Date: Tue, 18 Apr 2017 11:42:07 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Josh Triplett <josh@joshtriplett.org>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, tglx@linutronix.de,
        peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com,
        bobby.prani@gmail.com
Subject: Re: [PATCH v2 tip/core/rcu 04/11] rcu: Make RCU_FANOUT_LEAF help
 text more explicit about skew_tick
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On Mon, Apr 17, 2017 at 05:18:25PM -0700, Josh Triplett wrote:
> On Mon, Apr 17, 2017 at 04:28:51PM -0700, Paul E. McKenney wrote:
> > If you set RCU_FANOUT_LEAF too high, you can get lock contention
> > on the leaf rcu_node, and you should boot with the skew_tick kernel
> > parameter set in order to avoid this lock contention.  This commit
> > therefore upgrades the RCU_FANOUT_LEAF help text to explicitly state
> > this.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> >  init/Kconfig | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/init/Kconfig b/init/Kconfig
> > index a92f27da4a27..946e561e67b7 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -612,11 +612,17 @@ config RCU_FANOUT_LEAF
> >  	  initialization.  These systems tend to run CPU-bound, and thus
> >  	  are not helped by synchronized interrupts, and thus tend to
> >  	  skew them, which reduces lock contention enough that large
> > -	  leaf-level fanouts work well.
> > +	  leaf-level fanouts work well.  That said, setting leaf-level
> > +	  fanout to a large number will likely cause problematic
> > +	  lock contention on the leaf-level rcu_node structures unless
> > +	  you boot with the skew_tick kernel parameter.
> >  
> >  	  Select a specific number if testing RCU itself.
> >  
> > -	  Select the maximum permissible value for large systems.
> > +	  Select the maximum permissible value for large systems, but
> > +	  	please understand that you may also need to set the
> > +		skew_tick kernel boot parameter to avoid contention
> > +		on the rcu_node structure's locks.
> 
> Nit: the indentation seems wrong here.

Right you are!  Fixed, thank you!

							Thanx, Paul