RE: [PATCH iwl-net 1/2] igc: Fix Tx stop threshold to cover empty frame descriptors

[Loktionov, Aleksandr]

> -----Original Message-----
> From: Benoit DE RANCOURT <b2rancourt@xxxxxxxxx>
> Sent: Sunday, October 4, 2026 5:29 PM
> To: Nguyen, Anthony L <anthony.l.nguyen@xxxxxxxxx>; Kitszel,
> Przemyslaw <przemyslaw.kitszel@xxxxxxxxx>; intel-wired-
> lan@xxxxxxxxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx; Andrew Lunn <andrew+netdev@xxxxxxx>; David
> S. Miller <davem@xxxxxxxxxxxxx>; Eric Dumazet <edumazet@xxxxxxxxxx>;
> Jakub Kicinski <kuba@xxxxxxxxxx>; Paolo Abeni <pabeni@xxxxxxxxxx>;
> Gomes, Vinicius <vinicius.gomes@xxxxxxxxx>; Sasha Neftin
> <sasha.neftin@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Benoit DE
> RANCOURT <b2rancourt@xxxxxxxxx>
> Subject: [PATCH iwl-net 1/2] igc: Fix Tx stop threshold to cover empty
> frame descriptors
> 
> Commit db0b124f02ba ("igc: Enhance Qbv scheduling by using first flag
> bit") raised the number of free descriptors that igc_xmit_frame_ring()
> requires before mapping a packet from count + 3 to count + 5, to leave
> room for the empty frame (one context and one data descriptor) that
> may be inserted ahead of a launch time packet. DESC_NEEDED was left at
> MAX_SKB_FRAGS + 4. igc_tx_map() uses it to stop the queue in advance,
> and TX_WAKE_THRESHOLD is derived from it.
> 
> A queue left running after a transmit therefore only guarantees
> MAX_SKB_FRAGS + 4 free descriptors, while the next skb with a linear
> part and MAX_SKB_FRAGS fragments, each fitting in one data descriptor,
> requires MAX_SKB_FRAGS + 6. igc_xmit_frame_ring() then returns
> NETDEV_TX_BUSY, which the queue stop logic is meant to prevent.
> 
> With TSO this happens routinely. On an I226-V (8086:125c) running
> 7.2.8 with MAX_SKB_FRAGS = 17, routed and locally generated TCP
> traffic on a CPU-saturated router produced 10890 NETDEV_TX_BUSY
> returns in three runs of 180 seconds, all for TSO skbs with 16 or 17
> fragments, with
> 21 or 22 free descriptors in all but 5 cases. Together with deferred
> tail writes (xmit_more), these returns led to 25 Tx timeouts and
> adapter resets in the same runs:
> 
>   igc 0000:08:00.0 terra: NETDEV WATCHDOG: CPU: 1: transmit queue 2
> timed out 5353 ms
>   igc 0000:08:00.0 terra: Reset adapter
> 
> The following patch addresses the lost tail write itself.
> 
> Raise DESC_NEEDED to MAX_SKB_FRAGS + 6 to match the admission check.
> The additional reservation is currently unconditional, including when
> launch time is disabled; match that existing admission policy in the
> proactive stop threshold. With MAX_SKB_FRAGS = 17, the queue is now
> stopped below 23 free descriptors instead of 21. This also raises the
> completion-based wake threshold from 42 to 46 free descriptors,
> preserving the existing two-times-stop-threshold policy. Throughput
> and latency effects were not measured.
> 
> With this change alone, three runs of the same test on 7.2.8 produced
> no NETDEV_TX_BUSY and no Tx timeout over 8.8M NETDEV_TX_OK returns,
> including 613k TSO skbs with 17 fragments.
> 
> As before commit db0b124f02ba, an skb whose linear part or fragments
> exceed IGC_MAX_DATA_PER_TXD may still need more descriptors than
> DESC_NEEDED accounts for.
> 
> Fixes: db0b124f02ba ("igc: Enhance Qbv scheduling by using first flag
> bit")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: LLM bpftrace
> Signed-off-by: Benoit DE RANCOURT <b2rancourt@xxxxxxxxx>
> ---
>  drivers/net/ethernet/intel/igc/igc.h | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/igc/igc.h
> b/drivers/net/ethernet/intel/igc/igc.h
> index 17f213cc93e4..f1efadbd9f74 100644
> --- a/drivers/net/ethernet/intel/igc/igc.h
> +++ b/drivers/net/ethernet/intel/igc/igc.h
> @@ -575,9 +575,13 @@ enum igc_boards {
>  #define IGC_MAX_TXD_PWR		15
>  #define IGC_MAX_DATA_PER_TXD	BIT(IGC_MAX_TXD_PWR)
> 
> -/* Tx Descriptors needed, worst case */
>  #define TXD_USE_COUNT(S)	DIV_ROUND_UP((S), IGC_MAX_DATA_PER_TXD)
> -#define DESC_NEEDED	(MAX_SKB_FRAGS + 4)
> +
> +/* Tx descriptor budget for a head and MAX_SKB_FRAGS fragments,
> + * each fitting in one data descriptor: one context descriptor,
> + * two descriptors for an optional empty frame, and two spare
> entries.
> + */
> +#define DESC_NEEDED	(MAX_SKB_FRAGS + 6)
> 
>  struct igc_rx_buffer {
>  	union {
> 
> base-commit: a83267db14681b3be481e02a4d5a38177507006c
> --
> 2.55.0

Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@xxxxxxxxx>