Skip to content

Commit

Permalink
xen-netback: improve ring effeciency for guest RX
Browse files Browse the repository at this point in the history 
There was a bug that netback routines netbk/xenvif_skb_count_slots and
netbk/xenvif_gop_frag_copy disagreed with each other, which caused
netback to push wrong number of responses to netfront, which caused
netfront to eventually crash. The bug was fixed in 6e43fc0
("xen-netback: count number required slots for an skb more carefully").

Commit 6e43fc0 focused on backport-ability. The drawback with the
existing packing scheme is that the ring is not used effeciently, as
stated in 6e43fc0.

skb->data like:
    |        1111|222222222222|3333        |

is arranged as:
    |1111        |222222222222|3333        |

If we can do this:
    |111122222222|22223333    |
That would save one ring slot, which improves ring effeciency.

This patch effectively reverts 6e43fc0. That patch made count_slots
agree with gop_frag_copy, while this patch goes the other way around --
make gop_frag_copy agree with count_slots. The end result is that they
still agree with each other, and the ring is now arranged like:
    |111122222222|22223333    |

The patch that improves packing was first posted by Xi Xong and Matt
Wilson. I only rebase it on top of net-next and rewrite commit message,
so I retain all their SoBs. For more infomation about the original bug
please refer to email listed below and commit message of 6e43fc0.

Original patch:
http://lists.xen.org/archives/html/xen-devel/2013-07/msg00760.html

Signed-off-by: Xi Xiong <xixiong@amazon.com>
Reviewed-by: Matt Wilson <msw@amazon.com>
[ msw: minor code cleanups, rewrote commit message, adjusted code
  to count RX slots instead of meta structures ]
Signed-off-by: Matt Wilson <msw@amazon.com>
Cc: Annie Li <annie.li@oracle.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
[ liuw: rebased on top of net-next tree, rewrote commit message, coding
  style cleanup. ]
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
  • Loading branch information
Wei Liu authored and David S. Miller committed Sep 30, 2013
1 parent 6459082 commit 4f0581d
Showing 1 changed file with 61 additions and 83 deletions.
144 changes: 61 additions & 83 deletions drivers/net/xen-netback/netback.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -47,6 +47,14 @@
#include <asm/xen/hypercall.h>
#include <asm/xen/page.h>

/* SKB control block overlay is used to store useful information when
* doing guest RX.
*/
struct skb_cb_overlay {
int meta_slots_used;
int peek_slots_count;
};

/* Provide an option to disable split event channels at load time as
* event channels are limited resource. Split event channels are
* enabled by default.
Expand Down Expand Up @@ -212,90 +220,60 @@ static bool start_new_rx_buffer(int offset, unsigned long size, int head)
return false;
}

struct xenvif_count_slot_state {
unsigned long copy_off;
bool head;
};

unsigned int xenvif_count_frag_slots(struct xenvif *vif,
unsigned long offset, unsigned long size,
struct xenvif_count_slot_state *state)
{
unsigned count = 0;

offset &= ~PAGE_MASK;

while (size > 0) {
unsigned long bytes;

bytes = PAGE_SIZE - offset;

if (bytes > size)
bytes = size;

if (start_new_rx_buffer(state->copy_off, bytes, state->head)) {
count++;
state->copy_off = 0;
}

if (state->copy_off + bytes > MAX_BUFFER_OFFSET)
bytes = MAX_BUFFER_OFFSET - state->copy_off;

state->copy_off += bytes;

offset += bytes;
size -= bytes;

if (offset == PAGE_SIZE)
offset = 0;

state->head = false;
}

return count;
}

/*
* Figure out how many ring slots we're going to need to send @skb to
* the guest. This function is essentially a dry run of
* xenvif_gop_frag_copy.
*/
unsigned int xenvif_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
{
struct xenvif_count_slot_state state;
unsigned int count;
unsigned char *data;
unsigned i;
int i, copy_off;
struct skb_cb_overlay *sco;

state.head = true;
state.copy_off = 0;
count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);

/* Slot for the first (partial) page of data. */
count = 1;
copy_off = skb_headlen(skb) % PAGE_SIZE;

