linux-kernel - Re: [PATCH v2 tip/core/rcu 04/11] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew

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Message-Id: <20170418184207.GA3956@linux.vnet.ibm.com>
Date:   Tue, 18 Apr 2017 11:42:07 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH v2 tip/core/rcu 04/11] rcu: Make RCU_FANOUT_LEAF help
 text more explicit about skew_tick

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@...ux.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