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

From: Paul E. McKenney
Date: Fri Feb 20 2015 - 11:37:48 EST

On Fri, Feb 20, 2015 at 10:11:07AM +0100, Peter Zijlstra wrote:
> On Thu, Feb 19, 2015 at 09:08:50PM -0800, Paul E. McKenney wrote:
> > Hello!
> >
> > This series, possibly for v3.21, contains changes that allow in-kernel
> > code to specify that all subsequent synchronous grace-period primitives
> > (synchronize_rcu() and friends) be expedited. New rcu_expedite_gp()
> > and rcu_unexpedite_gp() primitives enable and disable expediting,
> > and these may be nested. Note that the rcu_expedited boot/sysfs
> > variable, if non-zero, causes expediting to happen regardless of calls
> > to rcu_expedite_gp().
> >
> > Because one of the use cases for these primitives is to expedite
> > grace periods during the in-kernel portion of boot, a new Kconfig
> > parameter named CONFIG_RCU_EXPEDITE_BOOT causes the kernel to act
> > as if rcu_expedite_gp() was called very early in boot. At the end
> > of boot (presumably just before init is spawned), a call to
> > rcu_end_inkernel_boot() will provide the matching rcu_unexpedite_gp()
> > if required.
> 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.

> 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.

Thanx, Paul

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