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

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Thu, Jul 09, 2026 at 09:19:06PM +0000, Harshitha Ramamurthy wrote:
> From: Eddie Phillips <eddiephillips@google.com>
> 
> When the system is under extreme memory pressure, page allocations can
> fail during the Rx buffer refill loop. If the number of buffers posted
> to hardware falls below a critical low threshold and the refill loop
> exits due to allocation failures, the queue can stall:
> 
> 1. The device drops incoming packets because there are no descriptors.
> 2. Since no packets are processed, no Rx completions are generated.
> 3. Because no completions occur, NAPI is never scheduled, preventing
>    the refill loop from running again even after memory is freed.
> 
> This results in a permanent queue stall.
> 
> Resolve this by introducing a starvation recovery timer for each Rx queue.
> If the number of buffers posted to hardware falls below a critical low
> threshold, start a timer to periodically reschedule NAPI. Once NAPI runs
> and successfully refills the queue above the threshold, the timer is
> not rescheduled.
> 
> The threshold is set to 32 because a single maximum-sized Receive Segment
> Coalescing (RSC) packet can consume up to 19 descriptors in the Rx path.
> Lower thresholds (such as 8 or 16) would be insufficient to process a
> complete maximum-sized RSC packet, risking packet drops or unexpected
> hardware behavior under memory pressure. Setting the threshold to 32
> guarantees a safe margin to handle at least one full RSC packet.
> 
> Cc: stable@vger.kernel.org
> Fixes: 9b8dd5e5ea48 ("gve: DQO: Add RX path")
> Reviewed-by: Jordan Rhee <jordanrhee@google.com>
> Signed-off-by: Eddie Phillips <eddiephillips@google.com>
> Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
> ---
> Changes in v2:
> - Link to v1: https://lore.kernel.org/netdev/20260701005341.3699161-1-hramamurthy@google.com/
> - Relocated the starvation timer to the end of gve_rx_ring to avoid polluting
> hotpath cachelines
> - Decoupled timer lifecycle from allocation cycles by moving initialization
> and shutdown to start/stop pathways instead of setup/remove pathways.
> - Added explicit rationale for the 32-descriptor threshold
> (GVE_RX_BUF_THRESH_DQO) ensuring it is safe for maximum-sized RSC packets.
> - Removed addition of a stat tracking critical low buffer events
> 
>  drivers/net/ethernet/google/gve/gve.h        |  3 +++
>  drivers/net/ethernet/google/gve/gve_rx_dqo.c | 34 ++++++++++++++++++++++++++++++++++
>  2 files changed, 37 insertions(+)
> 

[...]

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

nit: redundant braces

>  }
>  
>  static void gve_rx_skb_csum(struct sk_buff *skb,
> -- 
> 2.55.0.795.g602f6c329a-goog
>