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

[Bobby Eshleman <bobbyeshleman@gmail.com>]

On Tue, Aug 04, 2026 at 11:23:53AM +0300, Tariq Toukan wrote:
> 
> 
> On 04/08/2026 2:47, Bobby Eshleman wrote:
> > From: Bobby Eshleman <bobbyeshleman@meta.com>
> > 
> > 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 <bobbyeshleman@meta.com>
> 
> Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
> 
> Thanks.

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

> 
> > ---
> > 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
> > 
> > Changes in v2:
> > - remove misleading comment (Tariq)
> > - Link to v1: https://lore.kernel.org/r/20260728-mlx5-bql-v1-1-99a0ab77ede7@meta.com
> > ---
> >   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,
>