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

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Message-Id: <20170213195249.GN30506@linux.vnet.ibm.com>
Date:   Mon, 13 Feb 2017 11:52:49 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH RFC v2 tip/core/rcu] Maintain special bits at bottom of
 ->dynticks counter

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@...radead.org> 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