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net: dst: Switch to rcuref_t reference counting 

Under high contention dst_entry::__refcnt becomes a significant bottleneck.

atomic_inc_not_zero() is implemented with a cmpxchg() loop, which goes into
high retry rates on contention.

Switch the reference count to rcuref_t which results in a significant
performance gain. Rename the reference count member to __rcuref to reflect
the change.

The gain depends on the micro-architecture and the number of concurrent
operations and has been measured in the range of +25% to +130% with a
localhost memtier/memcached benchmark which amplifies the problem
massively.

Running the memtier/memcached benchmark over a real (1Gb) network
connection the conversion on top of the false sharing fix for struct
dst_entry::__refcnt results in a total gain in the 2%-5% range over the
upstream baseline.

Reported-by: Wangyang Guo <wangyang.guo@intel.com>
Reported-by: Arjan Van De Ven <arjan.van.de.ven@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230307125538.989175656@linutronix.de
Link: https://lore.kernel.org/r/20230323102800.215027837@linutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Thomas Gleixner 2023-03-23 21:55:32 +01:00 committed by Jakub Kicinski
parent d288a162dd
commit bc9d3a9f2a
7 changed files with 23 additions and 38 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
include/net/dst.h
View File

@ -16,6 +16,7 @@
#include <linux/bug.h>
#include <linux/jiffies.h>
#include <linux/refcount.h>
#include <linux/rcuref.h>
#include <net/neighbour.h>
#include <asm/processor.h>
#include <linux/indirect_call_wrapper.h>
@ -61,11 +62,11 @@ struct dst_entry {
unsigned short trailer_len; /* space to reserve at tail */
/*
* __refcnt wants to be on a different cache line from
* __rcuref wants to be on a different cache line from
* input/output/ops or performance tanks badly
*/
#ifdef CONFIG_64BIT
atomic_t __refcnt; /* 64-bit offset 64 */
rcuref_t __rcuref; /* 64-bit offset 64 */
#endif
int __use;
unsigned long lastuse;
@ -75,16 +76,16 @@ struct dst_entry {
__u32 tclassid;
#ifndef CONFIG_64BIT
struct lwtunnel_state *lwtstate;
atomic_t __refcnt; /* 32-bit offset 64 */
rcuref_t __rcuref; /* 32-bit offset 64 */
#endif
netdevice_tracker dev_tracker;
/*
* Used by rtable and rt6_info. Moves lwtstate into the next cache
* line on 64bit so that lwtstate does not cause false sharing with
* __refcnt under contention of __refcnt. This also puts the
* __rcuref under contention of __rcuref. This also puts the
* frequently accessed members of rtable and rt6_info out of the
* __refcnt cache line.
* __rcuref cache line.
*/
struct list_head rt_uncached;
struct uncached_list *rt_uncached_list;
@ -238,10 +239,10 @@ static inline void dst_hold(struct dst_entry *dst)
{
/*
* If your kernel compilation stops here, please check
* the placement of __refcnt in struct dst_entry
* the placement of __rcuref in struct dst_entry
*/
BUILD_BUG_ON(offsetof(struct dst_entry, __refcnt) & 63);
WARN_ON(atomic_inc_not_zero(&dst->__refcnt) == 0);
BUILD_BUG_ON(offsetof(struct dst_entry, __rcuref) & 63);
WARN_ON(!rcuref_get(&dst->__rcuref));
}
static inline void dst_use_noref(struct dst_entry *dst, unsigned long time)
@ -305,7 +306,7 @@ static inline void skb_dst_copy(struct sk_buff *nskb, const struct sk_buff *oskb
*/
static inline bool dst_hold_safe(struct dst_entry *dst)
{
return atomic_inc_not_zero(&dst->__refcnt);
return rcuref_get(&dst->__rcuref);
}
/**

2
include/net/sock.h
View File

@ -2131,7 +2131,7 @@ sk_dst_get(struct sock *sk)
rcu_read_lock();
dst = rcu_dereference(sk->sk_dst_cache);
if (dst && !atomic_inc_not_zero(&dst->__refcnt))
if (dst && !rcuref_get(&dst->__rcuref))
dst = NULL;
rcu_read_unlock();
return dst;

2
net/bridge/br_nf_core.c
View File

@ -73,7 +73,7 @@ void br_netfilter_rtable_init(struct net_bridge *br)
{
struct rtable *rt = &br->fake_rtable;
atomic_set(&rt->dst.__refcnt, 1);
rcuref_init(&rt->dst.__rcuref, 1);
rt->dst.dev = br->dev;
dst_init_metrics(&rt->dst, br_dst_default_metrics, true);
rt->dst.flags = DST_NOXFRM | DST_FAKE_RTABLE;

