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

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Message-ID: <CALCETrXwUeaRbDziA=7vgY3_r9u3E2wLLRwAU=GEiNhYq9jJwg@mail.gmail.com>
Date:   Mon, 13 Feb 2017 11:08:55 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "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 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.

--Andy