Re: [dpdk-dev] [PATCH v6 3/4] net/ixgbe: cleanup Tx buffers

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

> > > Add support to the ixgbe driver for the API rte_eth_tx_done_cleanup to
> > > force free consumed buffers on Tx ring.
> > >
> > > Signed-off-by: Chenxu Di <chenxux.di@intel.com>
> > > ---
> > >  drivers/net/ixgbe/ixgbe_ethdev.c |   2 +
> > >  drivers/net/ixgbe/ixgbe_rxtx.c   | 116 +++++++++++++++++++++++++++++++
> > >  drivers/net/ixgbe/ixgbe_rxtx.h   |   2 +
> > >  3 files changed, 120 insertions(+)
> > >
> > > diff --git a/drivers/net/ixgbe/ixgbe_ethdev.c
> > > b/drivers/net/ixgbe/ixgbe_ethdev.c
> > > index 2c6fd0f13..0091405db 100644
> > > --- a/drivers/net/ixgbe/ixgbe_ethdev.c
> > > +++ b/drivers/net/ixgbe/ixgbe_ethdev.c
> > > @@ -601,6 +601,7 @@ static const struct eth_dev_ops ixgbe_eth_dev_ops
> > > = {  .udp_tunnel_port_add  = ixgbe_dev_udp_tunnel_port_add,
> > > .udp_tunnel_port_del  = ixgbe_dev_udp_tunnel_port_del,
> > >  .tm_ops_get           = ixgbe_tm_ops_get,
> > > +.tx_done_cleanup      = ixgbe_tx_done_cleanup,
> >
> > Don't see how we can have one tx_done_cleanup() for different tx functions?
> > Vector and scalar TX path use different  format for sw_ring[] entries.
> > Also offload and simile TX paths use different method to track used/free
> > descriptors, and use different functions to free them:
> > offload uses tx_entry next_id, last_id plus txq. last_desc_cleaned, while simple
> > TX paths use tx_next_dd.
> >
> 
> This patches will be not include function for Vector, and I will update my code to
> Make it work for offload and simple .
> >
> > >  };
> > >
> > >  /*
> > > @@ -649,6 +650,7 @@ static const struct eth_dev_ops ixgbevf_eth_dev_ops
> > = {
> > >  .reta_query           = ixgbe_dev_rss_reta_query,
> > >  .rss_hash_update      = ixgbe_dev_rss_hash_update,
> > >  .rss_hash_conf_get    = ixgbe_dev_rss_hash_conf_get,
> > > +.tx_done_cleanup      = ixgbe_tx_done_cleanup,
> > >  };
> > >
> > >  /* store statistics names and its offset in stats structure */ diff
> > > --git a/drivers/net/ixgbe/ixgbe_rxtx.c
> > > b/drivers/net/ixgbe/ixgbe_rxtx.c index fa572d184..520b9c756 100644
> > > --- a/drivers/net/ixgbe/ixgbe_rxtx.c
> > > +++ b/drivers/net/ixgbe/ixgbe_rxtx.c
> > > @@ -2306,6 +2306,122 @@ ixgbe_tx_queue_release_mbufs(struct
> > > ixgbe_tx_queue *txq)  }  }
> > >
> > > +int ixgbe_tx_done_cleanup(void *q, uint32_t free_cnt)
> >
> > That seems to work only for offload(full) TX path (ixgbe_xmit_pkts).
> > Simple(fast) path seems not covered by this function.
> >
> 
> Same as above
> 
> > > +{
> > > +struct ixgbe_tx_queue *txq = (struct ixgbe_tx_queue *)q; struct
> > > +ixgbe_tx_entry *sw_ring; volatile union ixgbe_adv_tx_desc *txr;
> > > +uint16_t tx_first; /* First segment analyzed. */
> > > +uint16_t tx_id;    /* Current segment being processed. */
> > > +uint16_t tx_last;  /* Last segment in the current packet. */ uint16_t
> > > +tx_next;  /* First segment of the next packet. */ int count;
> > > +
> > > +if (txq == NULL)
> > > +return -ENODEV;
> > > +
> > > +count = 0;
> > > +sw_ring = txq->sw_ring;
> > > +txr = txq->tx_ring;
> > > +
> > > +/*
> > > + * tx_tail is the last sent packet on the sw_ring. Goto the end
> > > + * of that packet (the last segment in the packet chain) and
> > > + * then the next segment will be the start of the oldest segment
> > > + * in the sw_ring.
> >
> > Not sure I understand the sentence above.
> > tx_tail is the value of TDT HW register (most recently armed by SW TD).
> > last_id  is the index of last descriptor for multi-seg packet.
> > next_id is just the index of next descriptor in HW TD ring.
> > How do you conclude that it will be the ' oldest segment in the sw_ring'?
> >
> 
> The tx_tail is the last sent packet on the sw_ring. While the xmit_cleanup or
> Tx_free_bufs will be call when the nb_tx_free < tx_free_thresh .
> So the sw_ring[tx_tail].next_id must be the begin of mbufs which are not used or
>  Already freed . then begin the loop until the mbuf is used and begin to free them.
> 
> 
> 
> > Another question why do you need to write your own functions?
> > Why can't you reuse existing ixgbe_xmit_cleanup() for full(offload) path and
> > ixgbe_tx_free_bufs() for simple path?
> > Yes,  ixgbe_xmit_cleanup() doesn't free mbufs, but at least it could be used to
> > determine finished TX descriptors.
> > Based on that you can you can free appropriate sw_ring[] entries.
> >
> 
> The reason why I don't reuse existing function is that they all free several mbufs
> While the free_cnt of the API rte_eth_tx_done_cleanup() is the number of packets.
> It also need to be done that check which mbuffs are from the same packet.

