From c9ea670c1dc7e3f111d8139f915082b60c9c1ffe Mon Sep 17 00:00:00 2001 From: Yuanhan Liu Date: Wed, 11 Jan 2017 12:27:11 +0800 Subject: [PATCH] net/virtio: fix performance regression due to TSO TSO is now enabled, but it's not actually being used by default in a simple L2 forward mode. In such case, we have to zero the virtio net headers, to inform the vhost backend that no offload is being used: hdr->csum_start = 0; hdr->csum_offset = 0; hdr->flags = 0; hdr->gso_type = 0; hdr->gso_size = 0; hdr->hdr_len = 0; Such writes could be very costly; it introduces severe cache issues: The above operations introduce cache write for each packet, which stalls the read operation from the vhost backend. The fact that virtio net header is initiated to zero in PMD driver init stage means that these costly writes are unnecessary and could be avoided: if (hdr->csum_start != 0) hdr->csum_start = 0; And that's what the macro ASSIGN_UNLESS_EQUAL does. With this, the performance drop introduced by TSO enabling is recovered: it could be up to 20% in micro benchmarking. Fixes: 58169a9c8153 ("net/virtio: support Tx checksum offload") Fixes: 696573046e9e ("net/virtio: support TSO") Cc: stable@dpdk.org Cc: Olivier Matz Cc: Maxime Coquelin Cc: Michael S. Tsirkin Signed-off-by: Yuanhan Liu Reviewed-by: Maxime Coquelin Reviewed-by: Olivier Matz --- drivers/net/virtio/virtio_rxtx.c | 18 ++++++++++++------ 1 file changed, 12 insertions(+), 6 deletions(-) diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c index b29565e590..21948372db 100644 --- a/drivers/net/virtio/virtio_rxtx.c +++ b/drivers/net/virtio/virtio_rxtx.c @@ -267,6 +267,12 @@ tx_offload_enabled(struct virtio_hw *hw) vtpci_with_feature(hw, VIRTIO_NET_F_HOST_TSO6); } +/* avoid write operation when necessary, to lessen cache issues */ +#define ASSIGN_UNLESS_EQUAL(var, val) do { \ + if ((var) != (val)) \ + (var) = (val); \ +} while (0) + static inline void virtqueue_enqueue_xmit(struct virtnet_tx *txvq, struct rte_mbuf *cookie, uint16_t needed, int use_indirect, int can_push) @@ -346,9 +352,9 @@ virtqueue_enqueue_xmit(struct virtnet_tx *txvq, struct rte_mbuf *cookie, break; default: - hdr->csum_start = 0; - hdr->csum_offset = 0; - hdr->flags = 0; + ASSIGN_UNLESS_EQUAL(hdr->csum_start, 0); + ASSIGN_UNLESS_EQUAL(hdr->csum_offset, 0); + ASSIGN_UNLESS_EQUAL(hdr->flags, 0); break; } @@ -364,9 +370,9 @@ virtqueue_enqueue_xmit(struct virtnet_tx *txvq, struct rte_mbuf *cookie, cookie->l3_len + cookie->l4_len; } else { - hdr->gso_type = 0; - hdr->gso_size = 0; - hdr->hdr_len = 0; + ASSIGN_UNLESS_EQUAL(hdr->gso_type, 0); + ASSIGN_UNLESS_EQUAL(hdr->gso_size, 0); + ASSIGN_UNLESS_EQUAL(hdr->hdr_len, 0); } } -- 2.20.1