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Commit 2a06b898 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
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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: 355b9855 ("netns: provide pure entropy for net_hash_mix()")
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Reported-by: default avatarkernel test robot <oliver.sang@intel.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 50c7d2ba

include/net/net_namespace.h

+17 −8
Original line number Diff line number Diff line
@@ -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>