At first, I don't see anything bad if tx_done_cleanup() will free only some segments from
the packet. As long as it is safe - there is no problem with that.
I think rte_eth_tx_done_cleanup() operates on mbuf, not packet quantities.
But in our case I think it doesn't matter, as ixgbe_xmit_cleanup()
mark TXDs as free only when HW is done with all TXDs for that packet.
As long as there is a way to reuse existing code and avoid duplication
(without introducing any degradation) - we should use it.
And I think there is a very good opportunity here to reuse existing
ixgbe_xmit_cleanup() for tx_done_cleanup() implementation.
Moreover because your code doesn't follow ixgbe_xmit_pkts()/ixgbe_xmit_cleanup()
logic and infrastructure, it introduces unnecessary scans over TXD ring,
and in some cases doesn't work as expected: 

+	while (1) {
+		tx_last = sw_ring[tx_id].last_id;
+
+		if (sw_ring[tx_last].mbuf) {
+			if (txr[tx_last].wb.status &
+					IXGBE_TXD_STAT_DD) {
...
+			} else {
+				/*
+				 * mbuf still in use, nothing left to
+				 * free.
+				 */
+				break;

It is not correct to expect that IXGBE_TXD_STAT_DD will be set on last TXD for *every* packet.
We set IXGBE_TXD_CMD_RS bit only on threshold packet last descriptor.
Plus ixgbe_xmit_cleanup() can cleanup TXD wb.status.

So I strongly recommend to reuse ixgbe_xmit_cleanup() here.
It would be much less error prone and will help to avoid code duplication.

