Re: [PATCH v1] ixgbe: remove vector pmd burst size restriction

On 31/07/15 11:21, Ananyev, Konstantin wrote:

quoted

-----Original Message-----
From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Zoltan Kiss
Sent: Friday, July 31, 2015 11:04 AM
To: Liang, Cunming; dev@dpdk.org
Subject: Re: [dpdk-dev] [PATCH v1] ixgbe: remove vector pmd burst size restriction



On 31/07/15 09:17, Cunming Liang wrote:

quoted

The patch removes the restriction of burst size on a constant 32.

On receive side, the burst size floor now aligns to RTE_IXGBE_DESCS_PER_LOOP power of 2.
According to this rule, the burst size less than 4 still won't receive anything.
(Before this change, the burst size less than 32 can't receive anything.)

On transmit side, the max burst size no longer bind with a constant, however it still
require to check the cross tx_rs_thresh violation.

There's no obvious performance drop found on both recv_pkts_vec
and recv_scattered_pkts_vec on burst size 32.

Signed-off-by: Cunming Liang <redacted>
---
   drivers/net/ixgbe/ixgbe_rxtx.c     |  3 ++-
   drivers/net/ixgbe/ixgbe_rxtx.h     |  4 +---
   drivers/net/ixgbe/ixgbe_rxtx_vec.c | 35 ++++++++++++++++-------------------
   3 files changed, 19 insertions(+), 23 deletions(-)

diff --git a/drivers/net/ixgbe/ixgbe_rxtx.c b/drivers/net/ixgbe/ixgbe_rxtx.c
index 3f808b3..dbdb761 100644
--- a/drivers/net/ixgbe/ixgbe_rxtx.c
+++ b/drivers/net/ixgbe/ixgbe_rxtx.c

@@ -4008,7 +4008,8 @@ ixgbe_set_rx_function(struct rte_eth_dev *dev)
   	 */
   	} else if (adapter->rx_vec_allowed) {
   		PMD_INIT_LOG(DEBUG, "Vector rx enabled, please make sure RX "
-				   "burst size no less than 32.");
+				    "burst size no less than 4 (port=%d).",
+			     dev->data->port_id);

I think it would be better to use RTE_IXGBE_DESCS_PER_LOOP instead of a
constant 4.

quoted

   		dev->rx_pkt_burst = ixgbe_recv_pkts_vec;
   	} else if (adapter->rx_bulk_alloc_allowed) {

diff --git a/drivers/net/ixgbe/ixgbe_rxtx.h b/drivers/net/ixgbe/ixgbe_rxtx.h
index 113682a..eb931fe 100644
--- a/drivers/net/ixgbe/ixgbe_rxtx.h
+++ b/drivers/net/ixgbe/ixgbe_rxtx.h

@@ -47,9 +47,7 @@
   	(uint64_t) ((mb)->buf_physaddr + RTE_PKTMBUF_HEADROOM)

   #ifdef RTE_IXGBE_INC_VECTOR
-#define RTE_IXGBE_VPMD_RX_BURST         32
-#define RTE_IXGBE_VPMD_TX_BURST         32
-#define RTE_IXGBE_RXQ_REARM_THRESH      RTE_IXGBE_VPMD_RX_BURST
+#define RTE_IXGBE_RXQ_REARM_THRESH      32
   #define RTE_IXGBE_TX_MAX_FREE_BUF_SZ    64
   #endif

diff --git a/drivers/net/ixgbe/ixgbe_rxtx_vec.c b/drivers/net/ixgbe/ixgbe_rxtx_vec.c
index 1c16dec..b72f817 100644
--- a/drivers/net/ixgbe/ixgbe_rxtx_vec.c
+++ b/drivers/net/ixgbe/ixgbe_rxtx_vec.c

@@ -199,13 +199,11 @@ desc_to_olflags_v(__m128i descs[4], struct rte_mbuf **rx_pkts)
   #endif

   /*
- * vPMD receive routine, now only accept (nb_pkts == RTE_IXGBE_VPMD_RX_BURST)
- * in one loop
+ * vPMD raw receive routine

I would keep some warning there, like "(if nb_pkts <
RTE_IXGBE_DESCS_PER_LOOP, won't receive anything)"

quoted

    *
    * Notice:
- * - nb_pkts < RTE_IXGBE_VPMD_RX_BURST, just return no packet
- * - nb_pkts > RTE_IXGBE_VPMD_RX_BURST, only scan RTE_IXGBE_VPMD_RX_BURST
- *   numbers of DD bit
+ * - floor align nb_pkts to a RTE_IXGBE_DESC_PER_LOOP power-of-two
+ * - 'nb_pkts < 4' causes 0 packet receiving

Again, RTE_IXGBE_DESCS_PER_LOOP would be better than 4

quoted

    * - don't support ol_flags for rss and csum err
    */
   static inline uint16_t

@@ -240,8 +238,7 @@ _recv_raw_pkts_vec(struct ixgbe_rx_queue *rxq, struct rte_mbuf **rx_pkts,
   	__m128i dd_check, eop_check;
   #endif /* RTE_NEXT_ABI */

-	if (unlikely(nb_pkts < RTE_IXGBE_VPMD_RX_BURST))
-		return 0;
+	nb_pkts = RTE_ALIGN_FLOOR(nb_pkts, RTE_IXGBE_DESCS_PER_LOOP);

   	/* Just the act of getting into the function from the application is
   	 * going to cost about 7 cycles */

