Re: need_heavy_qs flag for PREEMPT=y kernels

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Sun, Aug 11, 2019 at 11:21:42PM -0400, Joel Fernandes wrote:
> On Sun, Aug 11, 2019 at 02:13:18PM -0700, Paul E. McKenney wrote:
> [snip]
> > This leaves NO_HZ_FULL=y&&PREEMPT=y kernels.  In that case, RCU is
> > more aggressive about using resched_cpu() on CPUs that have not yet
> > reported a quiescent state for the current grace period.
> 
> Just wanted to ask something - how does resched_cpu() help for this case?
> 
> Consider a nohz_full CPU and a PREEMPT=y kernel. Say a single task is running
> in kernel mode with scheduler tick off. As we discussed, we have no help from
> cond_resched() (since its a PREEMPT=y kernel).  Because enough time has
> passed (jtsq*3), we send the CPU a re-scheduling IPI.
> 
> This seems not that useful. Even if we enter the scheduler due to the
> rescheduling flags set on that CPU, nothing will do the rcu_report_qs_rdp()
> or rcu_report_qs_rnp() on those CPUs, which are needed to propagate the
> quiescent state to the leaf node. Neither will anything to do a
> rcu_momentary_dyntick_idle() for that CPU. Without this, the grace period
> will still end up getting blocked.
> 
> Could you clarify which code paths on a nohz_full CPU running PREEMPT=y
> kernel actually helps to end the grace period when we call resched_cpu() on
> it?  Don't we need atleast do a rcu_momentary_dyntick_idle() from the
> scheduler IPI handler or from resched_cpu() for the benefit of a nohz_full
> CPU? Maybe I should do an experiment to see this all play out.

An experiment would be good!

> And I need to write down everything I learnt today before I go crazy... ;-)

I know that feeling!  ;-)

							Thanx, Paul