From d288a162dd1c73507da582966f17dd226e34a0c0 Mon Sep 17 00:00:00 2001 From: Wangyang Guo Date: Thu, 23 Mar 2023 21:55:29 +0100 Subject: net: dst: Prevent false sharing vs. dst_entry:: __refcnt 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 Signed-off-by: Arjan van de Ven Signed-off-by: Thomas Gleixner Reviewed-by: Eric Dumazet Reviewed-by: David Ahern Link: https://lore.kernel.org/r/20230323102800.042297517@linutronix.de Signed-off-by: Jakub Kicinski --- net/ipv6/route.c | 26 +++++++++++++------------- net/ipv6/xfrm6_policy.c | 4 ++-- 2 files changed, 15 insertions(+), 15 deletions(-) (limited to 'net/ipv6') diff --git a/net/ipv6/route.c b/net/ipv6/route.c index 244df77fac87..12e163dec34e 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -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 diff --git a/net/ipv6/xfrm6_policy.c b/net/ipv6/xfrm6_policy.c index ea435eba3053..2b493f8d0091 100644 --- a/net/ipv6/xfrm6_policy.c +++ b/net/ipv6/xfrm6_policy.c @@ -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); } -- cgit v1.2.3-58-ga151