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

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Message-Id: <20170413194228.GS3956@linux.vnet.ibm.com>
Date:   Thu, 13 Apr 2017 12:42:28 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 08:29:39PM +0200, Peter Zijlstra wrote:
> 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.

In other words, motivating the order of the skewed ticks to be guided
by hardware locality?

							Thanx, Paul