Re: [PATCH 1/6] rcu: Settle config for userspace extended quiescent state

[Frederic Weisbecker <fweisbec@gmail.com>]

On Fri, Jul 06, 2012 at 09:31:29AM -0700, Paul E. McKenney wrote:
> On Fri, Jul 06, 2012 at 02:00:13PM +0200, Frederic Weisbecker wrote:
> > Create a new config option under the RCU menu that put
> > CPUs under RCU extended quiescent state (as in dynticks
> > idle mode) when they run in userspace. This require
> > some contribution from architectures to hook into kernel
> > and userspace boundaries.
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Alessio Igor Bogani <abogani@kernel.org>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Avi Kivity <avi@redhat.com>
> > Cc: Chris Metcalf <cmetcalf@tilera.com>
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Geoff Levand <geoff@infradead.org>
> > Cc: Gilad Ben Yossef <gilad@benyossef.com>
> > Cc: Hakan Akkan <hakanakkan@gmail.com>
> > Cc: H. Peter Anvin <hpa@zytor.com>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: Josh Triplett <josh@joshtriplett.org>
> > Cc: Kevin Hilman <khilman@ti.com>
> > Cc: Max Krasnyansky <maxk@qualcomm.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Stephen Hemminger <shemminger@vyatta.com>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > ---
> >  arch/Kconfig     |   13 +++++++++++++
> >  init/Kconfig     |   10 ++++++++++
> >  kernel/rcutree.c |    4 ++++
> >  3 files changed, 27 insertions(+), 0 deletions(-)
> > 
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 8c3d957..c2e0ce4 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -274,4 +274,17 @@ config SECCOMP_FILTER
> > 
> >  	  See Documentation/prctl/seccomp_filter.txt for details.
> > 
> > +config HAVE_RCU_USER_QS
> > +	bool
> > +	help
> > +	  Provide kernel entry/exit hooks necessary for userspace
> > +	  RCU extended quiescent state. Syscalls and exceptions
> > +	  low level handlers must be wrapped with a call to rcu_user_exit()
> > +	  on entry and rcu_user_enter() before resuming userspace. Irqs
> > +	  entry don't need to call rcu_user_exit() because their high level
> > +	  handlers are protected inside rcu_irq_enter/rcu_irq_exit() but
> > +	  preemption or signal handling on irq exit still need to be protected
> > +	  with a call to rcu_user_exit(). rcu_user_enter() must then be
> > +	  called back on irq exit when the preempted task is back on the CPU.
> > +
> >  source "kernel/gcov/Kconfig"
> > diff --git a/init/Kconfig b/init/Kconfig
> > index d07dcf9..3a4af8f 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -441,6 +441,16 @@ config PREEMPT_RCU
> >  	  This option enables preemptible-RCU code that is common between
> >  	  the TREE_PREEMPT_RCU and TINY_PREEMPT_RCU implementations.
> > 
> > +config RCU_USER_QS
> > +	bool "Consider userspace as in RCU extended quiescent state"
> > +	depends on HAVE_RCU_USER_QS && SMP
> 
> OK, I'll bite...  Why the "SMP"?  RCU could make good use of knowing
> about user-mode executing even in UP kernels.

Because Tiny RCU doesn't implement rcu_user_enter()/exit yet. And it
doesn't need it for now.

To better express the constraint I should probably have used:

	depends on TREE_RCU || TREE_PREEMPT_RCU