Date: Fri, 20 Feb 2015 09:14:42 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com,
        oleg@redhat.com, bobby.prani@gmail.com, arjan@linux.intel.com
Subject: Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace
 periods
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On Fri, Feb 20, 2015 at 05:54:09PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 20, 2015 at 08:37:37AM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 20, 2015 at 10:11:07AM +0100, Peter Zijlstra wrote:
> > > So I though we wanted to get rid / limit the expedited stuff because its
> > > IPI happy, and here its spreading.
> > 
> > Well, at least it no longer IPIs idle CPUs.  ;-)
> > 
> > And this is during boot, when a few extra IPIs should not be a big deal.
> 
> Well the one application now is during boot; but you expose the
> interface for all to use, and therefore someone will.

I could make rcu_expedite_gp() and rcu_unexpedite_gp() be static,
I suppose.  Except that I need to test them with rcutorture.
I suppose I could put the declaration in rcutorture.c, but then
0day will tell me to made them static.  :-/

> > > Does it really make a machine boot much faster? Why are people using
> > > synchronous gp primitives if they care about speed? Should we not fix
> > > that instead?
> > 
> > The report I heard was that it provided 10-15% faster boot times.
> 
> That's not insignificant; got more details? I think we should really
> look at why people are using the sync primitives.

I must defer to the people who took the exact measurements.

But yes, once I have that info, I should add it to the commit log.

							Thanx, Paul

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