The TX mitigation logic only sets the Interrupt on Completion bit once
every tx_coal_frames descriptors (STMMAC_TX_FRAMES = 25), with the
tx_coal_timer hrtimer (STMMAC_COAL_TX_TIMER = 5000 us) as the only
fallback. TX skbs are freed exclusively from the TX completion path,
so any flow that keeps fewer than 25 frames in flight has all of its
skbs held for up to 5 ms after transmission.
Paced flows never queue enough frames to reach the frame threshold:
TCP Small Queues caps the amount of unfreed data at roughly two pacing
intervals worth, which at moderate pacing rates is only a couple of
packets. Every small burst then stalls until the coalesce timer fires,
and throughput collapses to approximately tsq_limit / tx_coal_timer
regardless of link capacity.
This is easily reproducible with BBR, which paces its output and thus
keeps only a few frames in flight at a time. On a YT6801
(dwmac-motorcomm) equipped Orange Pi 5 Pro, a BBR upload over a ~23 ms
RTT path is capped at 5.24 Mbit/s, while CUBIC reaches 207 Mbit/s on
the same path. BBR measures the stalled send rate as the path
bandwidth and locks its estimate near the floor, so the connection
never recovers. Lowering the coalesce settings with ethtool -C
(tx-usecs 100 tx-frames 1) lifts the same transfer to 447 Mbit/s,
confirming the mechanism.
Fix this by setting the IC bit on the last descriptor of every xmit
burst, i.e. whenever netdev_xmit_more() reports that no further frames
are pending in the current dequeue batch. Frame-based coalescing still
applies within a burst, bulk traffic keeps batching through qdisc bulk
dequeue and NAPI polling, and the coalesce timer becomes a pure
fallback instead of the primary completion mechanism for lightly
queued flows.
tx-frames 0 keeps its meaning of timer-based mitigation only.
Signed-off-by: Johan Alvarado <contact@c127.dev>
Link: https://patch.msgid.link/20260731194522.55069-1-contact@c127.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
d21a374995