回复: [PATCH 3/3] net/ixgbe: reduce redundant store operation

[Feifei Wang <Feifei.Wang2@arm.com>]

> -----邮件原件-----
> 发件人: Wang, Haiyue <haiyue.wang@intel.com>
> 发送时间: Monday, December 20, 2021 3:25 PM
> 收件人: Feifei Wang <Feifei.Wang2@arm.com>
> 抄送: dev@dpdk.org; nd <nd@arm.com>; Ruifeng Wang
> <Ruifeng.Wang@arm.com>
> 主题: RE: [PATCH 3/3] net/ixgbe: reduce redundant store operation
> 
> > -----Original Message-----
> > From: Feifei Wang <feifei.wang2@arm.com>
> > Sent: Monday, December 20, 2021 13:51
> > To: Wang, Haiyue <haiyue.wang@intel.com>
> > Cc: dev@dpdk.org; nd@arm.com; Feifei Wang <feifei.wang2@arm.com>;
> > Ruifeng Wang <ruifeng.wang@arm.com>
> > Subject: [PATCH 3/3] net/ixgbe: reduce redundant store operation
> >
> > For free buffer in ixgbe driver, it is unnecessary to store 'NULL'
> > into txep.mbuf. This is because when putting mbuf into Tx queue,
> > tx_tail is the sentinel. And when doing tx_free, tx_next_dd is the
> > sentinel. In all processes, mbuf==NULL is not a condition in check.
> > Thus reset of mbuf is unnecessary and can be omitted.
> >
> > Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
> > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > ---
> >  drivers/net/ixgbe/ixgbe_rxtx.c | 1 -
> >  1 file changed, 1 deletion(-)
> >
> > diff --git a/drivers/net/ixgbe/ixgbe_rxtx.c
> > b/drivers/net/ixgbe/ixgbe_rxtx.c index d7c80d4242..9f3f2e9b50 100644
> > --- a/drivers/net/ixgbe/ixgbe_rxtx.c
> > +++ b/drivers/net/ixgbe/ixgbe_rxtx.c
> > @@ -120,7 +120,6 @@ 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;
> 
> Not sure, but at least found:
> 
> static void __rte_cold
> ixgbe_tx_queue_release_mbufs(struct ixgbe_tx_queue *txq) {
> 	unsigned i;
> 
> 	if (txq->sw_ring != NULL) {
> 		for (i = 0; i < txq->nb_tx_desc; i++) {
> 			if (txq->sw_ring[i].mbuf != NULL) {  <--------------------
> -------- ?
> 				rte_pktmbuf_free_seg(txq-
> >sw_ring[i].mbuf);
> 				txq->sw_ring[i].mbuf = NULL;
> 			}
> 		}
> 	}
> }
> 
Thanks for your remind. I check the function"xx_tx_queue_release_mbufs" and "xx_tx_done_cleanup_full" which
have the check for 'sw_ring->buf == NULL'.  I find the scheme of free buffers in scalar path and vector path are different:
For scalar, it should support jumbo frame, so it cannot use 'tx_next_dd' to find free buffer index
For vector, free a packet means free a buffer, just use tx_next_dd can find the start index of free buffer.
At last, store operation of NULL for freed buffer is necessary for scalar path. And I will just keep the vector path path.

> >
> >  		if (unlikely(m == NULL))
> >  			continue;
> > --
> > 2.25.1