[Bug 2141880] [NEW] Please enable CONFIG_RCU_LAZY

Public bug reported:

Please enable CONFIG_RCU_LAZY=y and CONFIG_RCU_LAZY_DEFAULT_OFF=y in
Ubuntu RT kernels.

CONFIG_RCU_LAZY allows (but doesn't require) us to delay RCU grace
period start by 10s or so when the system is idle in order to conserve
battery power. GP start is expedited when system load appears. Android
uses CONFIG_RCU_LAZY, combined with full RCU offload, to avoid
unnecessary wakeups under load and saves about 10-15% power in the idle
static-image-display scenario. I'd like to use lazy RCU on my laptop for
the same reason. While the regular Ubuntu kernel enables
CONFIG_RCU_LAZY, the RT kernel does not.

With CONFIG_RCU_LAZY_DEFAULT_OFF=y, kernel defaults to disabling lazy
RCU, resulting in no change of RT kernel relative to today.

** Affects: linux (Ubuntu)
Importance: Undecided
Status: New

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https://bugs.launchpad.net/bugs/2141880

Title:
Please enable CONFIG_RCU_LAZY

Status in linux package in Ubuntu:
New

Bug description:
Please enable CONFIG_RCU_LAZY=y and CONFIG_RCU_LAZY_DEFAULT_OFF=y in
Ubuntu RT kernels.

CONFIG_RCU_LAZY allows (but doesn't require) us to delay RCU grace
period start by 10s or so when the system is idle in order to conserve
battery power. GP start is expedited when system load appears. Android
uses CONFIG_RCU_LAZY, combined with full RCU offload, to avoid
unnecessary wakeups under load and saves about 10-15% power in the
idle static-image-display scenario. I'd like to use lazy RCU on my
laptop for the same reason. While the regular Ubuntu kernel enables
CONFIG_RCU_LAZY, the RT kernel does not.

With CONFIG_RCU_LAZY_DEFAULT_OFF=y, kernel defaults to disabling lazy
RCU, resulting in no change of RT kernel relative to today.

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