Re: [PATCH iwl-next 7/8] ixgbe: limit ITR decrease in latency mode to prevent ACK overdrive

[Simon Horman <horms@kernel.org>]

On Fri, May 08, 2026 at 05:12:25AM +0200, Aleksandr Loktionov wrote:
> From: Alexander Duyck <alexander.h.duyck@intel.com>
> 
> When operating in latency mode and the computed ITR is lower than the
> current setting, the algorithm can reduce the interrupt rate too
> aggressively in a single step.  For a TCP workload this means the ACK
> stream (a latency-sensitive, low-packet-rate workload) can drive the
> moderation down to very high interrupt rates, starving CPU time from
> the sender side.
> 
> After the speed-based ITR calculation is complete, check whether the
> result is in latency mode and would decrease below the current setting.
> If so, limit the decrease to at most IXGBE_ITR_ADAPTIVE_MIN_INC (2 us)
> per update.  This ensures the number of interrupts grows by no more
> than 2x per adjustment step for latency-class workloads, dialling in
> smoothly rather than overshooting.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
> Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>

Reviewed-by: Simon Horman <horms@kernel.org>

> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index aea76b3..ba7b013 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -2888,6 +2888,17 @@ static void ixgbe_update_itr(struct ixgbe_q_vector *q_vector,
>  		break;
>  	}
>  
> +	/* In the case of a latency specific workload only allow us to
> +	 * reduce the ITR by at most 2us. By doing this we should dial
> +	 * in so that our number of interrupts is no more than 2x the number
> +	 * of packets for the least busy workload. So for example in the case
> +	 * of a TCP workload the ACK packets being received would set the
> +	 * interrupt rate as they are a latency specific workload.
> +	 */
> +	if ((itr & IXGBE_ITR_ADAPTIVE_LATENCY) && itr < ring_container->itr)
> +		itr = max_t(unsigned int, itr,
> +			    ring_container->itr - IXGBE_ITR_ADAPTIVE_MIN_INC);

nit: I expect this could be min(). And if it could, it should.

> +
>  clear_counts:
>  	/* write back value */
>  	ring_container->itr = itr;
> -- 
> 2.52.0
>