo *inc, struct hc_metrics_lite *hc_metrics_lite)
{
	struct hc_metrics *hc_entry;

	if (!V_tcp_use_hostcache) {
		bzero(hc_metrics_lite, sizeof(*hc_metrics_lite));
		return;
	}

	/*
	 * Find the right bucket.
	 */
	hc_entry = tcp_hc_lookup(inc);

	/*
	 * If we don't have an existing object.
	 */
	if (hc_entry == NULL) {
		bzero(hc_metrics_lite, sizeof(*hc_metrics_lite));
		return;
	}

	hc_metrics_lite->hc_mtu = atomic_load_32(&hc_entry->hc_mtu);
	hc_metrics_lite->hc_ssthresh = atomic_load_32(&hc_entry->hc_ssthresh);
	hc_metrics_lite->hc_rtt = atomic_load_32(&hc_entry->hc_rtt);
	hc_metrics_lite->hc_rttvar = atomic_load_32(&hc_entry->hc_rttvar);
	hc_metrics_lite->hc_cwnd = atomic_load_32(&hc_entry->hc_cwnd);
	hc_metrics_lite->hc_sendpipe = atomic_load_32(&hc_entry->hc_sendpipe);
	hc_metrics_lite->hc_recvpipe = atomic_load_32(&hc_entry->hc_recvpipe);

	smr_exit(V_tcp_hostcache.smr);
}

/*
 * External function: look up an entry in the hostcache and return the
 * discovered path MTU.  Returns 0 if no entry is found or value is not
 * set.
 */
uint32_t
tcp_hc_getmtu(struct in_conninfo *inc)
{
	struct hc_metrics *hc_entry;
	uint32_t mtu;

	if (!V_tcp_use_hostcache)
		return (0);

	hc_entry = tcp_hc_lookup(inc);
	if (hc_entry == NULL) {
		return (0);
	}

	mtu = atomic_load_32(&hc_entry->hc_mtu);
	smr_exit(V_tcp_hostcache.smr);

	return (mtu);
}

/*
 * External function: update the MTU value of an entry in the hostcache.
 * Creates a new entry if none was found.
 */
void
tcp_hc_updatemtu(struct in_conninfo *inc, uint32_t mtu)
{
	struct hc_metrics_lite hcml = { .hc_mtu = mtu };

	return (tcp_hc_update(inc, &hcml));
}

/*
 * External function: update the TCP metrics of an entry in the hostcache.
 * Creates a new entry if none was found.
 */
void
tcp_hc_update(struct in_conninfo *inc, struct hc_metrics_lite *hcml)
{
	struct hc_head *hc_head;
	struct hc_metrics *hc_entry, *hc_prev;
	uint32_t v;
	bool new;

	if (!V_tcp_use_hostcache)
		return;

	hc_head = &V_tcp_hostcache.hashbase[HOSTCACHE_HASH(inc)];
	hc_prev = NULL;

	THC_LOCK(hc_head);
	CK_SLIST_FOREACH(hc_entry, &hc_head->hch_bucket, hc_q) {
		if (tcp_hc_cmp(hc_entry, inc))
			break;
		if (CK_SLIST_NEXT(hc_entry, hc_q) != NULL)
			hc_prev = hc_entry;
	}

	if (hc_entry != NULL) {
		if (atomic_load_int(&hc_entry->hc_expire) !=
		    V_tcp_hostcache.expire)
			atomic_store_int(&hc_entry->hc_expire,
			    V_tcp_hostcache.expire);
#ifdef	TCP_HC_COUNTERS
		hc_entry->hc_updates++;
#endif
		new = false;
	} else {
		/*
		 * Try to allocate a new entry.  If the bucket limit is
		 * reached, delete the least-used element, located at the end
		 * of the CK_SLIST.  During lookup we saved the pointer to
		 * the second to last element, in case if list has at least 2
		 * elements.  This will allow to delete last element without
		 * extra traversal.
		 *
		 * Give up if the row is empty.
		 */
		if (hc_head->hch_length >= V_tcp_hostcache.bucket_limit ||
		    atomic_load_int(&V_tcp_hostcache.cache_count) >=
		    V_tcp_hostcache.cache_limit) {
			if (hc_prev != NULL) {
				hc_entry = CK_SLIST_NEXT(hc_prev, hc_q);
				KASSERT(CK_SLIST_NEXT(hc_entry, hc_q) == NULL,
				    ("%s: %p is not one to last",
				    __func__, hc_prev));
				CK_SLIST_REMOVE_AFTER(hc_prev, hc_q);
			} else if ((hc_entry =
			    CK_SLIST_FIRST(&hc_head->hch_bucket)) != NULL) {
				KASSERT(CK_SLIST_NEXT(hc_entry, hc_q) == NULL,
				    ("%s: %p is not the only element",
				    __func__, hc_entry));
				CK_SLIST_REMOVE_HEAD(&hc_head->hch_bucket,
				    hc_q);
			} else {
				THC_UNLOCK(hc_head);
				return;
			}
			KASSERT(hc_head->hch_length > 0 &&
			    hc_head->hch_length <= V_tcp_hostcache.bucket_limit,
			    ("tcp_hostcache: bucket length violated at %p",
			    hc_head));
			hc_head->hch_length--;
			atomic_subtract_int(&V_tcp_hostcache.cache_count, 1);
			TCPSTAT_INC(tcps_hc_bucketoverflow);
			uma_zfree_smr(V_tcp_hostcache.zone, hc_entry);
		}

