Re: [PATCH v20 25/25] app/pdump: preserve VLAN tags in captured packets

Bruce Richardson
2026-03-25T16:22:59Z

On Wed, Mar 25, 2026 at 09:19:21AM -0700, Stephen Hemminger wrote:
> On Wed, 25 Mar 2026 10:36:56 +0100
> Morten Brørup <mb@smartsharesystems.com> wrote:
>
> > If an application clones packets instead of copying them, it is probably for performance reasons.
> > If the drivers start copying those clones, it may defeat the performance purpose.
> >
> > <brainstorming>
> > Maybe segmentation can be used instead of copying the full packet:
> > Make the "copy" packet of two (or more) segments, where the header is copied into a new mbuf (where the VLAN tag is added), and the remaining part of the packet uses an indirect mbuf referring to the "original" packet at the offset after the header.
> > </brainstorming>
> >
> > Furthermore...
> > If drivers start copying packets in the Tx function, the Tx queue should have its own mbuf pool to allocate these mbufs from.
> > Drivers should not steal mbufs from the pools used by the packets being transmitted.
> > E.g. if a segmented packet has a small mbuf for the first few bytes, followed by a large mbuf (from another pool) for the remaining bytes.
> > Or if the "original" mbuf comes from a mempool allocated on different CPU socket, the "copy" would too.
>
>
> The problem with the Tx function is how backpressure gets handled.
> Not sure that it is documented well enough that if a packet is not sent
> due to backpressure, the mbuf in the array may still have been replaced.

Most drivers should be able to check for space in a Tx ring, or whatever
other backpressure mechanism is being used, before modifying a buffer.

/Bruce