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: <20170413174631.56ycg545gwbsb4q2@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Apr 2017 19:46:31 +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 10:31:00AM -0700, Paul E. McKenney wrote:
> On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote:
> > On Thu, Apr 13, 2017 at 09:55:16AM -0700, Paul E. McKenney wrote:
> > > > > To avoid people tuning huge machines having to wait for me to give
> > > > > them an answer as to why they are suffering lock contention after
> > > > > cranking up the value of RCU_FANOUT_LEAF.
> > 
> > So is there a good reason to increase FANOUT_LEAF ?
> 
> 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.

Is that perhaps a good moment to look at aligning the cpus in said nodes
with the cache topology?