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

["Di, ChenxuX" <chenxux.di@intel.com>]

> -----Original Message-----
> From: Ananyev, Konstantin
> Sent: Monday, January 6, 2020 7:36 AM
> To: Di, ChenxuX <chenxux.di@intel.com>; dev@dpdk.org
> Cc: Yang, Qiming <qiming.yang@intel.com>
> Subject: RE: [dpdk-dev] [PATCH v6 3/4] net/ixgbe: cleanup Tx buffers
> 
> 
> > > > Add support to the ixgbe driver for the API
> > > > rte_eth_tx_done_cleanup to force free consumed buffers on Tx ring.

[snip]

> > > > + * 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
> 

At first.
The function ixgbe_xmit_cleanup(struct ixgbe_tx_queue *txq) will cleanup  TXD wb.status.
the number of status cleanuped is always txq->tx_rs_thresh.

The  API rte_eth_tx_done_cleanup() in rte_eth_dev.h show that 
	@param free_cnt
 	*   Maximum number of packets to free. Use 0 to indicate all possible packets
 	*   should be freed. Note that a packet may be using multiple mbufs.
a number must be set while ixgbe_xmit_cleanup and ixgbe_tx_free_bufs only have one parameter txq.
And what should do is not only free buffers and status but also check which bufs are from 
 One packet and count the packet freed. 
So I think it can't be implemented that reuse function xmit_cleanup without change it.
And create a new function with the code of xmit_cleanup will cause many duplication.

Above all , it seem not a perfect idea to reuse ixgbe_xmit_cleanup().

Second.
The function in patch is copy from code in igb_rxtx.c. it already updated in 2017,
The commit id is 8d907d2b79f7a54c809f1c44970ff455fa2865e1.
I trust the logic of code is right.
Actually it don't complete for ixgbe, i40e and ice, while it don't change the value of 
last_desc_cleaned and tx_next_dd. And it's beginning prefer last_desc_cleaned or
 tx_next_dd(for offload or simple) to tx_tail. 

So, I suggest to use the old function and fix the issue.

> >
> >
> > > >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
> > >
> >
>