Reducing boot time in embedded Linux

A pragmatic guide to boot-time optimization
Chris Simmonds
Embedded World 2018
A pragmatic guide to boot-time optimization 1 Copyright © 2011-2018, 2net Ltd

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About Chris Simmonds
• Consultant and trainer
• Author of Mastering Embedded Linux Programming
• Working with embedded Linux since 1999
• Android since 2009
• Speaker at many conferences and workshops
"Looking after the Inner Penguin" blog at http://2net.co.uk/
https://uk.linkedin.com/in/chrisdsimmonds/
https://google.com/+chrissimmonds

Overview
• Shorter boot time is always desirable
• It is always possible to reduce boot time
• This is software: there is always more you can do
• So, the real issues are:
• How much effort do you want to put in?
• How big a mess will you leave when you are done?

Where it all goes wrong
• The more effort you put in to reducing boot time, the higher the future
maintenance burden
Before After

Getting it right
• This is a pragmatic guide to reducing boot time
• Using the three guiding principles
• Measure
• Evaluate
• Modify
• With inspiration from Andrew Murray’s presentation "12 Lessons learnt
in boot time reduction"

Example system
• BeagleBone Black (single core Cortex A-8,
1GHz)
• Built using Yocto Project Pyro
• Qt app (qt4e-demo-image)
• systemd init

Measuring boot time
Commonly used tools:
• Grabserial: adds timstamps to all serial console output
• Bootchart: graphs running time of user-space processes
• Bootgraph: graphs running time of kernel functions
You can also change outputs (GPIOs) at certain points in the code and use
an oscilloscope to monitor them. This is the only way you can measure
the time from power-on

Grabserial
• Get it from https://elinux.org/Grabserial (written Tim Bird)
• You will need to install python-serial as well
sudo apt install python-serial
Then:
grabserial -d /dev/ttyUSB0 -t -m "U-Boot SPL" -e 30 -o boot.log
-d = serial device
-t = print timestamps
-m = match string before starting to log
-e = number of seconds to capture for
(useful when redirecting to a file: makes sure file is closed cleanly
and all output is recorded)
-o [filename] = output to file

Grabserial output
[0.000000 0.000000]
[0.000398 0.000398] U-Boot SPL 2017.01 (Oct 17 2017 - 11:10:21)
[0.095607 0.095607] Trying to boot from MMC1MMC partition switch failed
[0.188577 0.092970] *** Warning - MMC partition switch failed, using default
[0.190962 0.002385]
[0.203591 0.012629] reading u-boot.img
[0.204300 0.000709] reading u-boot.img
[0.259620 0.055320]
[0.259727 0.000107]
[0.259781 0.000054] U-Boot 2017.01 (Oct 17 2017 - 11:10:21 +0100)
[0.261334 0.001553]
[0.261384 0.000050] CPU : AM335X-GP rev 2.0
[...]
11.756176 0.080002] Started qtdemo.

Baseline measurement
3.52 4.53 3.70
11.76
U-Boot Kernel User space
From power-on to starting qtdemo app, in seconds

Reducing user-space boot time
• User-space is often simplest to change, so we start here
• Typical modifications
• Easy: running order in init
• Quite easy: build optimizations, e.g. compile flags
• Difficult: library optimizations to reduce load time

Measuring init boot time using Bootchart
• Two parts
• bootchartd captures process stats, starting with init
• part of Busybox, and also a package in Yocto Project/OpenEmbedded
• bootchart analyses ofﬂine, and creates a graph
• http://www.bootchart.org
• Launch it in place of init; it will fork and then exec /sbin/init
init=/sbin/bootchartd
• Logs are put in /var/log/bootchart.tgz
• Example on next slide: you are not expected to be able to read it!

Bootchart

Optimizing user space
• Careful analysis of bootchart shows that qtdemo is not started until 3.5
seconds after init begins
• Since this is the most important thing to run, we need to move it up the
batting order
• Some options:
• If using systemV init, use a lower number for the "S" script, e.g. "S01"
• If using systemd, change the dependencies of the unit (example at end
of slides)
• Or, run qtdemo ahead of init (*)
(*) be aware that the rootfs will most likely be ro, that other fs will not be
mounted, and that the network will not be started

Running the app as init
• This Script runs qtdemo in the background and then execs the normal
init so that it is still PID 1:
#!/bin/sh
echo "Starting qtdemo"
/usr/bin/qtdemoE -qws &
echo "qtdemo started"
sleep 1
exec /sbin/init
• Change kernel command line:
init=/usr/bin/run-qtdemo.sh

First pass
3.52 4.53 0.12
8.17
Launching qtdemo ahead of init saves 3.5 s

Reducing kernel boot time
• Easy: reduce console messages by adding quiet to cmdline
• Moderate: slim down kernel by removing drivers, ﬁlesystems,
subsysems
• Moderate: slim down device tree by removing hardware interfaces that
are not used
• Tricky: set about optimizing badly behaved drivers

Measuring kernel boot time: Bootgraph
• Bootgraph analyses kernel functions during boot
• Conﬁgure kernel with CONFIG_PRINTK_TIME and CONFIG_KALLSYMS
• Add initcall_debug to kernel cmdline
• Then boot, and get the kernel log
dmesg > boot.log
• Back on the host, create the graph using
linux/scripts/bootgraph.pl boot.log > boot.svg

Bootgraph
calibrate_xor_blocks
__omap_hwmod_setup_all
init_ladder
init_menu
customize_machine
param_sysfs_init
pm_sysrq_init
raid6_select_algo
edma_init
tracer_init_tracefs
event_trace_init
chr_dev_init
slab_sysfs_init
serial_omap_init
brd_init
mtdoops_init
cpsw_driver_init
am335x_child_init
omap_hsmmc_driver_init
inet6_init
init_machine_late
init_btrfs_fs
clk_debug_init
deferred_probe_initcall
0.17
0.35
0.52
0.7
0.88
1.05
1.23
1.41
1.58
1.76
1.94
2.11
2.29
2.47
2.64
raid6_select_algo
• Note: almost 2 seconds in raid6_select_algo
• RAID6 is selected by CONFIG_BTRFS_FS, which we are not using. So, rip it
out!

