need_heavy_qs flag for PREEMPT=y kernels

[Joel Fernandes <joel@joelfernandes.org>]

Hi Paul, everyone,

I noticed on reading code that the need_heavy_qs check and
rcu_momentary_dyntick_idle() is only called for !PREEMPT kernels. Don't we
need to call this for PREEMPT kernels for the benefit of nohz_full CPUs?

Consider the following events:
1. Kernel is PREEMPT=y configuration.
2. CPU 2 is a nohz_full CPU running only a single task and the tick is off.
3. CPU 2 is running only in kernel mode and does not enter user mode or idle.
4. Grace period thread running on CPU 3 enter the fqs loop.
5. Enough time passes and it sets the need_heavy_qs for CPU2.
6. CPU 2 is still in kernel mode but does cond_resched().
7. cond_resched() does not call rcu_momentary_dyntick_idle() because PREEMPT=y.

Is 7. not calling rcu_momentary_dyntick_idle() a lost opportunity for the FQS
loop to detect that the CPU has crossed a quiescent point?

Is this done so that cond_resched() is fast for PREEMPT=y kernels?

thanks,

 - Joel