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Message-ID: <453a50f7-ab7d-3474-cab6-2e83a3c3acbf@nvidia.com> Date: Wed, 31 Aug 2022 16:35:40 +0800 From: Gavin Li <gavinl@...dia.com> To: Si-Wei Liu <si-wei.liu@...cle.com>, stephen@...workplumber.org, davem@...emloft.net, jesse.brandeburg@...el.com, alexander.h.duyck@...el.com, kuba@...nel.org, sridhar.samudrala@...el.com, jasowang@...hat.com, loseweigh@...il.com, netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org, virtio-dev@...ts.oasis-open.org, mst@...hat.com Cc: gavi@...dia.com, parav@...dia.com Subject: Re: [virtio-dev] [PATCH v3 2/2] virtio-net: use mtu size as buffer length for big packets On 8/31/2022 4:33 PM, Si-Wei Liu wrote: > External email: Use caution opening links or attachments > > > On 8/29/2022 7:26 PM, Gavin Li wrote: >> Currently add_recvbuf_big() allocates MAX_SKB_FRAGS segments for big >> packets even when GUEST_* offloads are not present on the device. >> However, if guest GSO is not supported, it would be sufficient to >> allocate segments to cover just up the MTU size and no further. >> Allocating the maximum amount of segments results in a large waste of >> buffer space in the queue, which limits the number of packets that can >> be buffered and can result in reduced performance. >> >> Therefore, if guest GSO is not supported, use the MTU to calculate the >> optimal amount of segments required. >> >> When guest offload is enabled at runtime, RQ already has packets of >> bytes >> less than 64K. So when packet of 64KB arrives, all the packets of such >> size will be dropped. and RQ is now not usable. >> >> So this means that during set_guest_offloads() phase, RQs have to be >> destroyed and recreated, which requires almost driver reload. >> >> If VIRTIO_NET_F_CTRL_GUEST_OFFLOADS has been negotiated, then it should >> always treat them as GSO enabled. >> >> Below is the iperf TCP test results over a Mellanox NIC, using vDPA for >> 1 VQ, queue size 1024, before and after the change, with the iperf >> server running over the virtio-net interface. >> >> MTU(Bytes)/Bandwidth (Gbit/s) >> Before After >> 1500 22.5 22.4 >> 9000 12.8 25.9 >> >> Signed-off-by: Gavin Li <gavinl@...dia.com> >> Reviewed-by: Gavi Teitz <gavi@...dia.com> >> Reviewed-by: Parav Pandit <parav@...dia.com> >> --- >> changelog: >> v2->v3 >> - Addressed comments from Si-Wei >> - Simplify the condition check to enable the optimization >> v1->v2 >> - Addressed comments from Jason, Michael, Si-Wei. >> - Remove the flag of guest GSO support, set sg_num for big packets and >> use it directly >> - Recalculate sg_num for big packets in virtnet_set_guest_offloads >> - Replace the round up algorithm with DIV_ROUND_UP >> --- >> drivers/net/virtio_net.c | 37 ++++++++++++++++++++++++------------- >> 1 file changed, 24 insertions(+), 13 deletions(-) >> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c >> index e1904877d461..d2721e71af18 100644 >> --- a/drivers/net/virtio_net.c >> +++ b/drivers/net/virtio_net.c >> @@ -225,6 +225,9 @@ struct virtnet_info { >> /* I like... big packets and I cannot lie! */ >> bool big_packets; >> >> + /* number of sg entries allocated for big packets */ >> + unsigned int big_packets_sg_num; > Sorry for nit picking, but in my opinion big_packets_num_skbfrags might > be a better name than big_packets_sg_num, where the comment could be > written as "number of skb fragments for big packets" rather than "number > of sg entries allocated for big packets". ACK > > Reviewed-by: Si-Wei Liu <si-wei.liu@...cle.com> > >> + >> /* Host will merge rx buffers for big packets (shake it! shake >> it!) */ >> bool mergeable_rx_bufs; >> >> @@ -1331,10 +1334,10 @@ static int add_recvbuf_big(struct >> virtnet_info *vi, struct receive_queue *rq, >> char *p; >> int i, err, offset; >> >> - sg_init_table(rq->sg, MAX_SKB_FRAGS + 2); >> + sg_init_table(rq->sg, vi->big_packets_sg_num + 2); >> >> - /* page in rq->sg[MAX_SKB_FRAGS + 1] is list tail */ >> - for (i = MAX_SKB_FRAGS + 1; i > 1; --i) { >> + /* page in rq->sg[vi->big_packets_sg_num + 1] is list tail */ >> + for (i = vi->big_packets_sg_num + 1; i > 1; --i) { >> first = get_a_page(rq, gfp); >> if (!first) { >> if (list) >> @@ -1365,7 +1368,7 @@ static int add_recvbuf_big(struct virtnet_info >> *vi, struct receive_queue *rq, >> >> /* chain first in list head */ >> first->private = (unsigned long)list; >> - err = virtqueue_add_inbuf(rq->vq, rq->sg, MAX_SKB_FRAGS + 2, >> + err = virtqueue_add_inbuf(rq->vq, rq->sg, >> vi->big_packets_sg_num + 2, >> first, gfp); >> if (err < 0) >> give_pages(rq, first); >> @@ -3690,13 +3693,27 @@ static bool virtnet_check_guest_gso(const >> struct virtnet_info *vi) >> virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_UFO)); >> } >> >> +static void virtnet_set_big_packets_fields(struct virtnet_info *vi, >> const int mtu) >> +{ >> + bool guest_gso = virtnet_check_guest_gso(vi); >> + >> + /* If device can receive ANY guest GSO packets, regardless of mtu, >> + * allocate packets of maximum size, otherwise limit it to only >> + * mtu size worth only. >> + */ >> + if (mtu > ETH_DATA_LEN || guest_gso) { >> + vi->big_packets = true; >> + vi->big_packets_sg_num = guest_gso ? MAX_SKB_FRAGS : >> DIV_ROUND_UP(mtu, PAGE_SIZE); >> + } >> +} >> + >> static int virtnet_probe(struct virtio_device *vdev) >> { >> int i, err = -ENOMEM; >> struct net_device *dev; >> struct virtnet_info *vi; >> u16 max_queue_pairs; >> - int mtu; >> + int mtu = 0; >> >> /* Find if host supports multiqueue/rss virtio_net device */ >> max_queue_pairs = 1; >> @@ -3784,10 +3801,6 @@ static int virtnet_probe(struct virtio_device >> *vdev) >> INIT_WORK(&vi->config_work, virtnet_config_changed_work); >> spin_lock_init(&vi->refill_lock); >> >> - /* If we can receive ANY GSO packets, we must allocate large >> ones. */ >> - if (virtnet_check_guest_gso(vi)) >> - vi->big_packets = true; >> - >> if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF)) >> vi->mergeable_rx_bufs = true; >> >> @@ -3853,12 +3866,10 @@ static int virtnet_probe(struct virtio_device >> *vdev) >> >> dev->mtu = mtu; >> dev->max_mtu = mtu; >> - >> - /* TODO: size buffers correctly in this case. */ >> - if (dev->mtu > ETH_DATA_LEN) >> - vi->big_packets = true; >> } >> >> + virtnet_set_big_packets_fields(vi, mtu); >> + >> if (vi->any_header_sg) >> dev->needed_headroom = vi->hdr_len; >> >
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