[N/U][PATCH 1/1] make lazy RCU a boot time option

[Andrea Righi]

BugLink: https://bugs.launchpad.net/bugs/2045492

[Impact]

With LP: #2023007 we have decided to enable CONFIG_RCU_LAZY in the
lowlatency kernel to improve power consumption, but this option can
potentially introduce performance regressions in some cases, due to the
fact that RCU callbacks are now batched and flushed all at once after a
timed delay.

It would be definitely safer to have a way to enable/disable lazy RCUs
at boot time. In this way we could provide a simple kernel command line
option that can be used in all those cases where the lowlatency kernel
is required, but we don't want to risk performance regressions.

[Test case]

In this context providing a single test case is not relevant. After
applying the fix any performance benchmark can be used to evaluate if
lazy RCU feature should be enabled at boot time or not (according to the
specific context where the lowlatency kernel is going to be
used/deployed).

[Fix]

Apply this patch to the *generic* kernel:
https://lore.kernel.org/lkml/20231203011252.233748-1-qyousef@layalina.io/T/#u

We want to apply this to the generic kernel, not just lowlatency,
because in this way *all* derivatives will have the possibility to get
this feature, in case some of them want to enable lazy RCUs (even
generic itself).

Then make sure that lowlatency (or any other kernel with
CONFIG_RCU_LAZY=y) also has CONFIG_RCU_LAZY_DEFAULT_OFF not set (so that
the previous behavior is not changed).

[Regression potential]

We may receive reports of small performance regressions vs power
consumption regressions, depending on the rcutree.enable_rcu_lazy
command line option that is used. In such case we should suggest the
user to test both with lazy RCU disabled or enabled.