| Message ID | 20230228132910.934296889@linutronix.de (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | net, refcount: Address dst_entry reference count scalability issues | expand |
On Tue, Feb 28, 2023 at 3:33 PM Thomas Gleixner <tglx@linutronix.de> wrote: > > From: Wangyang Guo <wangyang.guo@intel.com> > > 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 <tglx@linutronix.de> 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); }
--- 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 {