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

[Peter Zijlstra <peterz@infradead.org>]

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.

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?