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net: reorder 'struct net' fields to avoid false sharing 

Intel test robot reported a ~7% regression on TCP_CRR tests
that they bisected to the cited commit.

Indeed, every time a new TCP socket is created or deleted,
the atomic counter net->count is touched (via get_net(net)
and put_net(net) calls)

So cpus might have to reload a contended cache line in
net_hash_mix(net) calls.

We need to reorder 'struct net' fields to move @hash_mix
in a read mostly cache line.

We move in the first cache line fields that can be
dirtied often.

We probably will have to address in a followup patch
the __randomize_layout that was added in linux-4.13,
since this might break our placement choices.

Fixes: 355b985537 ("netns: provide pure entropy for net_hash_mix()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2019-10-18 15:20:05 -07:00 committed by David S. Miller
parent 50c7d2ba9d
commit 2a06b8982f
1 changed files with 17 additions and 8 deletions
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25
include/net/net_namespace.h
View File

@ -52,6 +52,9 @@ struct bpf_prog;
#define NETDEV_HASHENTRIES (1 << NETDEV_HASHBITS)
struct net {
/* First cache line can be often dirtied.
* Do not place here read-mostly fields.
*/
refcount_t passive; /* To decide when the network
* namespace should be freed.
*/
@ -60,7 +63,13 @@ struct net {
*/
spinlock_t rules_mod_lock;
u32 hash_mix;
unsigned int dev_unreg_count;
unsigned int dev_base_seq; /* protected by rtnl_mutex */
int ifindex;
spinlock_t nsid_lock;
atomic_t fnhe_genid;
struct list_head list; /* list of network namespaces */
struct list_head exit_list; /* To linked to call pernet exit
@ -76,11 +85,11 @@ struct net {
#endif
struct user_namespace *user_ns; /* Owning user namespace */
struct ucounts *ucounts;
spinlock_t nsid_lock;
struct idr netns_ids;
struct ns_common ns;
struct list_head dev_base_head;
struct proc_dir_entry *proc_net;
struct proc_dir_entry *proc_net_stat;
@ -93,17 +102,18 @@ struct net {
struct uevent_sock *uevent_sock; /* uevent socket */
struct list_head dev_base_head;
struct hlist_head *dev_name_head;
struct hlist_head *dev_index_head;
unsigned int dev_base_seq; /* protected by rtnl_mutex */
int ifindex;
unsigned int dev_unreg_count;
/* Note that @hash_mix can be read millions times per second,
* it is critical that it is on a read_mostly cache line.
*/
u32 hash_mix;
struct net_device *loopback_dev; /* The loopback */
/* core fib_rules */
struct list_head rules_ops;
struct net_device *loopback_dev; /* The loopback */
struct netns_core core;
struct netns_mib mib;
struct netns_packet packet;
@ -171,7 +181,6 @@ struct net {
struct sock *crypto_nlsk;
#endif
struct sock *diag_nlsk;
atomic_t fnhe_genid;
} __randomize_layout;
#include <linux/seq_file_net.h>