Re: [dpdk-dev] [PATCH] eventdev: fix Rx adapter stalls on event device backpressure

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

On 2021-11-09 07:26, Kundapura, Ganapati wrote:
> Hi Mattias,
>
>> -----Original Message-----
>> From: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>> Sent: 08 November 2021 19:14
>> To: jerinj@marvell.com; Jayatheerthan, Jay <jay.jayatheerthan@intel.com>
>> Cc: dev@dpdk.org; Kundapura, Ganapati <ganapati.kundapura@intel.com>;
>> stable@dpdk.org
>> Subject: Re: [PATCH] eventdev: fix Rx adapter stalls on event device
>> backpressure
>>
>> On 2021-11-08 14:25, Mattias Rönnblom wrote:
>>> In the Eventdev Ethernet RX Adapter, correctly handle the case where
>>> the circular enqueue buffer head and tail index points to the same
>>> element (i.e., the buffer is full) and the buffer has wrapped.
>>>
>>> This bug may be triggered in case there is backpressure from the event
>>> device to the RX adapter.
>>>
>>> Fixes: 8113fd15e229 ("eventdev/eth_rx: make enqueue buffer circular")
>>> Cc: ganapati.kundapura@intel.com
>>> Cc: stable@dpdk.org
>>
>> Disregard the stable cc. This bug does not appear in any released DPDK
>> version (e.g., 21.08).
>>
>>
>>> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>>> ---
>>>    lib/eventdev/rte_event_eth_rx_adapter.c | 22 ++++++++++++++--------
>>>    1 file changed, 14 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/lib/eventdev/rte_event_eth_rx_adapter.c
>> b/lib/eventdev/rte_event_eth_rx_adapter.c
>>> index 56318b5a6f..809416d9b7 100644
>>> --- a/lib/eventdev/rte_event_eth_rx_adapter.c
>>> +++ b/lib/eventdev/rte_event_eth_rx_adapter.c
>>> @@ -777,19 +777,25 @@ rxa_flush_event_buffer(struct
>> event_eth_rx_adapter *rx_adapter,
>>>    		       struct eth_event_enqueue_buffer *buf,
>>>    		       struct rte_event_eth_rx_adapter_stats *stats)
>>>    {
>>> -	uint16_t count = buf->last ? buf->last - buf->head : buf->count;
>>> +	uint16_t count = buf->count;
>>> +	uint16_t n = 0;
>>>
>>>    	if (!count)
>>>    		return 0;
>>>
>>> -	uint16_t n = rte_event_enqueue_new_burst(rx_adapter-
>>> eventdev_id,
>>> -					rx_adapter->event_port_id,
>>> -					&buf->events[buf->head],
>>> -					count);
>>> -	if (n != count)
>>> -		stats->rx_enq_retry++;
>>> +	if (buf->last)
>>> +		count = buf->last - buf->head;
>>> +
>>> +	if (count) {
>>> +		n = rte_event_enqueue_new_burst(rx_adapter-
>>> eventdev_id,
>>> +						rx_adapter->event_port_id,
>>> +						&buf->events[buf->head],
>>> +						count);
>>> +		if (n != count)
>>> +			stats->rx_enq_retry++;
>>>
>>> -	buf->head += n;
>>> +		buf->head += n;
>>> +	}
>>>
>>>    	if (buf->last && n == count) {
>>>    		uint16_t n1;
> When head = tail, count is the number of events in the event buffer i.e count = buf->count and last = 0
> Last is the marker used in case of roll over.
> In case of tail roll over and head is not, events are processed from head to last and zero to tail.
> Looks like change is same as the original.
> Could you please clarify more on this change and also clarify if you were able to reproduce the backpressure issue?


The enqueue buffer state I encountered was last != 0 and head == tail, 
and size != 0. In that case, the function returns early, since count == 
0, even though there are events stored from 0 to tail. head, tail, last 
and size were all 192, from what I remember.


For reasons I didn't analyze, it only seem to occur when the event 
port's enqueue burst size was larger than 32 (the RX burst used against 
the RX adapter's ethdev queues), and there was backpressure from the 
event device.