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s390/qeth: fix GSO throughput regression

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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: 5722963a8e ("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>
This commit is contained in:
Julian Wiedmann 2017-12-01 10:14:50 +01:00 committed by David S. Miller
parent bc3ab70584
commit 6d69b1f1eb
4 changed files with 38 additions and 0 deletions
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3
drivers/s390/net/qeth_core.h
View file

@ -987,6 +987,9 @@ struct qeth_cmd_buffer *qeth_get_setassparms_cmd(struct qeth_card *,
int qeth_set_features(struct net_device *, netdev_features_t);
void qeth_recover_features(struct net_device *dev);
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);

31
drivers/s390/net/qeth_core_main.c
View file

@ -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>
@ -6438,6 +6443,32 @@ netdev_features_t qeth_fix_features(struct net_device *dev,
}
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;

2
drivers/s390/net/qeth_l2_main.c
View file

@ -960,6 +960,7 @@ static const struct net_device_ops qeth_l2_netdev_ops = {
.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,
@ -1010,6 +1011,7 @@ static int qeth_l2_setup_netdev(struct qeth_card *card)
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;

2
drivers/s390/net/qeth_l3_main.c
View file

@ -2918,6 +2918,7 @@ static const struct net_device_ops qeth_l3_osa_netdev_ops = {
.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,
@ -2958,6 +2959,7 @@ static int qeth_l3_setup_netdev(struct qeth_card *card)
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) {