tree 6dc7829e3529fae2f519ab8dc1a665f8109cc311
parent 5afff48bdf7481570c9385a8a674a81ffb8f09ee
author Paul E. McKenney <paulmck@linux.vnet.ibm.com> 1424371892 -0800
committer Paul E. McKenney <paulmck@linux.vnet.ibm.com> 1424980983 -0800

rcu: Add Kconfig option to expedite grace periods during boot

This commit adds a CONFIG_RCU_EXPEDITE_BOOT Kconfig parameter
that emulates a very early boot rcu_expedite_gp().  A late-boot
call to rcu_end_inkernel_boot() will provide the corresponding
rcu_unexpedite_gp().  The late-boot call to rcu_end_inkernel_boot()
should be made just before init is spawned.

According to Arjan:

> To show the boot time, I'm using the timestamp of the "Write protecting"
> line, that's pretty much the last thing we print prior to ring 3 execution.
>
> A kernel with default RCU behavior (inside KVM, only virtual devices)
> looks like this:
>
> [    0.038724] Write protecting the kernel read-only data: 10240k
>
> a kernel with expedited RCU (using the command line option, so that I
> don't have to recompile between measurements and thus am completely
> oranges-to-oranges)
>
> [    0.031768] Write protecting the kernel read-only data: 10240k
>
> which, in percentage, is an 18% improvement.

Reported-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Arjan van de Ven <arjan@linux.intel.com>