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xen-netback: worse-case estimate in xenvif_rx_action is underestimating

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The worse-case estimate for skb ring slot usage in xenvif_rx_action()
fails to take fragment page_offset into account. The page_offset does,
however, affect the number of times the fragmentation code calls
start_new_rx_buffer() (i.e. consume another slot) and the worse-case
should assume that will always return true. This patch adds the page_offset
into the DIV_ROUND_UP for each frag.

Unfortunately some frontends aggressively limit the number of requests
they post into the shared ring so to avoid an estimate that is 'too'
pessimal it is capped at MAX_SKB_FRAGS.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Paul Durrant 2014-03-28 11:39:06 +00:00 committed by David S. Miller
parent 0576eddf24
commit a02eb4732c
1 changed files with 20 additions and 1 deletions
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21
drivers/net/xen-netback/netback.c
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@ -493,9 +493,28 @@ static void xenvif_rx_action(struct xenvif *vif)
PAGE_SIZE);
for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
unsigned int size;
unsigned int offset;
size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
max_slots_needed += DIV_ROUND_UP(size, PAGE_SIZE);
offset = skb_shinfo(skb)->frags[i].page_offset;
/* For a worse-case estimate we need to factor in
* the fragment page offset as this will affect the
* number of times xenvif_gop_frag_copy() will
* call start_new_rx_buffer().
*/
max_slots_needed += DIV_ROUND_UP(offset + size,
PAGE_SIZE);
}
/* To avoid the estimate becoming too pessimal for some
* frontends that limit posted rx requests, cap the estimate
* at MAX_SKB_FRAGS.
*/
if (max_slots_needed > MAX_SKB_FRAGS)
max_slots_needed = MAX_SKB_FRAGS;
/* We may need one more slot for GSO metadata */
if (skb_is_gso(skb) &&
(skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4 ||
skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6))