RE: [PATCH] net/i40e: Fast release optimizations

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Wednesday, 10 December 2025 18.01
> 
> On Mon, Aug 11, 2025 at 02:47:17PM +0200, Morten Brørup wrote:
> > > From: Morten Brørup [mailto:mb@smartsharesystems.com]
> > > Sent: Tuesday, 24 June 2025 08.13
> > >
> > > When fast releasing mbufs, the mbufs are not accessed, so do not
> prefetch
> > > them.
> > > This saves a mbuf load operation for each fast released TX mbuf.
> > >
> > > When fast release of mbufs is enabled for a TX queue, cache the
> mbuf
> > > mempool pointer in the TX queue structure.
> > > This saves one mbuf load operation for each burst of fast released
> TX
> > > mbufs.
> > >
> > > The txep->mbuf pointer is not used after the mbuf has been freed,
> so do
> > > not reset the pointer.
> > > This saves a txep store operation for each TX mbuf freed.
> > >
> > > Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> > > ---
> >
> > After further consideration, I think this patch should be split in
> two:
> > 1. Remove superfluous code: prefetching mbufs and resetting txep-
> >mbuf pointers.
> > 2. Cache the mbuf mempool pointer for FAST_FREE.
> >
> Hi Morten,
> 
> any plans for a new version of this patch (split or otherwise)? This is
> popped up again on my list in patchwork and want to decide what to do
> with
> it for this release. :-)
> 
> /Bruce

I'll give it another shot.

I took another look at it today, and it looks like the txep->mbuf (i.e. sw_ring[].mbuf) is used by other cleanup functions, so not NULL'ing it needs more detailed review.

Anyway, prefetching the mbufs is a complete waste for fast free, where the mbufs are not being accessed at all. So there's still something to gain here.

Also, my assumption that txq->offloads is hotter than txq->fast_free_mp seems not to hold either, so I'll play around with that too.

I'm considering sending a series of patches with very small steps, so the individual changes are easy to review.

-Morten