Re: [dpdk-dev] [PATCH] net/ice: add ability to reduce the Rx latency

["Zhang, AlvinX" <alvinx.zhang@intel.com>]

> -----Original Message-----
> From: Kevin Traynor <ktraynor@redhat.com>
> Sent: Saturday, September 18, 2021 1:25 AM
> To: Zhang, AlvinX <alvinx.zhang@intel.com>; Zhang, Qi Z
> <qi.z.zhang@intel.com>; Guo, Junfeng <junfeng.guo@intel.com>
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] net/ice: add ability to reduce the Rx latency
> 
> On 14/09/2021 02:31, Alvin Zhang wrote:
> > This patch adds a devarg parameter to enable/disable reducing the Rx
> > latency.
> >
> > Signed-off-by: Alvin Zhang <alvinx.zhang@intel.com>
> > ---
> >  doc/guides/nics/ice.rst      |  8 ++++++++
> >  drivers/net/ice/ice_ethdev.c | 26 +++++++++++++++++++++++---
> > drivers/net/ice/ice_ethdev.h |  1 +
> >  3 files changed, 32 insertions(+), 3 deletions(-)
> >
> > diff --git a/doc/guides/nics/ice.rst b/doc/guides/nics/ice.rst index
> > 5bc472f..3db0430 100644
> > --- a/doc/guides/nics/ice.rst
> > +++ b/doc/guides/nics/ice.rst
> > @@ -219,6 +219,14 @@ Runtime Config Options
> >
> >    These ICE_DBG_XXX are defined in ``drivers/net/ice/base/ice_type.h``.
> >
> > +- ``Reduce Rx interrupts and latency`` (default ``0``)
> > +
> > +  vRAN workloads require low latency DPDK interface for the front
> > + haul  interface connection to Radio. Now we can reduce Rx interrupts
> > + and  latency by specify ``1`` for parameter ``rx-low-latency``::
> > +
> > +    -a 0000:88:00.0,rx-low-latency=1
> > +
> 
> When would a user select this and when not? What is the trade off?
> 
> The text is a bit unclear. It looks below like it reduces the interrupt latency, but
> not the number of interrupts. Maybe I got it wrong.

Yes, it reduces the interrupt latency,
We will refine the doc in next patch.

> 
> 
> >  Driver compilation and testing
> >  ------------------------------
> >
> > diff --git a/drivers/net/ice/ice_ethdev.c
> > b/drivers/net/ice/ice_ethdev.c index a4cd39c..85662e4 100644
> > --- a/drivers/net/ice/ice_ethdev.c
> > +++ b/drivers/net/ice/ice_ethdev.c
> > @@ -29,12 +29,14 @@
> >  #define ICE_PIPELINE_MODE_SUPPORT_ARG  "pipeline-mode-support"
> >  #define ICE_PROTO_XTR_ARG         "proto_xtr"
> >  #define ICE_HW_DEBUG_MASK_ARG     "hw_debug_mask"
> > +#define ICE_RX_LOW_LATENCY        "rx-low-latency"
> >
> >  static const char * const ice_valid_args[] = {
> >  	ICE_SAFE_MODE_SUPPORT_ARG,
> >  	ICE_PIPELINE_MODE_SUPPORT_ARG,
> >  	ICE_PROTO_XTR_ARG,
> >  	ICE_HW_DEBUG_MASK_ARG,
> > +	ICE_RX_LOW_LATENCY,
> >  	NULL
> >  };
> >
> > @@ -1827,6 +1829,9 @@ static int ice_parse_devargs(struct rte_eth_dev
> *dev)
> >  	if (ret)
> >  		goto bail;
> >
> > +	ret = rte_kvargs_process(kvlist, ICE_RX_LOW_LATENCY,
> > +				 &parse_bool, &ad->devargs.rx_low_latency);
> > +
> >  bail:
> >  	rte_kvargs_free(kvlist);
> >  	return ret;
> > @@ -3144,8 +3149,9 @@ static int ice_init_rss(struct ice_pf *pf)  {
> >  	struct ice_hw *hw = ICE_VSI_TO_HW(vsi);
> >  	uint32_t val, val_tx;
> > -	int i;
> > +	int rx_low_latency, i;
> >
> > +	rx_low_latency = vsi->adapter->devargs.rx_low_latency;
> >  	for (i = 0; i < nb_queue; i++) {
> >  		/*do actual bind*/
> >  		val = (msix_vect & QINT_RQCTL_MSIX_INDX_M) | @@ -3155,8
> +3161,21 @@
> > static int ice_init_rss(struct ice_pf *pf)
> >
> >  		PMD_DRV_LOG(INFO, "queue %d is binding to vect %d",
> >  			    base_queue + i, msix_vect);
> > +
> >  		/* set ITR0 value */
> > -		ICE_WRITE_REG(hw, GLINT_ITR(0, msix_vect), 0x2);
> > +		if (rx_low_latency) {
> > +			/**
> > +			 * Empirical configuration for optimal real time
> > +			 * latency reduced interrupt throttling to 2us
> > +			 */
> > +			ICE_WRITE_REG(hw, GLINT_ITR(0, msix_vect), 0x1);
> 
> Why not set this to 0? "Setting the INTERVAL to zero enables immediate
> interrupt."
> 
> > +			ICE_WRITE_REG(hw, QRX_ITR(base_queue + i),
> > +				      QRX_ITR_NO_EXPR_M);
> > +		} else {
> > +			ICE_WRITE_REG(hw, GLINT_ITR(0, msix_vect), 0x2);
> > +			ICE_WRITE_REG(hw, QRX_ITR(base_queue + i), 0);
> > +		}
> > +
> >  		ICE_WRITE_REG(hw, QINT_RQCTL(base_queue + i), val);
> >  		ICE_WRITE_REG(hw, QINT_TQCTL(base_queue + i), val_tx);
> >  	}
> > @@ -5314,7 +5333,8 @@ static int ice_xstats_get_names(__rte_unused
> struct rte_eth_dev *dev,
> >  			      ICE_HW_DEBUG_MASK_ARG "=0xXXX"
> >  			      ICE_PROTO_XTR_ARG
> "=[queue:]<vlan|ipv4|ipv6|ipv6_flow|tcp|ip_offset>"
> >  			      ICE_SAFE_MODE_SUPPORT_ARG "=<0|1>"
> > -			      ICE_PIPELINE_MODE_SUPPORT_ARG "=<0|1>");
> > +			      ICE_PIPELINE_MODE_SUPPORT_ARG "=<0|1>"
> > +			      ICE_RX_LOW_LATENCY "=<0|1>");
> >
> >  RTE_LOG_REGISTER_SUFFIX(ice_logtype_init, init, NOTICE);
> > RTE_LOG_REGISTER_SUFFIX(ice_logtype_driver, driver, NOTICE); diff
> > --git a/drivers/net/ice/ice_ethdev.h b/drivers/net/ice/ice_ethdev.h
> > index b4bf651..c61cc1f 100644
> > --- a/drivers/net/ice/ice_ethdev.h
> > +++ b/drivers/net/ice/ice_ethdev.h
> > @@ -463,6 +463,7 @@ struct ice_pf {
> >   * Cache devargs parse result.
> >   */
> >  struct ice_devargs {
> > +	int rx_low_latency;
> >  	int safe_mode_support;
> >  	uint8_t proto_xtr_dflt;
> >  	int pipe_mode_support;
> >