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Miller" , Jakub Kicinski Cc: netdev , Eric Dumazet , Eric Dumazet Subject: [PATCH net-next 1/9] inet: shrink inet_timewait_death_row by 48 bytes Date: Wed, 31 Mar 2021 10:52:05 -0700 Message-Id: <20210331175213.691460-2-eric.dumazet@gmail.com> X-Mailer: git-send-email 2.31.0.291.g576ba9dcdaf-goog In-Reply-To: <20210331175213.691460-1-eric.dumazet@gmail.com> References: <20210331175213.691460-1-eric.dumazet@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org From: Eric Dumazet struct inet_timewait_death_row uses two cache lines, because we want tw_count to use a full cache line to avoid false sharing. Rework its definition and placement in netns_ipv4 so that: 1) We add 60 bytes of padding after tw_count to avoid false sharing, knowing that tcp_death_row will have ____cacheline_aligned_in_smp attribute. 2) We do not risk padding before tcp_death_row, because we move it at the beginning of netns_ipv4, even if new fields are added later. 3) We do not waste 48 bytes of padding after it. Note that I have not changed dccp. pahole result for struct netns_ipv4 before/after the patch : /* size: 832, cachelines: 13, members: 139 */ /* sum members: 721, holes: 12, sum holes: 95 */ /* padding: 16 */ /* paddings: 2, sum paddings: 55 */ -> /* size: 768, cachelines: 12, members: 139 */ /* sum members: 673, holes: 11, sum holes: 39 */ /* padding: 56 */ /* paddings: 2, sum paddings: 7 */ /* forced alignments: 1 */ Signed-off-by: Eric Dumazet --- include/net/netns/ipv4.h | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h index 9c8dd424d79b1d1100b6cbaa64d9fda9352a0b3a..1085ed4e0788d7cd432dfc7e5604ef3cd66dc337 100644 --- a/include/net/netns/ipv4.h +++ b/include/net/netns/ipv4.h @@ -32,14 +32,18 @@ struct inet_hashinfo; struct inet_timewait_death_row { atomic_t tw_count; + char tw_pad[L1_CACHE_BYTES - sizeof(atomic_t)]; - struct inet_hashinfo *hashinfo ____cacheline_aligned_in_smp; + struct inet_hashinfo *hashinfo; int sysctl_max_tw_buckets; }; struct tcp_fastopen_context; struct netns_ipv4 { + /* Please keep tcp_death_row at first field in netns_ipv4 */ + struct inet_timewait_death_row tcp_death_row ____cacheline_aligned_in_smp; + #ifdef CONFIG_SYSCTL struct ctl_table_header *forw_hdr; struct ctl_table_header *frags_hdr; @@ -175,7 +179,6 @@ struct netns_ipv4 { int sysctl_tcp_comp_sack_nr; unsigned long sysctl_tcp_comp_sack_delay_ns; unsigned long sysctl_tcp_comp_sack_slack_ns; - struct inet_timewait_death_row tcp_death_row; int sysctl_max_syn_backlog; int sysctl_tcp_fastopen; const struct tcp_congestion_ops __rcu *tcp_congestion_control; -- 2.31.0.291.g576ba9dcdaf-goog