Second pass
3.52 1.52 0.12
5.16
• Remove BTRFS: saves 1860 ms
• Add "quiet" to cmdline: saves 700 ms
• Cut out unused drivers (e.g. WiFi), reducing zImage from 5.6 MiB to
3.2 MiB: saves 450 ms
• Total saving: 3.01 s

Reducing bootloader boot time
• Easy: remove the boot delay
• Easy(ish): simplify boot scripts
• Medium: compile out functions you don’t need
• Harder: use falcon mode

Remove boot delay
• U-Boot usually conﬁgured with a delay, during which you can press
space bar to stop booting
Press SPACE to abort autoboot in 2 seconds
• Set the display to zero
setenv bootdelay 0
• You can still hit space bar to halt boot if you are quick enough
OK, this is a bit trivial, but it still needs to be done

Simplify boot scripts
• Bootscript for BeagleBone is c. 100 lines
• Testing all possible sources of boot images takes time
• Loading image from ﬁlesystems takes time
• Reduce the script to the cases you support
• Loading from raw MMC sectors is fastest

Falcon mode
• Most SoCs have three stages of bootloader
• 1: on-chip ROMcode, which loads SPL into static RAM
• 2: SPL, which has to be small enough to ﬁt in SRAM. Main job is to set
up DRAM and load TPL into it
• 3: TPL: the main bootloader, which loads the kernel (4th stage)
• U-Boot Falcon mode jams everything into the SPL, skipping the TPL
• No user interaction: uses pre-built environment block
• More info in extra slides

Third pass
0.97 1.52 0.12
2.83
• Remove boot delay: saves 2000 ms
• Simplify boot script: saves: 230 ms
• Total saving: 2.32 s

Keep it up!
• Add boot time calculation as a metric in your CI
• Make boot time a criterion when adding new features

Conclusion
• Boot time reduction need not require massive changes to the platform
code
• Bake the changes you do make into the build system
• Look to the future

• Questions?

Systemd unit
• A systemd unit that will be run early
• Dependent on basic target; has no default dependencies
[Unit]
Description=qtdemo
DefaultDependencies=no
[Service]
ExecStart=/usr/bin/qtdemoE -qws
[Install]
WantedBy=basic.target

U-Boot Falcon mode
• See doc/README.falcon
• Steps
• Conﬁgure U-Boot
• Copy to SD card
• Boot into U-Boot and prepare environment
• Save environment in MMC

MMC layout
0x00000000
0x00010000
0x00040000
0x00060000
0x000e0000
0x00120000
0x01000000
MBR
Device tree +
arguments
SPL
U-Boot
U-Boot env
Kernel
• Typically, place images directory in MMC
storage
• This is the layout of the ﬁrst 16MiB
(128K sectors)

Conﬁgure U-Boot
• Tell U-Boot where to put things
• Add to include/configs/am335x_evm.h:
#define CONFIG_SYS_MMC_ENV_DEV 0
#define CONFIG_ENV_OFFSET 0xe0000
• Build U-Boot in the normal way
$ export CROSS_COMPILE=arm-buildroot-linux-gnueabihf-
$ make am335x_boneblack_defconfig
$ make

Copy images to micro SD card
• Using the layout from the diagram shown earlier
• Note: requires a uImage, not zImage
$ sudo dd if=am335x-boneblack.dtb of=/dev/mmcblk0 bs=1 seek=65536
$ sudo dd if=MLO of=/dev/mmcblk0 bs=1 seek=262144
$ sudo dd if=u-boot.img of=/dev/mmcblk0 bs=1 seek=393216
$ sudo dd if=uImage of=/dev/mmcblk0 bs=1 seek=1179648

Prepare Falcon boot 1/2
=> mmc read 82000000 900 2000
=> mmc read 83000000 80 180
=> spl export fdt 82000000 - 83000000
## Booting kernel from Legacy Image at 82000000 ...
Image Name: Linux-4.10.17-yocto-standard
Created: 2017-10-21 13:12:02 UTC
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 3365704 Bytes = 3.2 MiB
Load Address: 80008000
Entry Point: 80008000
Verifying Checksum ... OK
## Flattened Device Tree blob at 83000000
Booting using the fdt blob at 0x83000000
Loading Kernel Image ... OK
Loading Device Tree to 8fff4000, end 8ffffc86 ... OK
Loading Device Tree to 8ffe5000, end 8fff3c86 ... OK
Argument image is now in RAM: 0x8ffe5000

Prepare Falcon boot 2/2
• Write the modiﬁed FDT and parameter block back to MMC:
=> mmc write 0x8ffe5000 80 180
• Enable falcon mode:
=> setenv boot_os 1
• Now re boot and it should jump straight from SPL to kernel:
U-Boot SPL 2017.05-rc3-dirty (Oct 21 2017 - 11:27:01)
Trying to boot from MMC1
Booting Linux on physical CPU 0x0
Linux version 4.10.17-yocto-standard (oe-user@oe-host)

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