Messages in this thread Patch in this message |  | | Date | Mon, 16 Dec 2013 16:10:42 +0000 | From | Zoltan Kiss <> | Subject | Re: [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@citrix.com> >> >> --- >> drivers/net/xen-netback/netback.c | 99 +++++++++++++++++++++++++++++++++++-- >> 1 file changed, 94 insertions(+), 5 deletions(-) >> >> diff --git a/drivers/net/xen-netback/netback.c b/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. OK
|  |