Date: Thu, 13 Apr 2017 20:23:09 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com,
        bobby.prani@gmail.com
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 11:19:26AM -0700, Paul E. McKenney wrote:

> First get me some system-level data showing that the current layout is
> causing a real problem.  RCU's fastpath code doesn't come anywhere near
> the rcu_node tree, so in the absence of such data, I of course remain
> quite doubtful that there is a real need.  And painfully aware of the
> required increase in complexity.
> 
> But if there is a real need demonstrated by real system-level data,
> I will of course make the needed changes, as I have done many times in
> the past in response to other requests.

I read what you wrote here:

> > > Increasing it reduces the number of rcu_node structures, and thus the
> > > number of cache misses during grace-period initialization and cleanup.
> > > This has proven necessary in the past on large machines having long
> > > memory latencies.  And there are starting to be some pretty big machines
> > > running in production, and even for typical commerical workloads.

to mean you had exactly that pain. Or am I now totally not understanding
you?