		/*
		 * Allocate a new entry, or balk if not possible.
		 */
		hc_entry = uma_zalloc_smr(V_tcp_hostcache.zone, M_NOWAIT);
		if (hc_entry == NULL) {
			THC_UNLOCK(hc_head);
			return;
		}

		/*
		 * Initialize basic information of hostcache entry.
		 */
		bzero(hc_entry, sizeof(*hc_entry));
		if (inc->inc_flags & INC_ISIPV6) {
			hc_entry->ip6 = inc->inc6_faddr;
			hc_entry->ip6_zoneid = inc->inc6_zoneid;
		} else
			hc_entry->ip4 = inc->inc_faddr;
		hc_entry->hc_expire = V_tcp_hostcache.expire;
		new = true;
	}

	/*
	 * Fill in data.  Use atomics, since an existing entry is
	 * accessible by readers in SMR section.
	 */
	if (hcml->hc_mtu != 0) {
		atomic_store_32(&hc_entry->hc_mtu, hcml->hc_mtu);
	}
	if (hcml->hc_rtt != 0) {
		if (hc_entry->hc_rtt == 0)
			v = hcml->hc_rtt;
		else
			v = ((uint64_t)hc_entry->hc_rtt +
			    (uint64_t)hcml->hc_rtt) / 2;
		atomic_store_32(&hc_entry->hc_rtt, v);
		TCPSTAT_INC(tcps_cachedrtt);
	}
	if (hcml->hc_rttvar != 0) {
	        if (hc_entry->hc_rttvar == 0)
			v = hcml->hc_rttvar;
		else
			v = ((uint64_t)hc_entry->hc_rttvar +
			    (uint64_t)hcml->hc_rttvar) / 2;
		atomic_store_32(&hc_entry->hc_rttvar, v);
		TCPSTAT_INC(tcps_cachedrttvar);
	}
	if (hcml->hc_ssthresh != 0) {
		if (hc_entry->hc_ssthresh == 0)
			v = hcml->hc_ssthresh;
		else
			v = (hc_entry->hc_ssthresh + hcml->hc_ssthresh) / 2;
		atomic_store_32(&hc_entry->hc_ssthresh, v);
		TCPSTAT_INC(tcps_cachedssthresh);
	}
	if (hcml->hc_cwnd != 0) {
		if (hc_entry->hc_cwnd == 0)
			v = hcml->hc_cwnd;
		else
			v = ((uint64_t)hc_entry->hc_cwnd +
			    (uint64_t)hcml->hc_cwnd) / 2;
		atomic_store_32(&hc_entry->hc_cwnd, v);
		/* TCPSTAT_INC(tcps_cachedcwnd); */
	}
	if (hcml->hc_sendpipe != 0) {
		if (hc_entry->hc_sendpipe == 0)
			v = hcml->hc_sendpipe;
		else
			v = ((uint64_t)hc_entry->hc_sendpipe +
			    (uint64_t)hcml->hc_sendpipe) /2;
		atomic_store_32(&hc_entry->hc_sendpipe, v);
		/* TCPSTAT_INC(tcps_cachedsendpipe); */
	}
	if (hcml->hc_recvpipe != 0) {
		if (hc_entry->hc_recvpipe == 0)
			v = hcml->hc_recvpipe;
		else
			v = ((uint64_t)hc_entry->hc_recvpipe +
			    (uint64_t)hcml->hc_recvpipe) /2;
		atomic_store_32(&hc_entry->hc_recvpipe, v);
		/* TCPSTAT_INC(tcps_cachedrecvpipe); */
	}

