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: <20170413181926.GP3956@linux.vnet.ibm.com>
Date:   Thu, 13 Apr 2017 11:19:26 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 07:46:31PM +0200, Peter Zijlstra wrote:
> 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?

We have been here before, haven't we?  Over and over again.  ;-)

As always...

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.

							Thanx, Paul