net: reorder struct Qdisc for better SMP performance (5e140dfc) · Commits · 戴 / test

dev_queue_xmit() needs to dirty fields "state", "q", "bstats" and "qstats" On x86_64 arch, they currently span three cache lines, involving more cache line ping pongs than necessary, making longer holding of queue spinlock. We can reduce this to one cache line, by grouping all read-mostly fields at the beginning of structure. (Or should I say, all highly modified fields at the end :) ) Before patch : offsetof(struct Qdisc, state)=0x38 offsetof(struct Qdisc, q)=0x48 offsetof(struct Qdisc, bstats)=0x80 offsetof(struct Qdisc, qstats)=0x90 sizeof(struct Qdisc)=0xc8 After patch : offsetof(struct Qdisc, state)=0x80 offsetof(struct Qdisc, q)=0x88 offsetof(struct Qdisc, bstats)=0xa0 offsetof(struct Qdisc, qstats)=0xac sizeof(struct Qdisc)=0xc0 Signed-off-by:

Eric Dumazet <dada1@cosmosbay.com> Signed-off-by:

David S. Miller <davem@davemloft.net>

include/linux/gen_stats.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -22,7 +22,7 @@ struct gnet_stats_basic
		{
		__u64 bytes;
		__u32 packets;
		};
		} __attribute__ ((packed));

		/**
		* struct gnet_stats_rate_est - rate estimator

include/net/sch_generic.h

+12 −9

Original line number	Diff line number	Diff line
		@@ -49,18 +49,10 @@ struct Qdisc
		int padded;
		struct Qdisc_ops *ops;
		struct qdisc_size_table *stab;
		struct list_head list;
		u32 handle;
		u32 parent;
		atomic_t refcnt;
		unsigned long state;
		struct sk_buff *gso_skb;
		struct sk_buff_head q;
		struct netdev_queue *dev_queue;
		struct Qdisc *next_sched;
		struct list_head list;

		struct gnet_stats_basic bstats;
		struct gnet_stats_queue qstats;
		struct gnet_stats_rate_est rate_est;
		int (reshape_fail)(struct sk_buff skb,
		struct Qdisc *q);
		@@ -71,6 +63,17 @@ struct Qdisc
		* and it will live until better solution will be invented.
		*/
		struct Qdisc *__parent;
		struct netdev_queue *dev_queue;
		struct Qdisc *next_sched;

		struct sk_buff *gso_skb;
		/*
		* For performance sake on SMP, we put highly modified fields at the end
		*/
		unsigned long state;
		struct sk_buff_head q;
		struct gnet_stats_basic bstats;
		struct gnet_stats_queue qstats;
		};

		struct Qdisc_class_ops

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