em/igb if_transmit (drbr) and ALTQ

[Andre Oppermann]

On 12.11.2012 20:15, Karim wrote:
> Hi all,
>
> I have been following the current discussions on igb tx/rx locking with great interest and I though
> I would point out (as it was before pointed out in kern/138392) that any driver setting if_start to
> NULL and using if_transmit (multi-queue or not) will effectively break ALTQ.
>
> The reason for this can be found in tbr_timeout() in altq_subr.c where the token bucket regulator
> goes through all ALTQ registered interface and tries to 'kick' them into sending through if_start().
>
> tbr_timeout() :
>
> ...
>
>          for (ifp = TAILQ_FIRST(&V_ifnet); ifp;
>              ifp = TAILQ_NEXT(ifp, if_list)) {
>              /* read from if_snd unlocked */
>              if (!TBR_IS_ENABLED(&ifp->if_snd))
>                  continue;
>              active++;
>              if (!IFQ_IS_EMPTY(&ifp->if_snd) &&
>                  ifp->if_start != NULL)                     /* if_start is NULL if if_transmit is
> used in em/igb driver */
>                  (*ifp->if_start)(ifp);
>          }
>
> ...
>
> As you can see if_start is NULL on those new multi-queue enabled drivers which has for net effect to
> 'break' ALTQ's token bucket regulator.
>
> I am writing this because I am interested in your comments on how this can be fixed properly looking
> forward. The whole range of suggestions; from 'don't compile with EM_MULTIQUEUE defined' to 'here is
> how you can make ALTQ use drbr' will help.

This whole area needs some serious re-consideration for 10.0.
Even without the if_start issue ALTQ is somewhat broken because
the DMA rings nowadays are just too deep.

See this recent message to this list for some thoughts:
  http://lists.freebsd.org/pipermail/freebsd-net/2012-November/033780.html

On top of refining the stack/driver boundary I'm thinking of a
dedicated ethernet layer to consolidate all the ethernet extensions,
have a single place to go and to prevent the drivers from re-inventing
the wheel all over every time.

I'm working towards it and leaning into various drivers while trying
to bring in the hybrid interrupt/polling mode with life-lock prevention.

It'll take a few weeks until I'm ready to show a stack/ethernet/driver
prototype for discussion.  Parts may surface earlier in my tcp_workqueue
svn branch.

-- 
Andre