Status of Linux Boot-time Work

## Overview

Tim Bird from Sony Electronics presents the status of Linux boot-time work, covering optimization techniques, recent upstream patches, and tools for boot-time measurement and tuning. He emphasizes that boot-time optimization is "death by a thousand cuts"—requiring many small improvements rather than silver bullets—and highlights that solutions are platform-specific with no generalized approach.

## Key Topics

The presentation reviews boot-time work across three main areas: bootloader, kernel, and user space. Recent talks covered U-Boot optimization by Simon Glass (achieving 82ms boot time via command-line removal and driver disabling), Red Hat's automotive techniques (kernel configs, compression optimization, systemd/Udev tuning, SE Linux policy trimming), and TI's work on early Ethernet initialization (requiring firmware pre-initialization and driver changes to recognize pre-initialized hardware). Additionally, talks addressed unified boot-time measurement across multi-processor systems, secure boot optimization (reducing overhead from 450ms to 40ms using hardware-accelerated SHA-256), Android prefetching (6–8% boot improvement), and probe deferral elimination techniques (achieving 240ms kernel boot reduction on BeagleBone).

## Recent Patches and Work In Progress

Recent upstream kernel patches include detailed probe instrumentation (adding visible probe function names to dmesg), systemd's udevadm trigger -i option (reducing node processing), async printk output for serial console (accepted in 6.18, requiring preempt RT), Red Hat's parallel module loading (300ms improvement), and boot cache patches (caching crypto algorithm selections). In-progress work includes DRM splash screen support (RFC recently submitted), falcon mode optimization in U-Boot, and systemd tuning for embedded systems. Bird notes probe deferral optimization through makefile adjustments and device-tree settings, though these remain controversial.

## Tim Bird's Boot-Time Wizard

Bird is developing the Boot Time Wizard, an early-alpha tool to automate boot-time tuning by measuring the impact of individual optimizations. The tool applies techniques (disabling device-tree nodes, systemd units, adjusting kernel configs and command lines), generates before/after reports, and helps identify "toothpaste effect" interactions where changes shift delays rather than eliminate them. Accompanying tools include grab_boot_data.sh (system-wide data collection), Boot Data Tool (diffing boot outputs), and a Boot Time Wiki collecting cross-platform boot data to identify patterns (e.g., platform-specific bottlenecks). Bird notes the tool requires his TTC board-farm management system but acknowledges need for broader compatibility with Lava or LabGrid.

## Takeaways

Key recommendations: use "quiet" on the kernel command line (100–200ms gain), trim kernel image to remove unnecessary drivers, and apply platform-specific optimizations. Bird stresses that successful boot-time work requires understanding Linux systems deeply and validating changes per-platform due to requirement variability (e.g., USB camera vs. Ethernet). He highlights that boot-time work remains unsolved after 20 years because it depends entirely on platform, hardware, and product requirements, not universal techniques. Resources include the Boot Times Special Interest Group (via Linux Embedded mailing list), eLinux wiki, and Bird's Boot Time Wiki for data collection across systems.