What's the point of RTE_MBUF_PREFETCH_TO_FREE()?

What's the point of RTE_MBUF_PREFETCH_TO_FREE()?

[Mike Stolarchuk]

It seems to be used to prefetch the mbuf in preparation for a free of an
mbuf.

But RTE_MBUF_PREFETCH_TO_FREE() currently does a prefetch for the
ffirst cache line in the mbuf,  but doesn't a free do a put to the mbuf's
mempool?

And isn't the mempool reference in the mbuf's second cache line?

regards,
mts.

Re: What's the point of RTE_MBUF_PREFETCH_TO_FREE()?

[Bruce Richardson]

On Fri, Nov 03, 2017 at 01:28:18PM -0700, Mike Stolarchuk wrote:
> It seems to be used to prefetch the mbuf in preparation for a free of an
> mbuf.
> 
> But RTE_MBUF_PREFETCH_TO_FREE() currently does a prefetch for the
> ffirst cache line in the mbuf,  but doesn't a free do a put to the mbuf's
> mempool?
> 
> And isn't the mempool reference in the mbuf's second cache line?
> 
Yes, that looks like a bug. Doing "git blame" shows that that macro
comes from the original versions of DPDK pre-dpdk.org, and hasn't been
modified since, so it was missed when the mbuf was expanded from 1 line
to 2. I also note that the high-performance ixgbe/i40e vector drivers do
not use this macro, only the slow paths in those drivers do, so I wonder
if it can be removed. If not, it should be fixed.

	/Bruce