ANDROID: rcu: Add a minimum time for marking boot as completed

On many systems, a great deal of boot (in userspace) happens after the
kernel thinks the boot has completed. It is difficult to determine if
the system has really booted from the kernel side. Some features like
lazy-RCU can risk slowing down boot time if, say, a callback has been
added that the boot synchronously depends on. Further expedited callbacks
can get unexpedited way earlier than it should be, thus slowing down
boot (as shown in the data below).

For these reasons, this commit adds a config option
'CONFIG_RCU_BOOT_END_DELAY' and a boot parameter rcupdate.boot_end_delay.
Userspace can also make RCU's view of the system as booted, by writing the
time in milliseconds to: /sys/module/rcupdate/parameters/android_rcu_boot_end_delay
Or even just writing a value of 0 to this sysfs node.
However, under no circumstance will the boot be allowed to end earlier
than just before init is launched.

The default value of CONFIG_RCU_BOOT_END_DELAY is chosen as 15s. This
suites ChromeOS and also a PREEMPT_RT system below very well, which need
no config or parameter changes, and just a simple application of this
patch. A system designer can also choose a specific value here to keep
RCU from marking boot completion.  As noted earlier, RCU's perspective
of the system as booted will not be marker until at least
android_rcu_boot_end_delay milliseconds have passed or an update is made
via writing a small value (or 0) in milliseconds to:
/sys/module/rcupdate/parameters/android_rcu_boot_end_delay.

One side-effect of this patch is, there is a risk that a real-time workload
launched just after the kernel boots will suffer interruptions due to expedited
RCU, which previous ended just before init was launched. However, to mitigate
such an issue (however unlikely), the user should either tune
CONFIG_RCU_BOOT_END_DELAY to a smaller value than 15 seconds or write a value
of 0 to /sys/module/rcupdate/parameters/android_rcu_boot_end_delay, once userspace
boots, and before launching the real-time workload.

Qiuxu also noted impressive boot-time improvements with earlier version
of patch. An excerpt from the data he shared:

1) Testing environment:
    OS            : CentOS Stream 8 (non-RT OS)
    Kernel     : v6.2
    Machine : Intel Cascade Lake server (2 sockets, each with 44 logical threads)
    Qemu  args  : -cpu host -enable-kvm, -smp 88,threads=2,sockets=2, …

2) OS boot time definition:
    The time from the start of the kernel boot to the shell command line
    prompt is shown from the console. [ Different people may have
    different OS boot time definitions. ]

3) Measurement method (very rough method):
    A timer in the kernel periodically prints the boot time every 100ms.
    As soon as the shell command line prompt is shown from the console,
    we record the boot time printed by the timer, then the printed boot
    time is the OS boot time.

4) Measured OS boot time (in seconds)
   a) Measured 10 times w/o this patch:
        8.7s, 8.4s, 8.6s, 8.2s, 9.0s, 8.7s, 8.8s, 9.3s, 8.8s, 8.3s
        The average OS boot time was: ~8.7s

   b) Measure 10 times w/ this patch:
        8.5s, 8.2s, 7.6s, 8.2s, 8.7s, 8.2s, 7.8s, 8.2s, 9.3s, 8.4s
        The average OS boot time was: ~8.3s.

(CHROMIUM tag rationale: Submitted upstream but got lots of pushback as
it may harm a PREEMPT_RT system -- the concern is VERY theoretical and
this improves things for ChromeOS. Plus we are not a PREEMPT_RT system.
So I am strongly suggesting this mostly simple change for ChromeOS.)

Bug: 258241771
Bug: 268129466
Test: boot
Tested-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Change-Id: Ibd262189d7f92dbcc57f1508efe90fcfba95a6cc
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4350228


Commit-Queue: Joel Fernandes <joelaf@google.com>
Commit-Queue: Vineeth Pillai <vineethrp@google.com>
Tested-by: Vineeth Pillai <vineethrp@google.com>
Tested-by: Joel Fernandes <joelaf@google.com>
Reviewed-by: Vineeth Pillai <vineethrp@google.com>
(cherry picked from commit 7968079ec77b320ee9d4115fe13048a8f7afbc02)
[Cherry picked from chromeos-5.15 tree. Minor tweaks to commit message
to match Android style. Prefix boot param with android_]
Signed-off-by: Qais Yousef <qyousef@google.com>