26
net/core/dst.c
View File

@ -66,7 +66,7 @@ void dst_init(struct dst_entry *dst, struct dst_ops *ops,
dst->tclassid = 0;
#endif
dst->lwtstate = NULL;
atomic_set(&dst->__refcnt, initial_ref);
rcuref_init(&dst->__rcuref, initial_ref);
dst->__use = 0;
dst->lastuse = jiffies;
dst->flags = flags;
@ -162,31 +162,15 @@ EXPORT_SYMBOL(dst_dev_put);
void dst_release(struct dst_entry *dst)
{
if (dst) {
int newrefcnt;
newrefcnt = atomic_dec_return(&dst->__refcnt);
if (WARN_ONCE(newrefcnt < 0, "dst_release underflow"))
net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
__func__, dst, newrefcnt);
if (!newrefcnt)
call_rcu_hurry(&dst->rcu_head, dst_destroy_rcu);
}
if (dst && rcuref_put(&dst->__rcuref))
call_rcu_hurry(&dst->rcu_head, dst_destroy_rcu);
}
EXPORT_SYMBOL(dst_release);
void dst_release_immediate(struct dst_entry *dst)
{
if (dst) {
int newrefcnt;
newrefcnt = atomic_dec_return(&dst->__refcnt);
if (WARN_ONCE(newrefcnt < 0, "dst_release_immediate underflow"))
net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
__func__, dst, newrefcnt);
if (!newrefcnt)
dst_destroy(dst);
}
if (dst && rcuref_put(&dst->__rcuref))
dst_destroy(dst);
}
EXPORT_SYMBOL(dst_release_immediate);

2
net/core/rtnetlink.c
View File

@ -843,7 +843,7 @@ int rtnl_put_cacheinfo(struct sk_buff *skb, struct dst_entry *dst, u32 id,
if (dst) {
ci.rta_lastuse = jiffies_delta_to_clock_t(jiffies - dst->lastuse);
ci.rta_used = dst->__use;
ci.rta_clntref = atomic_read(&dst->__refcnt);
ci.rta_clntref = rcuref_read(&dst->__rcuref);
}
if (expires) {
unsigned long clock;

6
net/ipv6/route.c
View File

@ -293,7 +293,7 @@ static const struct fib6_info fib6_null_entry_template = {
static const struct rt6_info ip6_null_entry_template = {
.dst = {
.__refcnt = ATOMIC_INIT(1),
.__rcuref = RCUREF_INIT(1),
.__use = 1,
.obsolete = DST_OBSOLETE_FORCE_CHK,
.error = -ENETUNREACH,
@ -307,7 +307,7 @@ static const struct rt6_info ip6_null_entry_template = {
static const struct rt6_info ip6_prohibit_entry_template = {
.dst = {
.__refcnt = ATOMIC_INIT(1),
.__rcuref = RCUREF_INIT(1),
.__use = 1,
.obsolete = DST_OBSOLETE_FORCE_CHK,
.error = -EACCES,
@ -319,7 +319,7 @@ static const struct rt6_info ip6_prohibit_entry_template = {
static const struct rt6_info ip6_blk_hole_entry_template = {
.dst = {
.__refcnt = ATOMIC_INIT(1),
.__rcuref = RCUREF_INIT(1),
.__use = 1,
.obsolete = DST_OBSOLETE_FORCE_CHK,
.error = -EINVAL,

4
net/netfilter/ipvs/ip_vs_xmit.c
View File

@ -339,7 +339,7 @@ __ip_vs_get_out_rt(struct netns_ipvs *ipvs, int skb_af, struct sk_buff *skb,
spin_unlock_bh(&dest->dst_lock);
IP_VS_DBG(10, "new dst %pI4, src %pI4, refcnt=%d\n",
&dest->addr.ip, &dest_dst->dst_saddr.ip,
atomic_read(&rt->dst.__refcnt));
rcuref_read(&rt->dst.__rcuref));
}
if (ret_saddr)
*ret_saddr = dest_dst->dst_saddr.ip;
@ -507,7 +507,7 @@ __ip_vs_get_out_rt_v6(struct netns_ipvs *ipvs, int skb_af, struct sk_buff *skb,
spin_unlock_bh(&dest->dst_lock);
IP_VS_DBG(10, "new dst %pI6, src %pI6, refcnt=%d\n",
&dest->addr.in6, &dest_dst->dst_saddr.in6,
atomic_read(&rt->dst.__refcnt));
rcuref_read(&rt->dst.__rcuref));
}
if (ret_saddr)
*ret_saddr = dest_dst->dst_saddr.in6;