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: <20170413182309.vmyivo3oqrtfhhxt@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Apr 2017 20:23:09 +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 11:19:26AM -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?