Konstantin 

> 
> 
> > >This is the first packet that will be
> > > + * attempted to be freed.
> > > + */
> > > +
> > > +/* Get last segment in most recently added packet. */ tx_last =
> > > +sw_ring[txq->tx_tail].last_id;
> > > +
> > > +/* Get the next segment, which is the oldest segment in ring. */
> > > +tx_first = sw_ring[tx_last].next_id;
> > > +
> > > +/* Set the current index to the first. */ tx_id = tx_first;
> > > +
> > > +/*
> > > + * Loop through each packet. For each packet, verify that an
> > > + * mbuf exists and that the last segment is free. If so, free
> > > + * it and move on.
> > > + */
> > > +while (1) {
> > > +tx_last = sw_ring[tx_id].last_id;
> > > +
> > > +if (sw_ring[tx_last].mbuf) {
> > > +if (!(txr[tx_last].wb.status &
> > > +IXGBE_TXD_STAT_DD))
> > > +break;
> > > +
> > > +/* Get the start of the next packet. */ tx_next =
> > > +sw_ring[tx_last].next_id;
> > > +
> > > +/*
> > > + * Loop through all segments in a
> > > + * packet.
> > > + */
> > > +do {
> > > +rte_pktmbuf_free_seg(sw_ring[tx_id].mbuf);
> > > +sw_ring[tx_id].mbuf = NULL;
> > > +sw_ring[tx_id].last_id = tx_id;
> > > +
> > > +/* Move to next segment. */
> > > +tx_id = sw_ring[tx_id].next_id;
> > > +
> > > +} while (tx_id != tx_next);
> > > +
> > > +/*
> > > + * Increment the number of packets
> > > + * freed.
> > > + */
> > > +count++;
> > > +
> > > +if (unlikely(count == (int)free_cnt)) break; } else {
> > > +/*
> > > + * There are multiple reasons to be here:
> > > + * 1) All the packets on the ring have been
> > > + *    freed - tx_id is equal to tx_first
> > > + *    and some packets have been freed.
> > > + *    - Done, exit
> > > + * 2) Interfaces has not sent a rings worth of
> > > + *    packets yet, so the segment after tail is
> > > + *    still empty. Or a previous call to this
> > > + *    function freed some of the segments but
> > > + *    not all so there is a hole in the list.
> > > + *    Hopefully this is a rare case.
> > > + *    - Walk the list and find the next mbuf. If
> > > + *      there isn't one, then done.
> > > + */
> > > +if (likely(tx_id == tx_first && count != 0)) break;
> > > +
> > > +/*
> > > + * Walk the list and find the next mbuf, if any.
> > > + */
> > > +do {
> > > +/* Move to next segment. */
> > > +tx_id = sw_ring[tx_id].next_id;
> > > +
> > > +if (sw_ring[tx_id].mbuf)
> > > +break;
> > > +
> > > +} while (tx_id != tx_first);
> > > +
> > > +/*
> > > + * Determine why previous loop bailed. If there
> > > + * is not an mbuf, done.
> > > + */
> > > +if (sw_ring[tx_id].mbuf == NULL)
> > > +break;
> > > +}
> > > +}
> > > +
> > > +return count;
> > > +}
> > > +
> > >  static void __attribute__((cold))
> > >  ixgbe_tx_free_swring(struct ixgbe_tx_queue *txq)  { diff --git
> > > a/drivers/net/ixgbe/ixgbe_rxtx.h b/drivers/net/ixgbe/ixgbe_rxtx.h
> > > index 505d344b9..2c3770af6 100644
> > > --- a/drivers/net/ixgbe/ixgbe_rxtx.h
> > > +++ b/drivers/net/ixgbe/ixgbe_rxtx.h
> > > @@ -285,6 +285,8 @@ int ixgbe_rx_vec_dev_conf_condition_check(struct
> > > rte_eth_dev *dev);  int ixgbe_rxq_vec_setup(struct ixgbe_rx_queue
> > > *rxq);  void ixgbe_rx_queue_release_mbufs_vec(struct ixgbe_rx_queue
> > > *rxq);
> > >
> > > +int ixgbe_tx_done_cleanup(void *txq, uint32_t free_cnt);
> > > +
> > >  extern const uint32_t ptype_table[IXGBE_PACKET_TYPE_MAX];
> > >  extern const uint32_t ptype_table_tn[IXGBE_PACKET_TYPE_TN_MAX];
> > >
> > > --
> > > 2.17.1
> >
>