Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace periods

From: Paul E. McKenney
Date: Fri Feb 20 2015 - 12:14:53 EST

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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