Re: [PATCH RFC v2 tip/core/rcu] Maintain special bits at bottom of ->dynticks counter

From: Paul E. McKenney
Date: Mon Feb 13 2017 - 14:52:59 EST

On Mon, Feb 13, 2017 at 11:08:55AM -0800, Andy Lutomirski wrote:
> On Mon, Feb 13, 2017 at 9:57 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Mon, Feb 13, 2017 at 09:01:04AM -0800, Paul E. McKenney wrote:
> >> > I think I've asked this before, but why does this live in the guts of
> >> > RCU?
> >> >
> >> > Should we lift this state tracking stuff out and make RCU and
> >> > NOHZ(_FULL) users of it, or doesn't that make sense (reason)?
> >>
> >> The dyntick-idle stuff is pretty specific to RCU. And what precisely
> >> would be helped by moving it?
> >
> > Maybe untangle the inter-dependencies somewhat. It just seems a wee bit
> > odd to have arch TLB invalidate depend on RCU implementation details
> > like this.
> This came out of a courtyard discussion at KS/LPC. The idea is that
> this optimzation requires an atomic op that could be shared with RCU
> and that we probably care a lot more about this optimization on
> kernels with context tracking enabled, so putting it in RCU has nice
> performance properties. Other than that, it doesn't make a huge
> amount of sense.
> Amusingly, Darwin appears to do something similar without an atomic
> op, and I have no idea why that's safe.

Given that they run on ARM, I have no idea either. Maybe they don't
need to be quite as bulletproof on idle-duration detection? Rumor has it
that their variant of RCU uses program-counter ranges, so they wouldn't
have the RCU tie-in -- just checks of program-counter ranges and
interesting dependencies on the compiler.

Thanx, Paul