Re: [PATCH v2] EXP rcu: Move expedited grace period (GP) work to RT kthread_worker

[Joel Fernandes]

On Fri, Apr 8, 2022 at 12:57 AM Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
[..]
> +config RCU_EXP_KTHREAD
> +       bool "Perform RCU expedited work in a real-time kthread"
> +       depends on RCU_BOOST && RCU_EXPERT

Doesn't this disable the fix if a system is not RCU_EXPERT ?  Please
see the definition of RCU_EXPERT:
"This option needs to be enabled if you wish to make expert-level
adjustments to RCU configuration."

I don't think a bug fix counts as an expert-level adjustment.

> +       default !PREEMPT_RT && NR_CPUS <= 32

What is the benefit of turning it off on PREEMPT_RT, even if
PREEMPT_RT does not use expedited GPs much post-boot? I would think in
the future if PREEMPT_RT ever uses expedited GPs, they would want this
feature even more. I'd rather be future-proof now as I don't see any
advantages of disabling it on !PREEMPT_RT (And a drawback that the fix
won't apply to those systems). Also will keep the config simple.

> +       help
> +         Use this option to further reduce the latencies of expedited
> +         grace periods at the expense of being more disruptive.
> +
> +         Accept the default if unsure.
> +
>  config RCU_NOCB_CPU
>         bool "Offload RCU callback processing from boot-selected CPUs"
>         depends on TREE_RCU
[...]

Thanks,

 - Joel