Re: [PATCH -rcu dev 3/3] RFC: rcu/tree: Read dynticks_nmi_nesting in advance

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Aug 16, 2019 at 09:52:42AM -0700, Paul E. McKenney wrote:
> On Fri, Aug 16, 2019 at 12:24:04PM -0400, Joel Fernandes wrote:
> > On Thu, Aug 15, 2019 at 10:53:11PM -0400, Joel Fernandes (Google) wrote:
> > > I really cannot explain this patch, but without it, the "else if" block
> > > just doesn't execute thus causing the tick's dep mask to not be set and
> > > causes the tick to be turned off.
> > > 
> > > I tried various _ONCE() macros but the only thing that works is this
> > > patch.
> > > 
> > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > ---
> > >  kernel/rcu/tree.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 856d3c9f1955..ac6bcf7614d7 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -802,6 +802,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
> > >  {
> > >  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> > >  	long incby = 2;
> > > +	int dnn = rdp->dynticks_nmi_nesting;
> > 
> > I believe the accidental sign extension / conversion from long to int was
> > giving me an illusion since things started working well. Changing the 'int
> > dnn' to 'long dnn' gives similar behavior as without this patch! At least I
> > know now. Please feel free to ignore this particular RFC patch while I debug
> > this more (over the weekend or early next week). The first 2 patches are
> > good, just ignore this one.
> 
> Ah, good point on the type!  So you were ending up with zero due to the
> low-order 32 bits of DYNTICK_IRQ_NONIDLE being zero, correct?  If so,
> the "!rdp->dynticks_nmi_nesting" instead needs to be something like
> "rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE", which sounds like
> it is actually worse then the earlier comparison against the constant 2.
> 
> Sounds like I should revert the -rcu commit 805a16eaefc3 ("rcu: Force
> nohz_full tick on upon irq enter instead of exit").

I can't find that patch so all I can say so far is that its title doesn't
inspire me much. Do you still need that change for some reason?

Thanks.