[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH net-next v2 6/9] xen-netback: Handle guests with too many frags

On 13/12/13 15:43, Wei Liu wrote:
On Thu, Dec 12, 2013 at 11:48:14PM +0000, Zoltan Kiss wrote:
Xen network protocol had implicit dependency on MAX_SKB_FRAGS. Netback has to
handle guests sending up to XEN_NETBK_LEGACY_SLOTS_MAX slots. To achieve that:
- create a new skb
- map the leftover slots to its frags (no linear buffer here!)
- chain it to the previous through skb_shinfo(skb)->frag_list
- map them
- copy the whole stuff into a brand new skb and send it to the stack
- unmap the 2 old skb's pages

Do you see performance regression with this approach?
Well, it was pretty hard to reproduce that behaviour even with NFS. I don't think it happens often enough that it causes a noticable performance regression. Anyway, it would be just as slow as the current grant copy with coalescing, maybe a bit slower due to the unmapping. But at least we use a core network function to do the coalescing. Or, if you mean the generic performance, if this problem doesn't appear, then no, I don't see performance regression.

Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>

  drivers/net/xen-netback/netback.c |   99 +++++++++++++++++++++++++++++++++++--
  1 file changed, 94 insertions(+), 5 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c 
index e26cdda..f6ed1c8 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -906,11 +906,15 @@ static struct gnttab_map_grant_ref 
*xenvif_get_requests(struct xenvif *vif,
        u16 pending_idx = *((u16 *)skb->data);
        int start;
        pending_ring_idx_t index;
-       unsigned int nr_slots;
+       unsigned int nr_slots, frag_overflow = 0;

        /* At this point shinfo->nr_frags is in fact the number of
         * slots, which can be as large as XEN_NETBK_LEGACY_SLOTS_MAX.
+       if (shinfo->nr_frags > MAX_SKB_FRAGS) {
+               frag_overflow = shinfo->nr_frags - MAX_SKB_FRAGS;
+               shinfo->nr_frags = MAX_SKB_FRAGS;
+       }
        nr_slots = shinfo->nr_frags;

It is also probably better to check whether shinfo->nr_frags is too
large which makes frag_overflow > MAX_SKB_FRAGS. I know skb should be
already be valid at this point but it wouldn't hurt to be more careful.
Ok, I've added this:
        /* At this point shinfo->nr_frags is in fact the number of
         * slots, which can be as large as XEN_NETBK_LEGACY_SLOTS_MAX.
+       if (shinfo->nr_frags > MAX_SKB_FRAGS) {
+               if (shinfo->nr_frags > XEN_NETBK_LEGACY_SLOTS_MAX) return NULL;
+               frag_overflow = shinfo->nr_frags - MAX_SKB_FRAGS;

        /* Skip first skb fragment if it is on same page as header fragment. */
@@ -926,6 +930,33 @@ static struct gnttab_map_grant_ref 
*xenvif_get_requests(struct xenvif *vif,

        BUG_ON(shinfo->nr_frags > MAX_SKB_FRAGS);

+       if (frag_overflow) {
+               struct sk_buff *nskb = alloc_skb(NET_SKB_PAD + NET_IP_ALIGN,
+                               GFP_ATOMIC | __GFP_NOWARN);
+               if (unlikely(nskb == NULL)) {
+                       netdev_err(vif->dev,
+                                  "Can't allocate the frag_list skb.\n");
+                       return NULL;
+               }
+               /* Packets passed to netif_rx() must have some headroom. */
+               skb_reserve(nskb, NET_SKB_PAD + NET_IP_ALIGN);

The code to call alloc_skb and skb_reserve is copied from other
location. I would like to have a dedicated function to allocate skb in
netback if possible.

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.