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: <20170419145053.GK3956@linux.vnet.ibm.com>
Date:   Wed, 19 Apr 2017 07:50:53 -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 Wed, Apr 19, 2017 at 03:22:26PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 13, 2017 at 11:42:32AM -0700, Paul E. McKenney wrote:
> 
> > I believe that you are missing the fact that RCU grace-period
> > initialization and cleanup walks through the rcu_node tree breadth
> > first, using rcu_for_each_node_breadth_first().
> 
> Indeed. That is the part I completely missed.
> 
> >                                                 This macro (shown below)
> > implements this breadth-first walk using a simple sequential traversal of
> > the ->node[] array that provides the structures making up the rcu_node
> > tree.  As you can see, this scan is completely independent of how CPU
> > numbers might be mapped to rcu_data slots in the leaf rcu_node structures.
> 
> So this code is clearly not a hotpath, but still its performance
> matters?
> 
> Seems like you cannot win here :/

Welcome to my world!!!  ;-)

But yes, running on 4096-CPU systems can put some serious stress on
some surprising areas.  Especially when those systems have cache-miss
latencies well in excess of a microsecond, and the users are nevertheless
expecting scheduling latencies well below 100 microseconds.

It was a fun challenge, I grant you that!

							Thanx, Paul