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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@vger.kernel.org > Assisted-by: LLM bpftrace > Signed-off-by: Benoit DE RANCOURT > --- > drivers/net/ethernet/intel/igc/igc.h | 8 ++++++-- > 1 file changed, 6 insertions(+), 2 deletions(-) > Tested-by: Avigail Dahan