Re: [PATCH net v2] gve: fix Rx queue stall on alloc failure

[Przemek Kitszel <przemyslaw.kitszel@intel.com>]

> @@ -400,6 +414,26 @@ void gve_rx_post_buffers_dqo(struct gve_rx_ring *rx)
>   	}
>   
>   	rx->fill_cnt += num_posted;
> +
> +	/* If the queue has fewer than GVE_RX_BUF_THRESH_DQO descriptors
> +	 * visible to the hardware, the hardware is in danger of starving
> +	 * and cannot trigger interrupts.
> +	 *
> +	 * We use a threshold of 32 because a single maximum-sized RSC
> +	 * packet can consume up to 19 descriptors in the Rx path. Lower
> +	 * thresholds (e.g., 8 or 16) would be unsafe as they could cause
> +	 * the device to drop/stall on a maximum-sized RSC packet.
> +	 *
> +	 * Start the timer to periodically reschedule NAPI and recover.
> +	 */
> +	num_bufs_avail_to_hw =
> +		((bufq->tail & ~(GVE_RX_BUF_THRESH_DQO - 1)) -
> +		 bufq->head) & bufq->mask;
> +
> +	if (num_bufs_avail_to_hw < GVE_RX_BUF_THRESH_DQO) {

nice bit-arith tricks, but perhaps a simpler condiion like:
	if (num_avail_slots + num_posted < GVE_RX_BUF_THRESH_DQO)
would be sufficient?

> +		mod_timer(&rx->starvation_timer,
> +			  jiffies + msecs_to_jiffies(GVE_RX_NAPI_RESCHED_MS));
> +	}
>   }