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

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

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.

> +	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
> +	  need 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 5541a07..64fc2cd 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -423,6 +423,7 @@ 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.
>   *
> @@ -436,6 +437,7 @@ void rcu_user_enter(void)
>  	rcu_eqs_enter(1);
>  }
>  EXPORT_SYMBOL_GPL(rcu_user_enter);
> +#endif
> 
> 
>  /**
> @@ -561,6 +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.
>   *
> @@ -572,6 +575,7 @@ void rcu_user_exit(void)
>  	rcu_eqs_exit(1);
>  }
>  EXPORT_SYMBOL_GPL(rcu_user_exit);
> +#endif
> 
>  /**
>   * rcu_user_exit_irq - inform RCU that we won't resume to userspace
> -- 
> 1.7.5.4
>