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

Message ID 1346360743-3628-4-git-send-email-paulmck@linux.vnet.ibm.com
State New
Headers show

Commit Message

Paul E. McKenney Aug. 30, 2012, 9:05 p.m.
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>
---
 arch/Kconfig             |   10 ++++++++++
 include/linux/rcupdate.h |    8 ++++++++
 init/Kconfig             |   10 ++++++++++
 kernel/rcutree.c         |    5 ++++-
 4 files changed, 32 insertions(+), 1 deletions(-)

Comments

Josh Triplett Aug. 31, 2012, 11:44 p.m. | #1
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?)

> +	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
>
Paul E. McKenney Sept. 5, 2012, 12:34 a.m. | #2
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
> > 
>
Josh Triplett Sept. 5, 2012, 12:46 a.m. | #3
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
Frederic Weisbecker Sept. 10, 2012, 9:35 p.m. | #4
On Tue, Sep 04, 2012 at 05:46:19PM -0700, Josh Triplett wrote:
> > 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.

Right! And in fact CONFIG_RCU_USER_QS is a temporary config. Once we'll have
CONFIG_NO_HZ_FULL, we won't need intermediate configs like this.
And CONFIG_NO_HZ_FULL will depend on SMP anyway.

Patch

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
+	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