Re: [dpdk-dev] [PATCH] ixgbe: fix checking for tx_free_thresh

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

The requirements for rte_eth_tx_burst(), which calls a driver specific 
function, in case of ixgbe, these two:

"It is the responsibility of the rte_eth_tx_burst() function to 
transparently free the memory buffers of packets previously sent. This 
feature is driven by the *tx_free_thresh* value supplied to the 
rte_eth_dev_configure() function at device configuration time. When the 
number of previously sent packets reached the "minimum transmit packets 
to free" threshold, the rte_eth_tx_burst() function must [attempt to] 
free the *rte_mbuf*  buffers of those packets whose transmission was 
effectively completed."

Also rte_eth_tx_queue_setup() uses the same description for tx_free_thresh:

"The *tx_free_thresh* value indicates the [minimum] number of network 
buffers that must be pending in the transmit ring to trigger their 
[implicit] freeing by the driver transmit function."

And all the other poll mode drivers are using this formula. Plus I've 
described a possible hang situation in the commit message.

On 28/05/15 11:50, Venkatesan, Venky wrote:
> NAK. This causes more (unsuccessful) cleanup attempts on the descriptor ring. What is motivating this change?
>
> Regards,
> Venky
>
>
>> On May 28, 2015, at 1:42 AM, Zoltan Kiss <zoltan.kiss@linaro.org> wrote:
>>
>> This check doesn't do what's required by rte_eth_tx_burst:
>> "When the number of previously sent packets reached the "minimum transmit
>> packets to free" threshold"
>>
>> This can cause problems when txq->tx_free_thresh + [number of elements in the
>> pool] < txq->nb_tx_desc.
>>
>> Signed-off-by: Zoltan Kiss <zoltan.kiss@linaro.org>
>> ---
>> drivers/net/ixgbe/ixgbe_rxtx.c     | 4 ++--
>> drivers/net/ixgbe/ixgbe_rxtx_vec.c | 2 +-
>> 2 files changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/ixgbe/ixgbe_rxtx.c b/drivers/net/ixgbe/ixgbe_rxtx.c
>> index 4f9ab22..b70ed8c 100644
>> --- a/drivers/net/ixgbe/ixgbe_rxtx.c
>> +++ b/drivers/net/ixgbe/ixgbe_rxtx.c
>> @@ -250,10 +250,10 @@ tx_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts,
>>
>>     /*
>>      * Begin scanning the H/W ring for done descriptors when the
>> -     * number of available descriptors drops below tx_free_thresh.  For
>> +     * number of in flight descriptors reaches tx_free_thresh. For
>>      * each done descriptor, free the associated buffer.
>>      */
>> -    if (txq->nb_tx_free < txq->tx_free_thresh)
>> +    if ((txq->nb_tx_desc - txq->nb_tx_free) > txq->tx_free_thresh)
>>         ixgbe_tx_free_bufs(txq);
>>
>>     /* Only use descriptors that are available */
>> diff --git a/drivers/net/ixgbe/ixgbe_rxtx_vec.c b/drivers/net/ixgbe/ixgbe_rxtx_vec.c
>> index abd10f6..f91c698 100644
>> --- a/drivers/net/ixgbe/ixgbe_rxtx_vec.c
>> +++ b/drivers/net/ixgbe/ixgbe_rxtx_vec.c
>> @@ -598,7 +598,7 @@ ixgbe_xmit_pkts_vec(void *tx_queue, struct rte_mbuf **tx_pkts,
>>     if (unlikely(nb_pkts > RTE_IXGBE_VPMD_TX_BURST))
>>         nb_pkts = RTE_IXGBE_VPMD_TX_BURST;
>>
>> -    if (txq->nb_tx_free < txq->tx_free_thresh)
>> +    if ((txq->nb_tx_desc - txq->nb_tx_free) > txq->tx_free_thresh)
>>         ixgbe_tx_free_bufs(txq);
>>
>>     nb_commit = nb_pkts = (uint16_t)RTE_MIN(txq->nb_tx_free, nb_pkts);
>> --
>> 1.9.1
>>