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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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.

							Thanx, Paul

> > +	help
> > +	  This option sets hooks on kernel / userspace boundaries and
> > +	  puts RCU in extended quiescent state when the CPU runs in
> > +	  userspace. It means that when a CPU runs in userspace, it is
> > +	  excluded from the global RCU state machine and thus doesn't
> > +	  to keep the timer tick on for RCU.
> > +
> >  config RCU_FANOUT
> >  	int "Tree-based hierarchical RCU fanout value"
> >  	range 2 64 if 64BIT
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 8fdea17..e287c4a 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -424,6 +424,7 @@ void rcu_idle_enter(void)
> >  }
> >  EXPORT_SYMBOL_GPL(rcu_idle_enter);
> >  
> > +#ifdef CONFIG_RCU_USER_QS
> >  /**
> >   * rcu_user_enter - inform RCU that we are resuming userspace.
> >   *
> > @@ -438,7 +439,6 @@ void rcu_user_enter(void)
> >  }
> >  EXPORT_SYMBOL_GPL(rcu_user_enter);
> >  
> > -
> >  /**
> >   * rcu_user_enter_irq - inform RCU that we are going to resume userspace
> >   * after the current irq returns.
> > @@ -459,6 +459,7 @@ void rcu_user_enter_irq(void)
> >  	rdtp->dynticks_nesting = 1;
> >  	local_irq_restore(flags);
> >  }
> > +#endif
> >  
> >  /**
> >   * rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle
> > @@ -562,6 +563,7 @@ void rcu_idle_exit(void)
> >  }
> >  EXPORT_SYMBOL_GPL(rcu_idle_exit);
> >  
> > +#ifdef CONFIG_RCU_USER_QS
> >  /**
> >   * rcu_user_exit - inform RCU that we are exiting userspace.
> >   *
> > @@ -595,6 +597,7 @@ void rcu_user_exit_irq(void)
> >  	rdtp->dynticks_nesting += DYNTICK_TASK_EXIT_IDLE;
> >  	local_irq_restore(flags);
> >  }
> > +#endif
> >  
> >  /**
> >   * rcu_irq_enter - inform RCU that current CPU is entering irq away from idle
> > -- 
> > 1.7.8
> > 
>