	/*
	 * Put it upfront.
	 */
	if (new) {
		CK_SLIST_INSERT_HEAD(&hc_head->hch_bucket, hc_entry, hc_q);
		hc_head->hch_length++;
		KASSERT(hc_head->hch_length <= V_tcp_hostcache.bucket_limit,
		    ("tcp_hostcache: bucket length too high at %p", hc_head));
		atomic_add_int(&V_tcp_hostcache.cache_count, 1);
		TCPSTAT_INC(tcps_hc_added);
	} else if (hc_entry != CK_SLIST_FIRST(&hc_head->hch_bucket)) {
		KASSERT(CK_SLIST_NEXT(hc_prev, hc_q) == hc_entry,
		    ("%s: %p next is not %p", __func__, hc_prev, hc_entry));
		CK_SLIST_REMOVE_AFTER(hc_prev, hc_q);
		CK_SLIST_INSERT_HEAD(&hc_head->hch_bucket, hc_entry, hc_q);
	}
	THC_UNLOCK(hc_head);
}

/*
 * Sysctl function: prints the list and values of all hostcache entries in
 * unsorted order.
 */
static int
sysctl_tcp_hc_list(SYSCTL_HANDLER_ARGS)
{
	const int linesize = 128;
	struct sbuf sb;
	int i, error, len;
	struct hc_metrics *hc_entry;
	char ip4buf[INET_ADDRSTRLEN];
#ifdef INET6
	char ip6buf[INET6_ADDRSTRLEN];
#endif

	if (jailed_without_vnet(curthread->td_ucred) != 0)
		return (EPERM);

	/* Optimize Buffer length query by sbin/sysctl */
	if (req->oldptr == NULL) {
		len = (atomic_load_int(&V_tcp_hostcache.cache_count) + 1) *
			linesize;
		return (SYSCTL_OUT(req, NULL, len));
	}

	error = sysctl_wire_old_buffer(req, 0);
	if (error != 0) {
		return(error);
	}

	/* Use a buffer sized for one full bucket */
	sbuf_new_for_sysctl(&sb, NULL, V_tcp_hostcache.bucket_limit *
		linesize, req);

	sbuf_printf(&sb,
		"\nIP address        MTU  SSTRESH      RTT   RTTVAR "
		"    CWND SENDPIPE RECVPIPE "
#ifdef	TCP_HC_COUNTERS
		"HITS  UPD  "
#endif
		"EXP\n");
	sbuf_drain(&sb);

#define msec(u) (((u) + 500) / 1000)
	for (i = 0; i < V_tcp_hostcache.hashsize; i++) {
		THC_LOCK(&V_tcp_hostcache.hashbase[i]);
		CK_SLIST_FOREACH(hc_entry,
		    &V_tcp_hostcache.hashbase[i].hch_bucket, hc_q) {
			sbuf_printf(&sb,
			    "%-15s %5u %8u %6lums %6lums %8u %8u %8u "
#ifdef	TCP_HC_COUNTERS
			    "%4lu %4lu "
#endif
			    "%4i\n",
			    hc_entry->ip4.s_addr ?
			        inet_ntoa_r(hc_entry->ip4, ip4buf) :
#ifdef INET6
				ip6_sprintf(ip6buf, &hc_entry->ip6),
#else
				"IPv6?",
#endif
			    hc_entry->hc_mtu,
			    hc_entry->hc_ssthresh,
			    msec((u_long)hc_entry->hc_rtt *
				(RTM_RTTUNIT / (hz * TCP_RTT_SCALE))),
			    msec((u_long)hc_entry->hc_rttvar *
				(RTM_RTTUNIT / (hz * TCP_RTTVAR_SCALE))),
			    hc_entry->hc_cwnd,
			    hc_entry->hc_sendpipe,
			    hc_entry->hc_recvpipe,
#ifdef	TCP_HC_COUNTERS
			    hc_entry->hc_hits,
			    hc_entry->hc_updates,
#endif
			    hc_entry->hc_expire);
		}
		THC_UNLOCK(&V_tcp_hostcache.hashbase[i]);
		sbuf_drain(&sb);
	}
#undef msec
	error = sbuf_finish(&sb);
	sbuf_delete(&sb);
	return(error);
}

