[Bug 1615685] Re: systemd-analyze stats for kernel startup time looks suspect

[Dimitri John Ledkov]

Systems that report more times, run systemd in the initrd.

We do not run systemd in the initrd.

To save time on the boot up, we are working on enabling more systems
boot initramfs-less, with an automatic fallback to a boot with
initramfs. In the near future most cloud instances, and some bare metal
systems will be doing that by default.

I will check if it is at all possible to fake the timings in the
Ubuntu's initramfs, to supply timings for systemd-analyze.

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Title:
  systemd-analyze stats for kernel startup time looks suspect

Status in systemd package in Ubuntu:
  New

Bug description:
  I've instrumented a kernel so I can clearly see when processes start
  and exit, this allows me to see when exactly the kernel has handed off
  control to userspace.  The time the kernel actually takes to
  initialize compared to the time systemd-analyze reports are different.

  $ systemd-analyze 
  Startup finished in 16.878s (kernel) + 43.152s (userspace) = 1min 30ms

  For example:

  [    2.286330] Freeing unused kernel memory: 1532K (ffffffffb1755000 - ffffffffb18d4000)
  [    2.296300] Write protecting the kernel read-only data: 12288k
  [    2.304356] Freeing unused kernel memory: 1872K (ffff9d9c6e42c000 - ffff9d9c6e600000)
  [    2.317659] Freeing unused kernel memory: 1208K (ffff9d9c6e8d2000 - ffff9d9c6ea00000)
  [    2.344896] x86/mm: Checked W+X mappings: passed, no W+X pages found.
  [    2.368095] exec: 1 (swapper/0) -> /init
  [    2.377284] _do_fork: 1 init
  [    2.381933] exit 53 init
  [    2.386444] _do_fork: 1 init
  [    2.390741] exit 54 init
  [    2.394899] _do_fork: 1 init

  and we have the initramfs initialization occurring, and then finally
  systemd starts:

  [   16.571630] exec: 1 (run-init) -> /sbin/init
  [   17.287342] systemd[1]: systemd 229 running in system mode. (+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ -LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN)
  [   17.307892] systemd[1]: Detected virtualization microsoft.
  [   17.314765] systemd[1]: Detected architecture x86-64.
  [   17.421589] systemd[1]: Set hostname to <boottest>.

  So it seems that systemd is accounting initramfs time in the kernel
  time stats.  It would be preferable to be able to report the kernel
  init time, the initramfs time and the userspace time rather just
  kernel + userspace.

  As it stands, the kernel passed off control to systemd at 16.571630
  seconds, so I have no no idea why systemd is reporting 16.878 seconds.

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