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net: dst: Prevent false sharing vs. dst_entry:: __refcnt

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dst_entry::__refcnt is highly contended in scenarios where many connections
happen from and to the same IP. The reference count is an atomic_t, so the
reference count operations have to take the cache-line exclusive.

Aside of the unavoidable reference count contention there is another
significant problem which is caused by that: False sharing.

perf top identified two affected read accesses. dst_entry::lwtstate and
rtable::rt_genid.

dst_entry:__refcnt is located at offset 64 of dst_entry, which puts it into
a seperate cacheline vs. the read mostly members located at the beginning
of the struct.

That prevents false sharing vs. the struct members in the first 64
bytes of the structure, but there is also

  dst_entry::lwtstate

which is located after the reference count and in the same cache line. This
member is read after a reference count has been acquired.

struct rtable embeds a struct dst_entry at offset 0. struct dst_entry has a
size of 112 bytes, which means that the struct members of rtable which
follow the dst member share the same cache line as dst_entry::__refcnt.
Especially

  rtable::rt_genid

is also read by the contexts which have a reference count acquired
already.

When dst_entry:__refcnt is incremented or decremented via an atomic
operation these read accesses stall. This was found when analysing the
memtier benchmark in 1:100 mode, which amplifies the problem extremly.

Move the rt[6i]_uncached[_list] members out of struct rtable and struct
rt6_info into struct dst_entry to provide padding and move the lwtstate
member after that so it ends up in the same cache line.

The resulting improvement depends on the micro-architecture and the number
of CPUs. It ranges from +20% to +120% with a localhost memtier/memcached
benchmark.

[ tglx: Rearrange struct ]

Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230323102800.042297517@linutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Wangyang Guo 2023-03-23 21:55:29 +01:00 committed by Jakub Kicinski
parent b133fffe57
commit d288a162dd
8 changed files with 42 additions and 35 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
include/net/dst.h
View File

@ -69,15 +69,28 @@ struct dst_entry {
#endif
int __use;
unsigned long lastuse;
struct lwtunnel_state *lwtstate;
struct rcu_head rcu_head;
short error;
short __pad;
__u32 tclassid;
#ifndef CONFIG_64BIT
struct lwtunnel_state *lwtstate;
atomic_t __refcnt; /* 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
* frequently accessed members of rtable and rt6_info out of the
* __refcnt cache line.
*/
struct list_head rt_uncached;
struct uncached_list *rt_uncached_list;
#ifdef CONFIG_64BIT
struct lwtunnel_state *lwtstate;
#endif
};
struct dst_metrics {

3
include/net/ip6_fib.h
View File

@ -217,9 +217,6 @@ struct rt6_info {
struct inet6_dev *rt6i_idev;
u32 rt6i_flags;
struct list_head rt6i_uncached;
struct uncached_list *rt6i_uncached_list;
/* more non-fragment space at head required */
unsigned short rt6i_nfheader_len;
};

2
include/net/ip6_route.h
View File

@ -100,7 +100,7 @@ static inline struct dst_entry *ip6_route_output(struct net *net,
static inline void ip6_rt_put_flags(struct rt6_info *rt, int flags)
{
if (!(flags & RT6_LOOKUP_F_DST_NOREF) ||
!list_empty(&rt->rt6i_uncached))
!list_empty(&rt->dst.rt_uncached))
ip6_rt_put(rt);
}

3
include/net/route.h
View File

@ -78,9 +78,6 @@ struct rtable {
/* Miscellaneous cached information */
u32 rt_mtu_locked:1,
rt_pmtu:31;
struct list_head rt_uncached;
struct uncached_list *rt_uncached_list;
};
static inline bool rt_is_input_route(const struct rtable *rt)

