Re: [RFC v1 2/2] rcu/tree: Remove dynticks_nmi_nesting counter

[Joel Fernandes]

Hi Paul,

On Thu, Aug 29, 2019 at 09:54:07AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 29, 2019 at 09:21:46AM -0700, Andy Lutomirski wrote:
> > On Thu, Aug 29, 2019 at 9:10 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Aug 29, 2019 at 10:43:55AM -0400, Joel Fernandes wrote:
> > >
> > > [ . . . ]
> > >
> > > > Paul, do we also nuke rcu_eqs_special_set()?  Currently I don't see anyone
> > > > using it. And also remove the bottom most bit of dynticks?
> > > >
> > > > Also what happens if a TLB flush broadcast is needed? Do we IPI nohz or idle
> > > > CPUs are the moment?
> > > >
> > > > All of this was introduced in:
> > > > b8c17e6664c4 ("rcu: Maintain special bits at bottom of ->dynticks counter")
> > >
> > > Adding Andy Lutomirski on CC.
> > >
> > > Andy, is this going to be used in the near term, or should we just get
> > > rid of it?
> > 
> > Let's get rid of it.  I'm not actually convinced it *can* be used as designed.
> > 
> > For those who forgot the history or weren't cc'd on all of it: I had
> > this clever idea about how we could reduce TLB flushes.  I implemented
> > some of it (but not the part that would have used this RCU feature),
> > and it exploded in nasty and subtle ways.  This caused me to learn
> > that speculative TLB fills were a problem that I had entirely failed
> > to account for.  Then PTI happened and thoroughly muddied the water.
> 
> Yeah, PTI was quite annoying.  Still is, from what I can see.  :-/
> 
> > So I think we should just drop this :(
> 
> OK, thank you!  I will put a tag into -rcu marking its removal in case
> it should prove useful whenever for whatever.
> 
> Joel, would you like to remove this, or would you rather that I did?
> It is in code you are working with right now, so if I do it, I need to
> wait until yours is finalized.  Which wouldn't be a problem.

I can remove it in my series, made a note to do so.

thanks,

 - Joel