Re: [PATCH tip/core/rcu 04/26] rcu: Settle config for userspace extended quiescent state

Josh Triplett
2012-09-05T00:46:34Z

On Tue, Sep 04, 2012 at 05:34:59PM -0700, Paul E. McKenney wrote:
> On Fri, Aug 31, 2012 at 04:44:01PM -0700, Josh Triplett wrote:
> > On Thu, Aug 30, 2012 at 02:05:21PM -0700, Paul E. McKenney wrote:
> > > From: Frederic Weisbecker <fweisbec@gmail.com>
> > >
> > > 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>
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >
> > One question below, but nonethelesss:
> >
> > Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> >
> > > arch/Kconfig | 10 ++++++++++
> > > include/linux/rcupdate.h | 8 ++++++++
> > > init/Kconfig | 10 ++++++++++
> > > kernel/rcutree.c | 5 ++++-
> > > 4 files changed, 32 insertions(+), 1 deletions(-)
> > >
> > > diff --git a/arch/Kconfig b/arch/Kconfig
> > > index 72f2fa1..1401a75 100644
> > > --- a/arch/Kconfig
> > > +++ b/arch/Kconfig
> > > @@ -281,4 +281,14 @@ 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 need to be wrapped inside
> > > + rcu_user_exit()-rcu_user_enter() through the slow path using
> > > + TIF_NOHZ flag. Exceptions handlers must be wrapped as well. Irqs
> > > + are already protected inside rcu_irq_enter/rcu_irq_exit() but
> > > + preemption or signal handling on irq exit still need to be protected.
> > > +
> > > source "kernel/gcov/Kconfig"
> > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > index 81d3d5c..e411117 100644
> > > --- a/include/linux/rcupdate.h
> > > +++ b/include/linux/rcupdate.h
> > > @@ -191,10 +191,18 @@ extern void rcu_idle_enter(void);
> > > extern void rcu_idle_exit(void);
> > > extern void rcu_irq_enter(void);
> > > extern void rcu_irq_exit(void);
> > > +
> > > +#ifdef CONFIG_RCU_USER_QS
> > > extern void rcu_user_enter(void);
> > > extern void rcu_user_exit(void);
> > > extern void rcu_user_enter_irq(void);
> > > extern void rcu_user_exit_irq(void);
> > > +#else
> > > +static inline void rcu_user_enter(void) { }
> > > +static inline void rcu_user_exit(void) { }
> > > +#endif /* CONFIG_RCU_USER_QS */
> > > +
> > > +
> > > extern void exit_rcu(void);
> > >
> > > /**
> > > diff --git a/init/Kconfig b/init/Kconfig
> > > index af6c7f8..f6a1830 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
> >
> > Does this actually depend on SMP, or does it depend on the non-TINY RCU
> > implementation? If the latter, it should depend on that rather than
> > SMP.
> >
> > (I assume that the tiny RCU implementation simply doesn't need all this
> > machinery because it doesn't need coordinated quiescence at all? Or
> > does tiny RCU still cause a periodic wakeup on UP?)
>
> It actually does depend on SMP. There has to be at least one CPU taking
> scheduling-clock interrupts in order to keep time computation accurate,
> so a de-facto UP system cannot adaptive-dynticks its sole CPU.

Ah. That seems like a removable limitation, albeit a difficult one.
Nonetheless, it makes sense to avoid providing the option when it won't
help.

However, once a config symbol for adaptive dynticks exists, perhaps that
symbol should depend on SMP and RCU_USER_QS should depend on that
instead, documenting the limitation in the right place and making it
easier to find and change eventually.

- Josh Triplett