20
net/ipv4/route.c
View File

@ -1508,20 +1508,20 @@ void rt_add_uncached_list(struct rtable *rt)
{
struct uncached_list *ul = raw_cpu_ptr(&rt_uncached_list);
rt->rt_uncached_list = ul;
rt->dst.rt_uncached_list = ul;
spin_lock_bh(&ul->lock);
list_add_tail(&rt->rt_uncached, &ul->head);
list_add_tail(&rt->dst.rt_uncached, &ul->head);
spin_unlock_bh(&ul->lock);
}
void rt_del_uncached_list(struct rtable *rt)
{
if (!list_empty(&rt->rt_uncached)) {
struct uncached_list *ul = rt->rt_uncached_list;
if (!list_empty(&rt->dst.rt_uncached)) {
struct uncached_list *ul = rt->dst.rt_uncached_list;
spin_lock_bh(&ul->lock);
list_del_init(&rt->rt_uncached);
list_del_init(&rt->dst.rt_uncached);
spin_unlock_bh(&ul->lock);
}
}
@ -1546,13 +1546,13 @@ void rt_flush_dev(struct net_device *dev)
continue;
spin_lock_bh(&ul->lock);
list_for_each_entry_safe(rt, safe, &ul->head, rt_uncached) {
list_for_each_entry_safe(rt, safe, &ul->head, dst.rt_uncached) {
if (rt->dst.dev != dev)
continue;
rt->dst.dev = blackhole_netdev;
netdev_ref_replace(dev, blackhole_netdev,
&rt->dst.dev_tracker, GFP_ATOMIC);
list_move(&rt->rt_uncached, &ul->quarantine);
list_move(&rt->dst.rt_uncached, &ul->quarantine);
}
spin_unlock_bh(&ul->lock);
}
@ -1644,7 +1644,7 @@ struct rtable *rt_dst_alloc(struct net_device *dev,
rt->rt_uses_gateway = 0;
rt->rt_gw_family = 0;
rt->rt_gw4 = 0;
INIT_LIST_HEAD(&rt->rt_uncached);
INIT_LIST_HEAD(&rt->dst.rt_uncached);
rt->dst.output = ip_output;
if (flags & RTCF_LOCAL)
@ -1675,7 +1675,7 @@ struct rtable *rt_dst_clone(struct net_device *dev, struct rtable *rt)
new_rt->rt_gw4 = rt->rt_gw4;
else if (rt->rt_gw_family == AF_INET6)
new_rt->rt_gw6 = rt->rt_gw6;
INIT_LIST_HEAD(&new_rt->rt_uncached);
INIT_LIST_HEAD(&new_rt->dst.rt_uncached);
new_rt->dst.input = rt->dst.input;
new_rt->dst.output = rt->dst.output;
@ -2859,7 +2859,7 @@ struct dst_entry *ipv4_blackhole_route(struct net *net, struct dst_entry *dst_or
else if (rt->rt_gw_family == AF_INET6)
rt->rt_gw6 = ort->rt_gw6;
INIT_LIST_HEAD(&rt->rt_uncached);
INIT_LIST_HEAD(&rt->dst.rt_uncached);
}
dst_release(dst_orig);

4
net/ipv4/xfrm4_policy.c
View File

@ -91,7 +91,7 @@ static int xfrm4_fill_dst(struct xfrm_dst *xdst, struct net_device *dev,
xdst->u.rt.rt_gw6 = rt->rt_gw6;
xdst->u.rt.rt_pmtu = rt->rt_pmtu;
xdst->u.rt.rt_mtu_locked = rt->rt_mtu_locked;
INIT_LIST_HEAD(&xdst->u.rt.rt_uncached);
INIT_LIST_HEAD(&xdst->u.rt.dst.rt_uncached);
rt_add_uncached_list(&xdst->u.rt);
return 0;
@ -121,7 +121,7 @@ static void xfrm4_dst_destroy(struct dst_entry *dst)
struct xfrm_dst *xdst = (struct xfrm_dst *)dst;
dst_destroy_metrics_generic(dst);
if (xdst->u.rt.rt_uncached_list)
if (xdst->u.rt.dst.rt_uncached_list)
rt_del_uncached_list(&xdst->u.rt);
xfrm_dst_destroy(xdst);
}

