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Subject[PATCH 5.3 127/140] net: reorder struct net fields to avoid false sharing
Date
From: Eric Dumazet <edumazet@google.com>

[ Upstream commit 2a06b8982f8f2f40d03a3daf634676386bd84dbc ]

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: 355b98553789 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/net/net_namespace.h | 25 +++++++++++++++++--------
1 file changed, 17 insertions(+), 8 deletions(-)

--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -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;

@@ -93,12 +102,14 @@ 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;
@@ -106,7 +117,6 @@ struct net {
struct list_head fib_notifier_ops; /* Populated by
* register_pernet_subsys()
*/
- 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 netns_xdp xdp;
#endif
struct sock *diag_nlsk;
- atomic_t fnhe_genid;
} __randomize_layout;

#include <linux/seq_file_net.h>

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