Re: [dpdk-dev] [PATCH v2] net/i40e: fix Rx instability with vector mode

["Zhang, Qi Z" <qi.z.zhang@intel.com>]

> -----Original Message-----
> From: Ananyev, Konstantin
> Sent: Friday, October 26, 2018 3:47 AM
> To: Xing, Beilei <beilei.xing@intel.com>; Zhang, Qi Z <qi.z.zhang@intel.com>
> Cc: dev@dpdk.org; stable@dpdk.org
> Subject: RE: [dpdk-dev] [PATCH v2] net/i40e: fix Rx instability with vector
> mode
> 
> 
> Hi,
> 
> >
> > Previously, there is instability during vector Rx if descriptor number
> > is not power of 2, e.g. process hang and some Rx packets are
> > unexpectedly empty. That's because vector Rx mode assumes Rx
> > descriptor number is power of 2 when doing bit mask.
> > This patch allows vector mode only when the number of Rx descriptor is
> > power of 2.
> >
> > Fixes: 8e109464c022 ("i40e: allow vector Rx and Tx usage")
> > Fixes: a3c83a2527e1 ("net/i40e: enable runtime queue setup")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Beilei Xing <beilei.xing@intel.com>
> > ---
> >
> > v2 changes:
> >  - rx_vec_allowed is global configuration, avoid overwrite.
> >
> >  doc/guides/nics/i40e.rst     |  7 +++++++
> >  drivers/net/i40e/i40e_rxtx.c | 13 ++++++++++++-
> >  2 files changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/doc/guides/nics/i40e.rst b/doc/guides/nics/i40e.rst index
> > ab3928a..bfacbd1 100644
> > --- a/doc/guides/nics/i40e.rst
> > +++ b/doc/guides/nics/i40e.rst
> > @@ -172,6 +172,13 @@ Runtime Config Options
> >
> >    -w 84:00.0,use-latest-supported-vec=1
> >
> > +Vector RX Pre-conditions
> > +~~~~~~~~~~~~~~~~~~~~~~~~
> > +For Vector RX it is assumed that the number of descriptor rings will
> > +be a power of 2. With this pre-condition, the ring pointer can easily
> > +scroll back to the head after hitting the tail without a conditional
> > +check. In addition Vector RX can use this assumption to do a bit mask
> using ``ring_size - 1``.
> > +
> >  Driver compilation and testing
> >  ------------------------------
> >
> > diff --git a/drivers/net/i40e/i40e_rxtx.c
> > b/drivers/net/i40e/i40e_rxtx.c index a827456..f6dcca0 100644
> > --- a/drivers/net/i40e/i40e_rxtx.c
> > +++ b/drivers/net/i40e/i40e_rxtx.c
> > @@ -1735,10 +1735,14 @@ i40e_dev_rx_queue_setup_runtime(struct
> rte_eth_dev *dev,
> >  		 * i40e_set_rx_function.
> >  		 */
> >  		ad->rx_bulk_alloc_allowed = true;
> > -		ad->rx_vec_allowed = true;
> >  		dev->data->scattered_rx = use_scattered_rx;
> >  		if (use_def_burst_func)
> >  			ad->rx_bulk_alloc_allowed = false;
> > +		/**
> > +		 * Vector mode is allowed only when number of Rx queue
> > +		 * descriptor is a power of 2.
> > +		 */
> > +		ad->rx_vec_allowed = !(rxq->nb_rx_desc & (rxq->nb_rx_desc - 1));
> >  		i40e_set_rx_function(dev);
> >  		return 0;
> >  	}
> 
> I think this is not complete - if it is not  the first function, we have to do:
> If (if (ad->rx_vec_allowed && (rxq->nb_rx_desc & (rxq->nb_rx_desc - 1) != 0)
> {
>       PMD_DRV_LOG(ERR, ...);
>        return -EINVAL;
> }

Yes, we should prevent downgrade if the device is already running.

> 
> BTW, here and below why not to use rte_is_power_of_2()?
> Konstantin
> 
> > @@ -1811,6 +1815,13 @@ i40e_dev_rx_queue_setup(struct rte_eth_dev
> *dev,
> >  		return -EINVAL;
> >  	}
> >
> > +	/**
> > +	 * Vector mode is allowed only when number of Rx queue
> > +	 * descriptor is a power of 2.
> > +	 */
> > +	if (ad->rx_vec_allowed)
> > +		ad->rx_vec_allowed = !(nb_desc & (nb_desc - 1));

Looks like we are not able to get back to vec mode once some queue is set to no power of 2, even if all queues has been re-setup to power of 2 again.
Seems we still need to check all other queue's configure here.

> > +
> >  	/* Free memory if needed */
> >  	if (dev->data->rx_queues[queue_idx]) {
> >  		i40e_dev_rx_queue_release(dev->data->rx_queues[queue_idx]);
> 
> 
> 
> 
> 
>