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Kitszel, >> Przemyslaw ; intel-wired- >> lan@lists.osuosl.org >> Cc: netdev@vger.kernel.org; Andrew Lunn ; David >> S. Miller ; Eric Dumazet ; >> Jakub Kicinski ; Paolo Abeni ; >> Gomes, Vinicius ; Sasha Neftin >> ; linux-kernel@vger.kernel.org; Benoit DE >> RANCOURT >> 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@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(-) >> >> 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 Reviewed-by: Dima Ruinskiy