4.14-stable review patch. If anyone has any objections, please let me know.
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From: Julian Wiedmann jwi@linux.vnet.ibm.com
[ Upstream commit 6d69b1f1eb7a2edf8a3547f361c61f2538e054bb ]
Using GSO with small MTUs currently results in a substantial throughput regression - which is caused by how qeth needs to map non-linear skbs into its IO buffer elements: compared to a linear skb, each GSO-segmented skb effectively consumes twice as many buffer elements (ie two instead of one) due to the additional header-only part. This causes the Output Queue to be congested with low-utilized IO buffers.
Fix this as follows: If the MSS is low enough so that a non-SG GSO segmentation produces order-0 skbs (currently ~3500 byte), opt out from NETIF_F_SG. This is where we anticipate the biggest savings, since an SG-enabled GSO segmentation produces skbs that always consume at least two buffer elements.
Larger MSS values continue to get a SG-enabled GSO segmentation, since 1) the relative overhead of the additional header-only buffer element becomes less noticeable, and 2) the linearization overhead increases.
With the throughput regression fixed, re-enable NETIF_F_SG by default to reap the significant CPU savings of GSO.
Fixes: 5722963a8e83 ("qeth: do not turn on SG per default") Reported-by: Nils Hoppmann niho@de.ibm.com Signed-off-by: Julian Wiedmann jwi@linux.vnet.ibm.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/s390/net/qeth_core.h | 3 +++ drivers/s390/net/qeth_core_main.c | 31 +++++++++++++++++++++++++++++++ drivers/s390/net/qeth_l2_main.c | 2 ++ drivers/s390/net/qeth_l3_main.c | 2 ++ 4 files changed, 38 insertions(+)
--- a/drivers/s390/net/qeth_core.h +++ b/drivers/s390/net/qeth_core.h @@ -985,6 +985,9 @@ struct qeth_cmd_buffer *qeth_get_setassp int qeth_set_features(struct net_device *, netdev_features_t); int qeth_recover_features(struct net_device *); netdev_features_t qeth_fix_features(struct net_device *, netdev_features_t); +netdev_features_t qeth_features_check(struct sk_buff *skb, + struct net_device *dev, + netdev_features_t features); int qeth_vm_request_mac(struct qeth_card *card); int qeth_push_hdr(struct sk_buff *skb, struct qeth_hdr **hdr, unsigned int len);
--- a/drivers/s390/net/qeth_core_main.c +++ b/drivers/s390/net/qeth_core_main.c @@ -19,6 +19,11 @@ #include <linux/mii.h> #include <linux/kthread.h> #include <linux/slab.h> +#include <linux/if_vlan.h> +#include <linux/netdevice.h> +#include <linux/netdev_features.h> +#include <linux/skbuff.h> + #include <net/iucv/af_iucv.h> #include <net/dsfield.h>
@@ -6505,6 +6510,32 @@ netdev_features_t qeth_fix_features(stru } EXPORT_SYMBOL_GPL(qeth_fix_features);
+netdev_features_t qeth_features_check(struct sk_buff *skb, + struct net_device *dev, + netdev_features_t features) +{ + /* GSO segmentation builds skbs with + * a (small) linear part for the headers, and + * page frags for the data. + * Compared to a linear skb, the header-only part consumes an + * additional buffer element. This reduces buffer utilization, and + * hurts throughput. So compress small segments into one element. + */ + if (netif_needs_gso(skb, features)) { + /* match skb_segment(): */ + unsigned int doffset = skb->data - skb_mac_header(skb); + unsigned int hsize = skb_shinfo(skb)->gso_size; + unsigned int hroom = skb_headroom(skb); + + /* linearize only if resulting skb allocations are order-0: */ + if (SKB_DATA_ALIGN(hroom + doffset + hsize) <= SKB_MAX_HEAD(0)) + features &= ~NETIF_F_SG; + } + + return vlan_features_check(skb, features); +} +EXPORT_SYMBOL_GPL(qeth_features_check); + static int __init qeth_core_init(void) { int rc; --- a/drivers/s390/net/qeth_l2_main.c +++ b/drivers/s390/net/qeth_l2_main.c @@ -963,6 +963,7 @@ static const struct net_device_ops qeth_ .ndo_stop = qeth_l2_stop, .ndo_get_stats = qeth_get_stats, .ndo_start_xmit = qeth_l2_hard_start_xmit, + .ndo_features_check = qeth_features_check, .ndo_validate_addr = eth_validate_addr, .ndo_set_rx_mode = qeth_l2_set_rx_mode, .ndo_do_ioctl = qeth_do_ioctl, @@ -1009,6 +1010,7 @@ static int qeth_l2_setup_netdev(struct q if (card->info.type == QETH_CARD_TYPE_OSD && !card->info.guestlan) { card->dev->hw_features = NETIF_F_SG; card->dev->vlan_features = NETIF_F_SG; + card->dev->features |= NETIF_F_SG; /* OSA 3S and earlier has no RX/TX support */ if (qeth_is_supported(card, IPA_OUTBOUND_CHECKSUM)) { card->dev->hw_features |= NETIF_F_IP_CSUM; --- a/drivers/s390/net/qeth_l3_main.c +++ b/drivers/s390/net/qeth_l3_main.c @@ -2923,6 +2923,7 @@ static const struct net_device_ops qeth_ .ndo_stop = qeth_l3_stop, .ndo_get_stats = qeth_get_stats, .ndo_start_xmit = qeth_l3_hard_start_xmit, + .ndo_features_check = qeth_features_check, .ndo_validate_addr = eth_validate_addr, .ndo_set_rx_mode = qeth_l3_set_multicast_list, .ndo_do_ioctl = qeth_do_ioctl, @@ -2963,6 +2964,7 @@ static int qeth_l3_setup_netdev(struct q card->dev->vlan_features = NETIF_F_SG | NETIF_F_RXCSUM | NETIF_F_IP_CSUM | NETIF_F_TSO; + card->dev->features |= NETIF_F_SG; } } } else if (card->info.type == QETH_CARD_TYPE_IQD) {