Re: [dpdk-dev] [PATCH] mbuf: add bulk free function

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Van Haaren, Harry
> Sent: Wednesday, September 11, 2019 12:30 PM
> To: Stephen Hemminger <stephen@networkplumber.org>; Morten Brørup <mb@smartsharesystems.com>
> Cc: olivier.matz@6wind.com; dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] mbuf: add bulk free function
> 
> > -----Original Message-----
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Stephen Hemminger
> > Sent: Wednesday, September 11, 2019 12:19 PM
> > To: Morten Brørup <mb@smartsharesystems.com>
> > Cc: olivier.matz@6wind.com; dev@dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH] mbuf: add bulk free function
> >
> > On Wed, 11 Sep 2019 09:19:08 +0000
> > Morten Brørup <mb@smartsharesystems.com> wrote:
> >
> > > Add function for freeing a bulk of mbufs.
> > >
> > > Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> > > ---
> > >  lib/librte_mbuf/rte_mbuf.h | 17 +++++++++++++++++
> > >  1 file changed, 17 insertions(+)
> > >
> > > diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> > > index 98225ec80..f2e174da1 100644
> > > --- a/lib/librte_mbuf/rte_mbuf.h
> > > +++ b/lib/librte_mbuf/rte_mbuf.h
> > > @@ -1907,6 +1907,23 @@ static inline void rte_pktmbuf_free(struct rte_mbuf
> > *m)
> > >  	}
> > >  }
> > >
> > > +/**
> > > + * Free a bulk of mbufs back into their original mempool.
> > > + *
> > > + *  @param mbufs
> > > + *    Array of pointers to mbufs
> > > + *  @param count
> > > + *    Array size
> > > + */
> > > +static inline void
> > > +rte_pktmbuf_free_bulk(struct rte_mbuf **mbufs, unsigned count)
> > > +{
> > > +	unsigned idx = 0;
> > > +
> > > +	for (idx = 0; idx < count; idx++)
> > > +		rte_pktmbuf_free(mbufs[idx]);
> > > +}
> > > +
> >
> > You can optimize this to use mempool bulk put operation.
> 
> I believe there's a nuance here - not all mbufs may come from the same mempool.
> The for() approach will free each to its "home" mempool.
> The bulk() approach may return mbufs to pools they didn't originate from.
> 
> For performance reasons it would be nice if they did, but we (in the DPDK library)
> should not blindly assume that.

I suppose Stephen is aware of that and suggests something similar to
What many PMDs are already doing. Let say in ixgbe:  
static __rte_always_inline int
ixgbe_tx_free_bufs(struct ixgbe_tx_queue *txq)
{
	....
	for (i = 0; i < txq->tx_rs_thresh; ++i, ++txep) {
                /* free buffers one at a time */
                m = rte_pktmbuf_prefree_seg(txep->mbuf);
                txep->mbuf = NULL;

                if (unlikely(m == NULL))
                        continue;

                if (nb_free >= RTE_IXGBE_TX_MAX_FREE_BUF_SZ ||
                    (nb_free > 0 && m->pool != free[0]->pool)) {
                        rte_mempool_put_bulk(free[0]->pool,
                                             (void **)free, nb_free);
                        nb_free = 0;
                }

                free[nb_free++] = m;
        }
}

Of course generic function will also need to go through all segments in each packet.
 
> We could consider adding a 2nd functions, rte_pktmbuf_free_bulk_to_single_mempool()
> or some better descriptive name.

Probably a good idea too.
Konstantin