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__netpoll_send_skb() parks an skb on npinfo->txq whenever the device cannot take it at once, and once the queue is non-empty every later skb goes straight there to keep ordering. queue_process() drains it from a workqueue and, unlike the direct path, never polls the device for completions: when the ring is stopped it backs off HZ/10. Nothing limits the queue length. A producer that outruns that drain therefore grows the queue until the host is out of memory. Observed with netconsole forwarding a GPU driver that logged one line at ~1e5/s after a firmware hang. The NIC was moving ~17k packets/s: completions for each burst surfaced tens of ms later, outside the one-tick window, so queue_process() slept HZ/10 per ring while ~1e5 lines/s kept arriving. The queue grew at ~170 MB/s, unreclaimable slab reached 51 GiB in five minutes and the OOM killer ran from kswapd with 341 MiB of anonymous memory on the whole box. What the queue held was the flood itself; the OOM report never left the host. Reproduced on the same host (netconsole over a 10G ConnectX-4 Lx) under the same slow-completion condition: 200k lines to /dev/kmsg in 0.12 s grew unreclaimable slab by 173 MiB, about 188k skbs, draining at ~8-10k packets/s. With prompt completions the same burst drains at line rate; a stall on the link while lines keep arriving faster than the drain reproduces the growth. Until the 2006 netpoll rework [1] the deferred path drained through dev_queue_xmit(), with the stack's own backpressure, and was capped at 16 skbs (MAX_QUEUE_DEPTH). That series moved it to a direct hard_start_xmit() with the HZ/10 back-off and made the queue per-device, dropping the cap on the way. Cap it at 1024 skbs per device and drop new skbs beyond that. The drop is counted in tx_dropped of the device whose queue is full and freed with SKB_DROP_REASON_FULL_RING. A netconsole target bound directly to that device also gets NET_XMIT_DROP and, with CONFIG_NETCONSOLE_DYNAMIC, counts it in xmit_drop_count. When a stacked device (bond, bridge, team, vlan, macvlan) passes the skb down and the lower device's queue is the one that fills, the return value does not reach netconsole and the lower device's tx_dropped is the record. The bound holds either way. Nothing is logged on the drop path because that would recurse into the console being drained. [1] https://lore.kernel.org/netdev/20061026225645.482978803@osdl.org/ Signed-off-by: Zack Gomez <zack.gomez@gmail.com> Reviewed-by: Breno Leitao <leitao@debian.org> Link: https://patch.msgid.link/20260925204537.2664119-1-zack.gomez@gmail.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. Quick Start ----------- * Report a bug: See Documentation/admin-guide/reporting-issues.rst * Get the latest kernel: https://kernel.org * Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst * Join the community: https://lore.kernel.org/ Essential Documentation ----------------------- All users should be familiar with: * Building requirements: Documentation/process/changes.rst * Code of Conduct: Documentation/process/code-of-conduct.rst * License: See COPYING Documentation can be built with make htmldocs or viewed online at: https://www.kernel.org/doc/html/latest/ Who Are You? ============ Find your role below: * New Kernel Developer: Getting started with kernel development * Academic Researcher: Studying kernel internals and architecture * Security Expert: Hardening and vulnerability analysis * Backport/Maintenance Engineer: Maintaining stable kernels * System Administrator: Configuring and troubleshooting * Maintainer: Leading subsystems and reviewing patches * Hardware Vendor: Writing drivers for new hardware * Distribution Maintainer: Packaging kernels for distros * AI Coding Assistant: LLMs and AI-powered development tools For Specific Users ================== New Kernel Developer -------------------- Welcome! Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst