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

["Jayatheerthan, Jay" <jay.jayatheerthan@intel.com>]

> -----Original Message-----
> From: Kundapura, Ganapati <ganapati.kundapura@intel.com>
> Sent: Wednesday, November 10, 2021 1:56 PM
> To: mattias.ronnblom <mattias.ronnblom@ericsson.com>; jerinj@marvell.com; Jayatheerthan, Jay <jay.jayatheerthan@intel.com>
> Cc: dev@dpdk.org; stable@dpdk.org
> Subject: RE: [PATCH] eventdev: fix Rx adapter stalls on event device backpressure
> 
> Hi Mattias,
> 
> > -----Original Message-----
> > From: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> > Sent: 09 November 2021 13:58
> > To: Kundapura, Ganapati <ganapati.kundapura@intel.com>;
> > jerinj@marvell.com; Jayatheerthan, Jay <jay.jayatheerthan@intel.com>
> > Cc: dev@dpdk.org; stable@dpdk.org
> > Subject: Re: [PATCH] eventdev: fix Rx adapter stalls on event device
> > backpressure
> >
> > 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.

Could you update the description to reflect last instead of tail ? last > 0 doesn’t mean the buffer is full. It is just a marker for event device enqueue when tail has rolled over and head hasn’t. Below would work, if you want to use it.

"In the Eventdev Ethernet RX Adapter, correctly handle the case where
the circular enqueue buffer head and last index points to the same
element."

> > >>>
> > >>> 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.
> >
> Issue seems to be head is catching up with last not tail when the tail is rolled over.
> When head = last, count is zero and returning without processing the events from 0 to tail.
> Approved. You can add my ack
> 
> >
> > 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.
> >