fastboot: add notes from previous schulungen

Signed-off-by: John Ogness <john.ogness@linutronix.de>

1 files changed, 81 insertions, 0 deletions

diff --git a/schulung_tools/notes/fastboot.txt b/schulung_tools/notes/fastboot.txt
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+This is a general procedure for improving Linux boot times. It only deals
+with Linux drivers and userspace.
+
+NOTE: Everything before Linux (hardware, bootloader) have very important
+      roles also. They also must be considered for a complete fast boot
+      concept.
+
+1. Add "poweroff -f" to the beginning of the boot script /etc/init.d/rcS
+   - this allows us to measure time with "time ./start-vm.sh"
+
+2. Do not enable audio in qemu: QEMU_AUDIO_DRV=none
+   - this reduces qemu startup time (we want to measure Linux, not qemu)
+
+3. Switch from NFS-Root to initramfs.
+   - this avoids ethernet setup and DHCP at boot
+   - this avoids network latencies during filesystem interaction
+   - initramfs uses /init as default start program, so we add a soft link
+     from /init to bin/busybox in the root filesystem
+   - removed "ip=dhcp root=... nfsroot=..." from kernel arguments
+
+4. Strip all libraries and binaries.
+   - this makes the initramfs *much* smaller
+   - smaller initramfs means faster "populate rootfs" on boot
+
+5. Remove libm dependency from busybox.
+   - the busybox features using libm are not needed for fast boot requirements
+   - removing libm means smaller initramfs
+   - less dependencies means faster busybox start time
+
+6. Add "printk.time=1 loglevel=8" to kernel arguments.
+   - ./start-vm.sh > boot.log
+   - browse boot.log to find any errors, unused features, slow non-important
+     features.
+   - disable or compile the drivers/features as modules
+
+7. Add "initcall_debug" to kernel arguments.
+   - now we can see the time consumed by each driver's init function
+   - if the device is in the device-tree, this time will also include
+     the device's probe time
+   - ./start-vm.sh > boot.log
+   - cat boot.log | linux-4.6/scripts/bootgraph.pl > boot.svg
+
+8. Remove (or change to modules) drivers not needed for fast boot requirements
+   - search kernel code to find init function
+   - look at the Makefile in that directory to see the corresponding
+     kernel config option
+   - remove amba_clcdfd_init (as example)
+   - change smsc911x_init to module (as example)
+
+9. Remove initcall_debug, consoles and set loglevel=0.
+   - we are done evaluating individual driver times
+   - the console is slow, so don't output anything to it
+   - maybe even disable the console in the kernel config
+
+10. Pre-configure loops per jiffy calibration.
+    - ./start-vm.sh | grep lpj=
+    - add lpj=xyz to kernel parameters
+
+11. Choose optimal kernel compression
+    - General setup -> Kernel compression mode
+    - set kernel compression to LZO
+
+12. Create fastpoweroff script to run instead of busybox-init.
+    - add kernel parameter "rdinit=/bin/fastpoweroff"
+    - now init is not started (since init did much more than just
+      run /etc/init.d/rcS)
+
+13. Create binary to run instead of script or busybox-init.
+    - change kernel parameter to "rdinit=/bin/poweroff"
+    - (busybox was hacked so "poweroff" automatically uses "-f")
+    - in #12 first a shell is loaded, then the script is parsed,
+      then poweroff is called
+    - now poweroff is directly called by kernel
+
+14. Add ld.so.cache for initramfs libraries.
+    - the dynamic loader now does not search for dependencies
+
+15. For systemd-based userspace, use systemd tools.
+    - systemd-analyze blame
+    - systemd-analyze plot > systemd.svg
+    - systemd-analyze critcal-chain app-qt