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This commit prevents tail-drop when a qdisc is present and the ptr_ring becomes full. Once the ring reaches capacity after a produce attempt, the netdev queue is stopped instead of dropping subsequent packets. If no qdisc is present, the previous tail-drop behavior is preserved. If producing an entry fails anyway due to a race, tun_net_xmit() drops the packet. Such races are expected because LLTX is enabled and the transmit path operates without the usual locking. The __tun_wake_queue() function of the consumer races with the producer for waking/stopping the netdev queue, which could result in a stalled queue. Therefore, an smp_mb__after_atomic() is introduced that pairs with the smp_mb() of the consumer. It follows the principle of store buffering described in tools/memory-model/Documentation/recipes.txt: - The producer in tun_net_xmit() first sets __QUEUE_STATE_DRV_XOFF, followed by an smp_mb__after_atomic() (= smp_mb()), and then reads the ring with __ptr_ring_check_produce(). - The consumer in __tun_wake_queue() first writes zero to the ring in __ptr_ring_consume(), followed by an smp_mb(), and then reads the queue status with netif_tx_queue_stopped(). => Following the aforementioned principle, it is impossible for the producer to see a full ring (and therefore not wake the queue on the re-check) while the consumer simultaneously fails to see a stopped queue (and therefore also does not wake it). Benchmarks: The benchmarks show a slight regression in raw transmission performance when using two sending threads. Packet loss also occurs only in the two-thread sending case; no packet loss was observed with a single sending thread. Test setup: AMD Ryzen 5 5600X at 4.3 GHz, 3200 MHz RAM, isolated QEMU threads; Average over 50 runs @ 100,000,000 packets. SRSO and spectre v2 mitigations disabled. Note for tap+vhost-net: XDP drop program active in VM -> ~2.5x faster; slower for tap due to more syscalls (high utilization of entry_SYSRETQ_unsafe_stack in perf) +--------------------------+--------------+----------------+----------+ | 1 thread | Stock | Patched with | diff | | sending | | fq_codel qdisc | | +------------+-------------+--------------+----------------+----------+ | TAP | Received | 1.132 Mpps | 1.123 Mpps | -0.8% | | +-------------+--------------+----------------+----------+ | | Lost/s | 3.765 Mpps | 0 pps | | +------------+-------------+--------------+----------------+----------+ | TAP | Received | 3.857 Mpps | 3.901 Mpps | +1.1% | | +-------------+--------------+----------------+----------+ | +vhost-net | Lost/s | 0.802 Mpps | 0 pps | | +------------+-------------+--------------+----------------+----------+ +--------------------------+--------------+----------------+----------+ | 2 threads | Stock | Patched with | diff | | sending | | fq_codel qdisc | | +------------+-------------+--------------+----------------+----------+ | TAP | Received | 1.115 Mpps | 1.081 Mpps | -3.0% | | +-------------+--------------+----------------+----------+ | | Lost/s | 8.490 Mpps | 391 pps | | +------------+-------------+--------------+----------------+----------+ | TAP | Received | 3.664 Mpps | 3.555 Mpps | -3.0% | | +-------------+--------------+----------------+----------+ | +vhost-net | Lost/s | 5.330 Mpps | 938 pps | | +------------+-------------+--------------+----------------+----------+ Co-developed-by: Tim Gebauer <tim.gebauer@tu-dortmund.de> Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de> Acked-by: Michael S. Tsirkin <mst@redhat.com> Link: https://patch.msgid.link/20260510151529.43895-5-simon.schippers@tu-dortmund.de Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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| kernel | ||
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| COPYING | ||
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| README | ||
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