[PATCH v7 1/4] net/i40e: use inline prefetch function

[Morten Brørup]

> From: Tyler Retzlaff [mailto:roretzla at linux.microsoft.com]
> Sent: Wednesday, 27 March 2024 19.15
> 
> On Tue, Mar 26, 2024 at 11:16:10AM +0100, Morten Brørup wrote:
> > > From: Tyler Retzlaff [mailto:roretzla at linux.microsoft.com]
> > > Sent: Wednesday, 20 March 2024 23.02
> > >
> > > Don't directly access the cacheline1 field in rte_mbuf struct for
> > > prefetch instead just use rte_mbuf_prefetch_part2() to prefetch.
> > >
> > > Signed-off-by: Tyler Retzlaff <roretzla at linux.microsoft.com>
> > > ---
> > >  drivers/net/i40e/i40e_rxtx_vec_avx512.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/net/i40e/i40e_rxtx_vec_avx512.c
> > > b/drivers/net/i40e/i40e_rxtx_vec_avx512.c
> > > index f3050cd..0238b03 100644
> > > --- a/drivers/net/i40e/i40e_rxtx_vec_avx512.c
> > > +++ b/drivers/net/i40e/i40e_rxtx_vec_avx512.c
> > > @@ -826,7 +826,7 @@
> > >  		free[0] = m;
> > >  		nb_free = 1;
> > >  		for (i = 1; i < n; i++) {
> > > -			rte_prefetch0(&txep[i + 3].mbuf->cacheline1);
> > > +			rte_mbuf_prefetch_part2(txep[i + 3].mbuf);
> >
> > @Yuying Zhang:
> > If this prefetch is for m->next, I think it can be omitted if m->next
> is in the first cache line:
> >
> > -			rte_prefetch0(&txep[i + 3].mbuf->cacheline1);
> > +#if RTE_IOVA_IN_MBUF
> > +			rte_mbuf_prefetch_part2(txep[i + 3].mbuf);
> > +#endif
> 
> Yuying Zhang any reply here to confirm?
> 
> If not I will leave it unconditionally prefetch.

I think you should leave it unconditionally prefetch under all circumstances.

An optimization to omit it (if it can be conditionally omitted) belongs in a separate patch. Also, Intel might want to test and document the performance improvement of such a patch.

> 
> >
> > If so, it belongs in a separate patch anyway.
> >
> > >  			m = rte_pktmbuf_prefree_seg(txep[i].mbuf);
> > >  			if (likely(m)) {
> > >  				if (likely(m->pool == free[0]->pool)) {
> > > --
> > > 1.8.3.1
> >
> > Reviewed-by: Morten Brørup <mb at smartsharesystems.com>