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:   Wed, 19 Apr 2017 15:22:26 +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 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 :/