• Paul Durrant's avatar
    xen-netback: improve guest-receive-side flow control · ca2f09f2
    Paul Durrant authored
    
    
    The way that flow control works without this patch is that, in start_xmit()
    the code uses xenvif_count_skb_slots() to predict how many slots
    xenvif_gop_skb() will consume and then adds this to a 'req_cons_peek'
    counter which it then uses to determine if the shared ring has that amount
    of space available by checking whether 'req_prod' has passed that value.
    If the ring doesn't have space the tx queue is stopped.
    xenvif_gop_skb() will then consume slots and update 'req_cons' and issue
    responses, updating 'rsp_prod' as it goes. The frontend will consume those
    responses and post new requests, by updating req_prod. So, req_prod chases
    req_cons which chases rsp_prod, and can never exceed that value. Thus if
    xenvif_count_skb_slots() ever returns a number of slots greater than
    xenvif_gop_skb() uses, req_cons_peek will get to a value that req_prod cannot
    possibly achieve (since it's limited by the 'real' req_cons) and, if this
    happens enough times, req_cons_peek gets more than a ring size ahead of
    req_cons and the tx queue then remains stopped forever waiting for an
    unachievable amount of space to become available in the ring.
    
    Having two routines trying to calculate the same value is always going to be
    fragile, so this patch does away with that. All we essentially need to do is
    make sure that we have 'enough stuff' on our internal queue without letting
    it build up uncontrollably. So start_xmit() makes a cheap optimistic check
    of how much space is needed for an skb and only turns the queue off if that
    is unachievable. net_rx_action() is the place where we could do with an
    accurate predicition but, since that has proven tricky to calculate, a cheap
    worse-case (but not too bad) estimate is all we really need since the only
    thing we *must* prevent is xenvif_gop_skb() consuming more slots than are
    available.
    
    Without this patch I can trivially stall netback permanently by just doing
    a large guest to guest file copy between two Windows Server 2008R2 VMs on a
    single host.
    
    Patch tested with frontends in:
    - Windows Server 2008R2
    - CentOS 6.0
    - Debian Squeeze
    - Debian Wheezy
    - SLES11
    Signed-off-by: default avatarPaul Durrant <paul.durrant@citrix.com>
    Cc: Wei Liu <wei.liu2@citrix.com>
    Cc: Ian Campbell <ian.campbell@citrix.com>
    Cc: David Vrabel <david.vrabel@citrix.com>
    Cc: Annie Li <annie.li@oracle.com>
    Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    Acked-by: default avatarWei Liu <wei.liu2@citrix.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    ca2f09f2