/* Need a slot for the GSO prefix for GSO extra data? */
if (skb_shinfo(skb)->gso_size)
count++;

data = skb->data;
while (data < skb_tail_pointer(skb)) {
unsigned long offset = offset_in_page(data);
unsigned long size = PAGE_SIZE - offset;
for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
unsigned long bytes;

if (data + size > skb_tail_pointer(skb))
size = skb_tail_pointer(skb) - data;
offset &= ~PAGE_MASK;

count += xenvif_count_frag_slots(vif, offset, size, &state);
while (size > 0) {
BUG_ON(offset >= PAGE_SIZE);
BUG_ON(copy_off > MAX_BUFFER_OFFSET);

data += size;
}
bytes = PAGE_SIZE - offset;

for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
if (bytes > size)
bytes = size;

if (start_new_rx_buffer(copy_off, bytes, 0)) {
count++;
copy_off = 0;
}

count += xenvif_count_frag_slots(vif, offset, size, &state);
if (copy_off + bytes > MAX_BUFFER_OFFSET)
bytes = MAX_BUFFER_OFFSET - copy_off;

copy_off += bytes;

offset += bytes;
size -= bytes;

if (offset == PAGE_SIZE)
offset = 0;
}
}

sco = (struct skb_cb_overlay *)skb->cb;
sco->peek_slots_count = count;
return count;
}

Expand Down Expand Up @@ -327,14 +305,11 @@ static struct xenvif_rx_meta *get_next_rx_buffer(struct xenvif *vif,
return meta;
}

/*
* Set up the grant operations for this fragment. If it's a flipping
* interface, we also set up the unmap request from here.
*/
/* Set up the grant operations for this fragment. */
static void xenvif_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
struct netrx_pending_operations *npo,
struct page *page, unsigned long size,
unsigned long offset, int *head)
unsigned long offset, int head, int *first)
{
struct gnttab_copy *copy_gop;
struct xenvif_rx_meta *meta;
Expand All @@ -358,12 +333,12 @@ static void xenvif_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
if (bytes > size)
bytes = size;

if (start_new_rx_buffer(npo->copy_off, bytes, *head)) {
if (start_new_rx_buffer(npo->copy_off, bytes, head)) {
/*
* Netfront requires there to be some data in the head
* buffer.
*/
BUG_ON(*head);
BUG_ON(*first);

meta = get_next_rx_buffer(vif, npo);
}
Expand Down Expand Up @@ -397,10 +372,10 @@ static void xenvif_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
}

/* Leave a gap for the GSO descriptor. */
if (*head && skb_shinfo(skb)->gso_size && !vif->gso_prefix)
if (*first && skb_shinfo(skb)->gso_size && !vif->gso_prefix)
vif->rx.req_cons++;

*head = 0; /* There must be something in this buffer now. */
*first = 0; /* There must be something in this buffer now. */

}
}
Expand All @@ -426,7 +401,7 @@ static int xenvif_gop_skb(struct sk_buff *skb,
struct xen_netif_rx_request *req;
struct xenvif_rx_meta *meta;
unsigned char *data;
int head = 1;
int first = 1;
int old_meta_prod;

old_meta_prod = npo->meta_prod;
Expand Down Expand Up @@ -462,7 +437,7 @@ static int xenvif_gop_skb(struct sk_buff *skb,
len = skb_tail_pointer(skb) - data;

xenvif_gop_frag_copy(vif, skb, npo,
virt_to_page(data), len, offset, &head);
virt_to_page(data), len, offset, 1, &first);
data += len;
}

Expand All @@ -471,7 +446,7 @@ static int xenvif_gop_skb(struct sk_buff *skb,
skb_frag_page(&skb_shinfo(skb)->frags[i]),
skb_frag_size(&skb_shinfo(skb)->frags[i]),
skb_shinfo(skb)->frags[i].page_offset,
&head);
0, &first);
}

return npo->meta_prod - old_meta_prod;
Expand Down Expand Up @@ -529,10 +504,6 @@ static void xenvif_add_frag_responses(struct xenvif *vif, int status,
}
}

struct skb_cb_overlay {
int meta_slots_used;
};

static void xenvif_kick_thread(struct xenvif *vif)
{
wake_up(&vif->wq);
Expand Down Expand Up @@ -563,19 +534,26 @@ void xenvif_rx_action(struct xenvif *vif)
count = 0;

while ((skb = skb_dequeue(&vif->rx_queue)) != NULL) {
RING_IDX old_rx_req_cons;

vif = netdev_priv(skb->dev);
nr_frags = skb_shinfo(skb)->nr_frags;

old_rx_req_cons = vif->rx.req_cons;
sco = (struct skb_cb_overlay *)skb->cb;
sco->meta_slots_used = xenvif_gop_skb(skb, &npo);

count += nr_frags + 1;
count += vif->rx.req_cons - old_rx_req_cons;

__skb_queue_tail(&rxq, skb);

skb = skb_peek(&vif->rx_queue);
if (skb == NULL)
break;
sco = (struct skb_cb_overlay *)skb->cb;

/* Filled the batch queue? */
/* XXX FIXME: RX path dependent on MAX_SKB_FRAGS */
if (count + MAX_SKB_FRAGS >= XEN_NETIF_RX_RING_SIZE)
if (count + sco->peek_slots_count >= XEN_NETIF_RX_RING_SIZE)
break;
}

Expand Down

0 comments on commit 4f0581d

Please sign in to comment.