Re: need_heavy_qs flag for PREEMPT=y kernels

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

On Mon, Aug 12, 2019 at 09:05:25PM -0400, Joel Fernandes wrote:
> On Mon, Aug 12, 2019 at 09:02:49PM -0400, Joel Fernandes wrote:
> [snip]
> > > 							Thanx, Paul
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 8c494a692728..ad906d6a74fb 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -651,6 +651,12 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
> > >  	 */
> > >  	if (rdp->dynticks_nmi_nesting != 1) {
> > >  		trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2, rdp->dynticks);
> > > +		if (tick_nohz_full_cpu(rdp->cpu) &&
> > > +		    rdp->dynticks_nmi_nesting == 2 &&
> > > +		    rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
> > > +			rdp->rcu_forced_tick = true;
> > > +			tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU);
> > > +		}
> > 
> > 
> > Instead of checking dynticks_nmi_nesting == 2 in rcu_nmi_exit_common(), can
> > we do the tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU)  from
> > rcu_nmi_enter_common() ? We could add this code there, under the "if
> > (rcu_dynticks_curr_cpu_in_eqs())".
> 
> Actually, the other way. if (!rcu_dynticks_curr_cpu_in_eqs()) then do the
> tick_dep_set_cpu().

That does sound more likely.  Still, need to work out what the
breakage is.

							Thanx, Paul