linux-kernel - Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew

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Date:   Thu, 13 Apr 2017 20:29:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text
 more explicit about skew_tick

On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote:
> On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote:

> > And I have vague memories of it actually causing lock contention, but
> > I've forgotten how that worked.
> 
> That is a new one on me.  I can easily see how not skewing ticks could
> cause serious lock contention, but am missing how skewed ticks would
> do so.

It could've been something like cacheline bouncing. Where with a
synchronized tick, the (global) cacheline would get used by all CPUs on
a node before heading out to the next node etc.. Where with a skewed
tick, it would forever bounce around.