enqueue_to_backlog() is showing up in kernel profiles on hosts with many cores, when RFS/RPS is used. The following softnet_data fields need to be updated: - input_queue_tail - input_pkt_queue (next, prev, qlen, lock) - backlog.state (if input_pkt_queue was empty) Unfortunately they are currenly using two cache lines: /* --- cacheline 3 boundary (192 bytes) --- */ call_single_data_t csd __attribute__((__aligned__(64))); /* 0xc0 0x20 */ struct softnet_data * rps_ipi_next; /* 0xe0 0x8 */ unsigned int cpu; /* 0xe8 0x4 */ unsigned int input_queue_tail; /* 0xec 0x4 */ struct sk_buff_head input_pkt_queue; /* 0xf0 0x18 */ /* --- cacheline 4 boundary (256 bytes) was 8 bytes ago --- */ struct napi_struct backlog __attribute__((__aligned__(8))); /* 0x108 0x1f0 */ Add one ____cacheline_aligned_in_smp to make sure they now are using a single cache line. Also, because napi_struct has written fields, make @state its first field. We want to make sure that cpus adding packets to sd->input_pkt_queue are not slowing down cpus processing their backlog because of false sharing. After this patch new layout is: /* --- cacheline 5 boundary (320 bytes) --- */ long int pad[3] __attribute__((__aligned__(64))); /* 0x140 0x18 */ unsigned int input_queue_tail; /* 0x158 0x4 */ /* XXX 4 bytes hole, try to pack */ struct sk_buff_head input_pkt_queue; /* 0x160 0x18 */ struct napi_struct backlog __attribute__((__aligned__(8))); /* 0x178 0x1f0 */ Signed-off-by: Eric Dumazet --- include/linux/netdevice.h | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h index 7f5aad5cc9a1994f95ba9037d3a4af27eef9d5e3..9c1e5042c5e7646c0aa9e8f4e160c78ea27a639a 100644 --- a/include/linux/netdevice.h +++ b/include/linux/netdevice.h @@ -377,6 +377,8 @@ struct napi_config { * Structure for NAPI scheduling similar to tasklet but with weighting */ struct napi_struct { + /* This field should be first or softnet_data.backlog needs tweaks. */ + unsigned long state; /* The poll_list must only be managed by the entity which * changes the state of the NAPI_STATE_SCHED bit. This means * whoever atomically sets that bit can add this napi_struct @@ -385,7 +387,6 @@ struct napi_struct { */ struct list_head poll_list; - unsigned long state; int weight; u32 defer_hard_irqs_count; int (*poll)(struct napi_struct *, int); @@ -3529,9 +3530,17 @@ struct softnet_data { call_single_data_t csd ____cacheline_aligned_in_smp; struct softnet_data *rps_ipi_next; unsigned int cpu; + + /* We force a cacheline alignment from here, to hold together + * input_queue_tail, input_pkt_queue and backlog.state. + * We add holes so that backlog.state is the last field + * of this cache line. + */ + long pad[3] ____cacheline_aligned_in_smp; unsigned int input_queue_tail; #endif struct sk_buff_head input_pkt_queue; + struct napi_struct backlog; struct numa_drop_counters drop_counters; -- 2.51.1.821.gb6fe4d2222-goog