dc82a33297
In production, we're seeing TX drops on veth devices when the ptr_ring fills up. This can occur when NAPI mode is enabled, though it's relatively rare. However, with threaded NAPI - which we use in production - the drops become significantly more frequent. The underlying issue is that with threaded NAPI, the consumer often runs on a different CPU than the producer. This increases the likelihood of the ring filling up before the consumer gets scheduled, especially under load, leading to drops in veth_xmit() (ndo_start_xmit()). This patch introduces backpressure by returning NETDEV_TX_BUSY when the ring is full, signaling the qdisc layer to requeue the packet. The txq (netdev queue) is stopped in this condition and restarted once veth_poll() drains entries from the ring, ensuring coordination between NAPI and qdisc. Backpressure is only enabled when a qdisc is attached. Without a qdisc, the driver retains its original behavior - dropping packets immediately when the ring is full. This avoids unexpected behavior changes in setups without a configured qdisc. With a qdisc in place (e.g. fq, sfq) this allows Active Queue Management (AQM) to fairly schedule packets across flows and reduce collateral damage from elephant flows. A known limitation of this approach is that the full ring sits in front of the qdisc layer, effectively forming a FIFO buffer that introduces base latency. While AQM still improves fairness and mitigates flow dominance, the latency impact is measurable. In hardware drivers, this issue is typically addressed using BQL (Byte Queue Limits), which tracks in-flight bytes needed based on physical link rate. However, for virtual drivers like veth, there is no fixed bandwidth constraint - the bottleneck is CPU availability and the scheduler's ability to run the NAPI thread. It is unclear how effective BQL would be in this context. This patch serves as a first step toward addressing TX drops. Future work may explore adapting a BQL-like mechanism to better suit virtual devices like veth. Reported-by: Yan Zhai <yan@cloudflare.com> Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org> Reviewed-by: Toshiaki Makita <toshiaki.makita1@gmail.com> Link: https://patch.msgid.link/174559294022.827981.1282809941662942189.stgit@firesoul Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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| LICENSES | ||
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
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| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
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| COPYING | ||
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| README | ||
README
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.