ixgbe & if_igb RX ring locking

[Luigi Rizzo]

On Sat, Oct 13, 2012 at 09:49:21PM +0400, Alexander V. Chernikov wrote:
> Hello list!
> 
> 
> Packets receiving code for both ixgbe and if_igb looks like the following:
> 
> 
> ixgbe_msix_que
> 
> -- ixgbe_rxeof()
>    {
>       IXGBE_RX_LOCK(rxr);
>         while
>         {
>            get_packet;
> 
>            -- ixgbe_rx_input()
>               {
>                  ++ IXGBE_RX_UNLOCK(rxr);
>                  if_input(packet);
>                  ++ IXGBE_RX_LOCK(rxr);
>               }
> 
>         }
>       IXGBE_RX_UNLOCK(rxr);
>     }
> 
> Lines marked with ++ appeared in r209068(igb) and r217593(ixgbe).
> 
> These lines probably do LORs masking (if any) well.
> However, such change introduce quite significant performance drop:
> 
> On my routing setup (nearly the same from previous -Intel 10G thread in 
> -net) adding lock/unlock causes 2.8MPPS decrease to 2.3MPPS which is 
> nearly 20%.

one option could be (same as it is done in the timer
routine in dummynet) to build a list of all the packets
that need to be sent to if_input(), and then call
if_input with the entire list outside the lock.

It would be even easier if we modify the various *_input()
routines to handle a list of mbufs instead of just one.

cheers
luigi

> So my questions are:
> 
> Can any real LORs happen in some complex setup? (I can't imagine any).
> If so: maybe we can somehow avoid/workaround such cases? (and consider 
> removing those locks).
> 
> 
> 
> -- 
> WBR, Alexander
> 
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