26
net/ipv6/route.c
View File

@ -139,20 +139,20 @@ void rt6_uncached_list_add(struct rt6_info *rt)
{
struct uncached_list *ul = raw_cpu_ptr(&rt6_uncached_list);
rt->rt6i_uncached_list = ul;
rt->dst.rt_uncached_list = ul;
spin_lock_bh(&ul->lock);
list_add_tail(&rt->rt6i_uncached, &ul->head);
list_add_tail(&rt->dst.rt_uncached, &ul->head);
spin_unlock_bh(&ul->lock);
}
void rt6_uncached_list_del(struct rt6_info *rt)
{
if (!list_empty(&rt->rt6i_uncached)) {
struct uncached_list *ul = rt->rt6i_uncached_list;
if (!list_empty(&rt->dst.rt_uncached)) {
struct uncached_list *ul = rt->dst.rt_uncached_list;
spin_lock_bh(&ul->lock);
list_del_init(&rt->rt6i_uncached);
list_del_init(&rt->dst.rt_uncached);
spin_unlock_bh(&ul->lock);
}
}
@ -169,7 +169,7 @@ static void rt6_uncached_list_flush_dev(struct net_device *dev)
continue;
spin_lock_bh(&ul->lock);
list_for_each_entry_safe(rt, safe, &ul->head, rt6i_uncached) {
list_for_each_entry_safe(rt, safe, &ul->head, dst.rt_uncached) {
struct inet6_dev *rt_idev = rt->rt6i_idev;
struct net_device *rt_dev = rt->dst.dev;
bool handled = false;
@ -188,7 +188,7 @@ static void rt6_uncached_list_flush_dev(struct net_device *dev)
handled = true;
}
if (handled)
list_move(&rt->rt6i_uncached,
list_move(&rt->dst.rt_uncached,
&ul->quarantine);
}
spin_unlock_bh(&ul->lock);
@ -334,7 +334,7 @@ static const struct rt6_info ip6_blk_hole_entry_template = {
static void rt6_info_init(struct rt6_info *rt)
{
memset_after(rt, 0, dst);
INIT_LIST_HEAD(&rt->rt6i_uncached);
INIT_LIST_HEAD(&rt->dst.rt_uncached);
}
/* allocate dst with ip6_dst_ops */
@ -2638,7 +2638,7 @@ struct dst_entry *ip6_route_output_flags(struct net *net,
dst = ip6_route_output_flags_noref(net, sk, fl6, flags);
rt6 = (struct rt6_info *)dst;
/* For dst cached in uncached_list, refcnt is already taken. */
if (list_empty(&rt6->rt6i_uncached) && !dst_hold_safe(dst)) {
if (list_empty(&rt6->dst.rt_uncached) && !dst_hold_safe(dst)) {
dst = &net->ipv6.ip6_null_entry->dst;
dst_hold(dst);
}
@ -2748,7 +2748,7 @@ INDIRECT_CALLABLE_SCOPE struct dst_entry *ip6_dst_check(struct dst_entry *dst,
from = rcu_dereference(rt->from);
if (from && (rt->rt6i_flags & RTF_PCPU ||
unlikely(!list_empty(&rt->rt6i_uncached))))
unlikely(!list_empty(&rt->dst.rt_uncached))))
dst_ret = rt6_dst_from_check(rt, from, cookie);
else
dst_ret = rt6_check(rt, from, cookie);
@ -6477,7 +6477,7 @@ static int __net_init ip6_route_net_init(struct net *net)
net->ipv6.ip6_null_entry->dst.ops = &net->ipv6.ip6_dst_ops;
dst_init_metrics(&net->ipv6.ip6_null_entry->dst,
ip6_template_metrics, true);
INIT_LIST_HEAD(&net->ipv6.ip6_null_entry->rt6i_uncached);
INIT_LIST_HEAD(&net->ipv6.ip6_null_entry->dst.rt_uncached);
#ifdef CONFIG_IPV6_MULTIPLE_TABLES
net->ipv6.fib6_has_custom_rules = false;
@ -6489,7 +6489,7 @@ static int __net_init ip6_route_net_init(struct net *net)
net->ipv6.ip6_prohibit_entry->dst.ops = &net->ipv6.ip6_dst_ops;
dst_init_metrics(&net->ipv6.ip6_prohibit_entry->dst,
ip6_template_metrics, true);
INIT_LIST_HEAD(&net->ipv6.ip6_prohibit_entry->rt6i_uncached);
INIT_LIST_HEAD(&net->ipv6.ip6_prohibit_entry->dst.rt_uncached);
net->ipv6.ip6_blk_hole_entry = kmemdup(&ip6_blk_hole_entry_template,
sizeof(*net->ipv6.ip6_blk_hole_entry),
@ -6499,7 +6499,7 @@ static int __net_init ip6_route_net_init(struct net *net)
net->ipv6.ip6_blk_hole_entry->dst.ops = &net->ipv6.ip6_dst_ops;
dst_init_metrics(&net->ipv6.ip6_blk_hole_entry->dst,
ip6_template_metrics, true);
INIT_LIST_HEAD(&net->ipv6.ip6_blk_hole_entry->rt6i_uncached);
INIT_LIST_HEAD(&net->ipv6.ip6_blk_hole_entry->dst.rt_uncached);
#ifdef CONFIG_IPV6_SUBTREES
net->ipv6.fib6_routes_require_src = 0;
#endif

4
net/ipv6/xfrm6_policy.c
View File

@ -89,7 +89,7 @@ static int xfrm6_fill_dst(struct xfrm_dst *xdst, struct net_device *dev,
xdst->u.rt6.rt6i_gateway = rt->rt6i_gateway;
xdst->u.rt6.rt6i_dst = rt->rt6i_dst;
xdst->u.rt6.rt6i_src = rt->rt6i_src;
INIT_LIST_HEAD(&xdst->u.rt6.rt6i_uncached);
INIT_LIST_HEAD(&xdst->u.rt6.dst.rt_uncached);
rt6_uncached_list_add(&xdst->u.rt6);
return 0;
@ -121,7 +121,7 @@ static void xfrm6_dst_destroy(struct dst_entry *dst)
if (likely(xdst->u.rt6.rt6i_idev))
in6_dev_put(xdst->u.rt6.rt6i_idev);
dst_destroy_metrics_generic(dst);
if (xdst->u.rt6.rt6i_uncached_list)
if (xdst->u.rt6.dst.rt_uncached_list)
rt6_uncached_list_del(&xdst->u.rt6);
xfrm_dst_destroy(xdst);
}