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Date: Wed, 19 Apr 2017 15:22:26 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
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Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text
 more explicit about skew_tick
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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 :/