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

[Zoltan Kiss <zoltan.kiss@linaro.org>]

On 31/07/15 11:21, Ananyev, Konstantin wrote:
>
>
>> -----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:
>>> 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 <cunming.liang@intel.com>
>>> ---
>>>    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.
>>>
>>>    		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)"
>>
>>>     *
>>>     * 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
>>
>>>     * - 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
>>
>>>     *
>>>     * 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
>
>>
>>
>>> @@ -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);
>>>