Re: [PATCH net v3] net/mlx5e: fix BQL reset on SQ re-activation

On Tue, Aug 04, 2026 at 11:23:53AM +0300, Tariq Toukan wrote:

On 04/08/2026 2:47, Bobby Eshleman wrote:

quoted

From: Bobby Eshleman <redacted>

mlx5e_queue_start() deactivates and re-activates all channels but closes
only the queue being restarted. mlx5e_activate_txqsq() then
unconditionally calls netdev_tx_reset_queue(), zeroing the BQL counters
of channels that kept their in-flight TX WQEs. The next completion then
over-charges and trips the BUG_ON() in dql_completed():

   kernel BUG at lib/dynamic_queue_limits.c:99!
   RIP: 0010:dql_completed+0x23d/0x280
   Call Trace:
    <IRQ>
    mlx5e_poll_tx_cq+0x668/0xa60
    mlx5e_napi_poll+0x5b/0x7b0
    net_rx_action+0x15a/0x580

Reset BQL only when the SQ has no bytes in flight (sq->cc == sq->pc).

In the case that reset is skipped, the outstanding WQEs will eventually
complete and rebalance the dql. The dql->limit is carried across the
reset.

Fixes: b2588ea40ec9 ("net/mlx5e: Implement queue mgmt ops and single channel swap")
Signed-off-by: Bobby Eshleman <redacted>

Reviewed-by: Tariq Toukan <tariqt@nvidia.com>

Thanks.

Thanks Tariq, and my apologies for forgetting to carry the R-b forward.

quoted

---
Changes in v3:
- Sample sq->cc/sq->pc before setting MLX5E_SQ_STATE_ENABLED, so a
   concurrent mlx5e_poll_tx_cq() cannot un-charge BQL between the check
   and netdev_tx_reset_queue() (Sashiko:
   https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260729-mlx5-bql-v2-1-cdf285e586df@meta.com)
- Link to v2: https://lore.kernel.org/r/20260729-mlx5-bql-v2-1-cdf285e586df@meta.com (local)

Changes in v2:
- remove misleading comment (Tariq)
- Link to v1: https://lore.kernel.org/r/20260728-mlx5-bql-v1-1-99a0ab77ede7@meta.com (local)
---
  drivers/net/ethernet/mellanox/mlx5/core/en_main.c | 4 +++-
  1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
index 7d47a1da8b6b..f0407a850ea8 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_main.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_main.c

@@ -1939,8 +1939,10 @@ int mlx5e_open_txqsq(struct mlx5e_channel *c, u32 tisn, int txq_ix,
  void mlx5e_activate_txqsq(struct mlx5e_txqsq *sq)
  {
  	sq->txq = netdev_get_tx_queue(sq->netdev, sq->txq_ix);
+	/* Reset BQL only when the SQ has no bytes in flight. */
+	if (sq->cc == sq->pc)
+		netdev_tx_reset_queue(sq->txq);
  	set_bit(MLX5E_SQ_STATE_ENABLED, &sq->state);
-	netdev_tx_reset_queue(sq->txq);
  	netif_tx_start_queue(sq->txq);
  	netif_queue_set_napi(sq->netdev, sq->txq_ix, NETDEV_QUEUE_TYPE_TX, sq->cq.napi);
  }

---
base-commit: d1000fd7995e51deec872d154e0a40d82f7a539f
change-id: 20260728-mlx5-bql-84e02a5a46c8

Best regards,