/*
 * Sysctl function: prints a histogram of the hostcache hashbucket
 * utilization.
 */
static int
sysctl_tcp_hc_histo(SYSCTL_HANDLER_ARGS)
{
	const int linesize = 50;
	struct sbuf sb;
	int i, error;
	int *histo;
	u_int hch_length;

	if (jailed_without_vnet(curthread->td_ucred) != 0)
		return (EPERM);

	histo = (int *)malloc(sizeof(int) * (V_tcp_hostcache.bucket_limit + 1),
			M_TEMP, M_NOWAIT|M_ZERO);
	if (histo == NULL)
		return(ENOMEM);

	for (i = 0; i < V_tcp_hostcache.hashsize; i++) {
		hch_length = V_tcp_hostcache.hashbase[i].hch_length;
		KASSERT(hch_length <= V_tcp_hostcache.bucket_limit,
		    ("tcp_hostcache: bucket limit exceeded at %u: %u",
		    i, hch_length));
		histo[hch_length]++;
	}

	/* Use a buffer for 16 lines */
	sbuf_new_for_sysctl(&sb, NULL, 16 * linesize, req);

	sbuf_printf(&sb, "\nLength\tCount\n");
	for (i = 0; i <= V_tcp_hostcache.bucket_limit; i++) {
		sbuf_printf(&sb, "%u\t%u\n", i, histo[i]);
	}
	error = sbuf_finish(&sb);
	sbuf_delete(&sb);
	free(histo, M_TEMP);
	return(error);
}

/*
 * Caller has to make sure the curvnet is set properly.
 */
static void
tcp_hc_purge_internal(int all)
{
	struct hc_head *head;
	struct hc_metrics *hc_entry, *hc_next, *hc_prev;
	int i;

	for (i = 0; i < V_tcp_hostcache.hashsize; i++) {
		head = &V_tcp_hostcache.hashbase[i];
		hc_prev = NULL;
		THC_LOCK(head);
		CK_SLIST_FOREACH_SAFE(hc_entry, &head->hch_bucket, hc_q,
		    hc_next) {
			KASSERT(head->hch_length > 0 && head->hch_length <=
			    V_tcp_hostcache.bucket_limit, ("tcp_hostcache: "
			    "bucket length out of range at %u: %u", i,
			    head->hch_length));
			if (all ||
			    atomic_load_int(&hc_entry->hc_expire) <= 0) {
				if (hc_prev != NULL) {
					KASSERT(hc_entry ==
					    CK_SLIST_NEXT(hc_prev, hc_q),
					    ("%s: %p is not next to %p",
					    __func__, hc_entry, hc_prev));
					CK_SLIST_REMOVE_AFTER(hc_prev, hc_q);
				} else {
					KASSERT(hc_entry ==
					    CK_SLIST_FIRST(&head->hch_bucket),
					    ("%s: %p is not first",
					    __func__, hc_entry));
					CK_SLIST_REMOVE_HEAD(&head->hch_bucket,
					    hc_q);
				}
				uma_zfree_smr(V_tcp_hostcache.zone, hc_entry);
				head->hch_length--;
				atomic_subtract_int(&V_tcp_hostcache.cache_count, 1);
			} else {
				atomic_subtract_int(&hc_entry->hc_expire,
				    V_tcp_hostcache.prune);
				hc_prev = hc_entry;
			}
		}
		THC_UNLOCK(head);
	}
}

/*
 * Expire and purge (old|all) entries in the tcp_hostcache.  Runs
 * periodically from the callout.
 */
static void
tcp_hc_purge(void *arg)
{
	CURVNET_SET((struct vnet *) arg);
	int all = 0;

	if (V_tcp_hostcache.purgeall) {
		if (V_tcp_hostcache.purgeall == 2)
			V_tcp_hostcache.hashsalt = arc4random();
		all = 1;
		V_tcp_hostcache.purgeall = 0;
	}

	tcp_hc_purge_internal(all);

	callout_reset(&V_tcp_hc_callout, V_tcp_hostcache.prune * hz,
	    tcp_hc_purge, arg);
	CURVNET_RESTORE();
}

/*
 * Expire and purge all entries in hostcache immediately.
 */
static int
sysctl_tcp_hc_purgenow(SYSCTL_HANDLER_ARGS)
{
	int error, val;

	val = 0;
	error = sysctl_handle_int(oidp, &val, 0, req);
	if (error || !req->newptr)
		return (error);

	if (val == 2)
		V_tcp_hostcache.hashsalt = arc4random();
	tcp_hc_purge_internal(1);

	callout_reset(&V_tcp_hc_callout, V_tcp_hostcache.prune * hz,
	    tcp_hc_purge, curvnet);

	return (0);
}