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

[Kevin Traynor <ktraynor@redhat.com>]

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.


>  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;
>