From: Wangyang Guo <[email protected]>
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.
Rearrange and pad the structure so that the lwtstate member is in the next
cache-line. This increases the struct size from 112 to 136 bytes on 64bit.
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 <[email protected]>
Signed-off-by: Arjan van De Ven <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: [email protected]
---
include/net/dst.h | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -69,15 +69,25 @@ 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;
+#ifdef CONFIG_64BIT
+ /*
+ * Ensure that lwtstate is not in the same cache line as __refcnt,
+ * because that would lead to false sharing under high contention
+ * of __refcnt. This also ensures that rtable::rt_genid is not
+ * sharing the same cache-line.
+ */
+ int pad2[6];
+ struct lwtunnel_state *lwtstate;
+#endif
};
struct dst_metrics {
On Tue, Feb 28, 2023 at 3:33 PM Thomas Gleixner <[email protected]> wrote:
>
> From: Wangyang Guo <[email protected]>
>
> 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.
This will probably increase struct rt6_info past the 4 cache line size, right ?
It would be nice to allow sharing the 'hot' cache line with seldom used fields.
Instead of mere pads, add some unions, and let rt6i_uncached/rt6i_uncached_list
use them.
Eric!
On Tue, Feb 28 2023 at 16:17, Eric Dumazet wrote:
> On Tue, Feb 28, 2023 at 3:33 PM Thomas Gleixner <[email protected]> wrote:
>> 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.
>
> This will probably increase struct rt6_info past the 4 cache line
> size, right ?
pahole says: /* size: 256, cachelines: 4, members: 11 */
> It would be nice to allow sharing the 'hot' cache line with seldom
> used fields.
Sure.
> Instead of mere pads, add some unions, and let rt6i_uncached/rt6i_uncached_list
> use them.
If I understand correctly, you suggest to move
rt6_info::rt6i_uncached[_list], rtable::rt_uncached[_list]
into struct dst_entry and fixup the usage sites, right?
I don't see why that would need a union. dst_entry::rt_uncached[_list]
would work for both, no?
Thanks,
tglx
On Tue, Feb 28 2023 at 22:31, Thomas Gleixner wrote:
> On Tue, Feb 28 2023 at 16:17, Eric Dumazet wrote:
>> Instead of mere pads, add some unions, and let rt6i_uncached/rt6i_uncached_list
>> use them.
>
> If I understand correctly, you suggest to move
>
> rt6_info::rt6i_uncached[_list], rtable::rt_uncached[_list]
>
> into struct dst_entry and fixup the usage sites, right?
>
> I don't see why that would need a union. dst_entry::rt_uncached[_list]
> would work for both, no?
So I came up with the below.
rt6_info shrinks from 232 to 224 bytes. rtable size is unchanged
Thanks,
tglx
---
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -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 {
--- a/include/net/ip6_fib.h
+++ b/include/net/ip6_fib.h
@@ -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;
};
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -104,7 +104,7 @@ static inline struct dst_entry *ip6_rout
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);
}
--- a/include/net/route.h
+++ b/include/net/route.h
@@ -81,9 +81,6 @@ 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)
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1508,20 +1508,20 @@ void rt_add_uncached_list(struct rtable
{
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_d
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_d
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(s
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);
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -91,7 +91,7 @@ static int xfrm4_fill_dst(struct xfrm_ds
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
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);
}
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -139,20 +139,20 @@ void rt6_uncached_list_add(struct rt6_in
{
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(
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(
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_hol
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
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
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);
@@ -6483,7 +6483,7 @@ static int __net_init ip6_route_net_init
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;
@@ -6495,7 +6495,7 @@ static int __net_init ip6_route_net_init
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),
@@ -6505,7 +6505,7 @@ static int __net_init ip6_route_net_init
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
--- a/net/ipv6/xfrm6_policy.c
+++ b/net/ipv6/xfrm6_policy.c
@@ -89,7 +89,7 @@ static int xfrm6_fill_dst(struct xfrm_ds
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
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);
}