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

[Josh Triplett <josh@joshtriplett.org>]

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
>