@@ -310,7 +307,7 @@ _recv_raw_pkts_vec(struct ixgbe_rx_queue *rxq, struct rte_mbuf **rx_pkts,
   	 * D. fill info. from desc to mbuf
   	 */
   #endif /* RTE_NEXT_ABI */
-	for (pos = 0, nb_pkts_recd = 0; pos < RTE_IXGBE_VPMD_RX_BURST;
+	for (pos = 0, nb_pkts_recd = 0; pos < nb_pkts;
   			pos += RTE_IXGBE_DESCS_PER_LOOP,
   			rxdp += RTE_IXGBE_DESCS_PER_LOOP) {
   		__m128i descs[RTE_IXGBE_DESCS_PER_LOOP];

@@ -450,13 +447,11 @@ _recv_raw_pkts_vec(struct ixgbe_rx_queue *rxq, struct rte_mbuf **rx_pkts,
   }

   /*
- * vPMD receive routine, now only accept (nb_pkts == RTE_IXGBE_VPMD_RX_BURST)
- * in one loop
+ * vPMD receive routine

Same note here as above

quoted

    *
    * Notice:
- * - nb_pkts < RTE_IXGBE_VPMD_RX_BURST, just return no packet
- * - nb_pkts > RTE_IXGBE_VPMD_RX_BURST, only scan RTE_IXGBE_VPMD_RX_BURST
- *   numbers of DD bit
+ * - floor align nb_pkts to a RTE_IXGBE_DESC_PER_LOOP power-of-two
+ * - 'nb_pkts < 4' causes 0 packet receiving
    * - don't support ol_flags for rss and csum err
    */
   uint16_t

@@ -470,12 +465,11 @@ static inline uint16_t
   reassemble_packets(struct ixgbe_rx_queue *rxq, struct rte_mbuf **rx_bufs,
   		uint16_t nb_bufs, uint8_t *split_flags)
   {
-	struct rte_mbuf *pkts[RTE_IXGBE_VPMD_RX_BURST]; /*finished pkts*/
+	struct rte_mbuf *pkts[nb_bufs]; /*finished pkts*/
   	struct rte_mbuf *start = rxq->pkt_first_seg;
   	struct rte_mbuf *end =  rxq->pkt_last_seg;
   	unsigned pkt_idx, buf_idx;

-
   	for (buf_idx = 0, pkt_idx = 0; buf_idx < nb_bufs; buf_idx++) {
   		if (end != NULL) {
   			/* processing a split packet */

@@ -535,14 +529,17 @@ reassemble_packets(struct ixgbe_rx_queue *rxq, struct rte_mbuf **rx_bufs,
    *
    * Notice:
    * - don't support ol_flags for rss and csum err
- * - now only accept (nb_pkts == RTE_IXGBE_VPMD_RX_BURST)
+ * - will floor align nb_pkts to a RTE_IXGBE_DESC_PER_LOOP power-of-two
+ * - 'nb_pkts < 4' causes 0 packet receiving
    */
   uint16_t
   ixgbe_recv_scattered_pkts_vec(void *rx_queue, struct rte_mbuf **rx_pkts,
   		uint16_t nb_pkts)
   {
   	struct ixgbe_rx_queue *rxq = rx_queue;
-	uint8_t split_flags[RTE_IXGBE_VPMD_RX_BURST] = {0};
+	uint8_t split_flags[nb_pkts];
+
+	memset(split_flags, 0, nb_pkts);

   	/* get some new buffers */
   	uint16_t nb_bufs = _recv_raw_pkts_vec(rxq, rx_pkts, nb_pkts,

After this _recv_raw_pkts_vec it checks 32 bytes in split_flags (4x8
bytes), that can overrun or miss some flags.
Btw. Bruce just fixed that part in "ixgbe: fix check for split packets"

Ah yes, missed that when reviewing, that code would be broken if nb_bufs > 32:

         const uint64_t *split_fl64 = (uint64_t *)split_flags;
         if (rxq->pkt_first_seg == NULL &&
                         split_fl64[0] == 0 && split_fl64[1] == 0 &&
                         split_fl64[2] == 0 && split_fl64[3] == 0)
                 return nb_bufs;

right?

Yes. And if nb_bufs < (32 -RTE_IXGBE_DESCS_PER_LOOP), it would address into memory outside the array.

Another thing, that I just thought about:
Right now we invoke ixgbe_rxq_rearm() only at the start of _recv_raw_pkts_vec().
Before it was ok, as _recv_raw_pkts_vec() would never try to read more then 32 RXDs.
But what would happen if nb_pkts > rxq->nb_desc and rxq->rxrearm_nb == 0?

Yes, that call would deplete the RX ring, the card wouldn't be able to receive more, so the receive function wouldn't be called again to rearm the ring.

I suppose,  _recv_raw_pkts_vec() can wrpa around RXD ring and 'receive' same packet twice?
So we probably better still limit nb_pkts <= 32 at _recv_raw_pkts_vec().

I agree we should limit nb_pkts. To avoid the above problem it would be enough to limit it to (rxq->nb_desc - 1), but I don't know if there is another factor here we should consider.

Konstantin

quoted

quoted

@@ -667,8 +664,8 @@ ixgbe_xmit_pkts_vec(void *tx_queue, struct rte_mbuf **tx_pkts,
   	uint64_t rs = IXGBE_ADVTXD_DCMD_RS|DCMD_DTYP_FLAGS;
   	int i;

-	if (unlikely(nb_pkts > RTE_IXGBE_VPMD_TX_BURST))
-		nb_pkts = RTE_IXGBE_VPMD_TX_BURST;
+	/* cross rx_thresh boundary is not allowed */
+	nb_pkts = RTE_MIN(nb_pkts, txq->tx_rs_thresh);

   	if (txq->nb_tx_free < txq->tx_free_thresh)
   		ixgbe_tx_free_bufs(txq);