linux-kernel - Re: [PATCH 07/18] nohz: Selectively enable context tracking on full dynticks CPUs

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Date:	Wed, 17 Jul 2013 14:27:17 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>,
	Kevin Hilman <khilman@...aro.org>
Subject: Re: [PATCH 07/18] nohz: Selectively enable context tracking on full
 dynticks CPUs

On Wed, 2013-07-17 at 18:44 +0200, Frederic Weisbecker wrote:
> The code is ready to do so in the context tracking subsystem, now

"do so"? Do what?

> we just need to pass our cpu range selection to it from the

Pass cpu range selection to what?

Pronouns are evil in technical documentation.

> full dynticks subsystem.
> 
> This way we can spare the overhead of RCU user extended quiescent
> state and vtime maintainance on these CPUs. Just keep in mind the
> context tracking itself is still necessary everywhere.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Li Zhong <zhong@...ux.vnet.ibm.com>
> Cc: Mike Galbraith <efault@....de>
> Cc: Kevin Hilman <khilman@...aro.org>
> ---
>  include/linux/context_tracking.h |    2 ++
>  init/Kconfig                     |    2 +-
>  kernel/context_tracking.c        |    5 +++++
>  kernel/time/Kconfig              |    1 -
>  kernel/time/tick-sched.c         |    6 ++++++
>  5 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> index 12045ce..2c2b73aa 100644
> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -34,6 +34,8 @@ static inline bool context_tracking_active(void)
>  	return __this_cpu_read(context_tracking.active);
>  }
>  
> +extern void context_tracking_cpu_set(int cpu);
> +
>  extern void user_enter(void);
>  extern void user_exit(void);
>  
> diff --git a/init/Kconfig b/init/Kconfig
> index 247084b..914da3f 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -527,7 +527,7 @@ config RCU_USER_QS
>  config CONTEXT_TRACKING_FORCE
>  	bool "Force context tracking"
>  	depends on CONTEXT_TRACKING
> -	default CONTEXT_TRACKING
> +	default y if !NO_HZ_FULL

Why the if !NO_HZ_FULL?

That selects this anyway. Oh wait, you changed this.

>  	help
>  	  Probe on user/kernel boundaries by default in order to
>  	  test the features that rely on it such as userspace RCU extended
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index 7b095de..72bcb25 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -26,6 +26,11 @@ DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
>  #endif
>  };
>  
> +void context_tracking_cpu_set(int cpu)
> +{
> +	per_cpu(context_tracking.active, cpu) = true;
> +}
> +
>  /**
>   * user_enter - Inform the context tracking that the CPU is going to
>   *              enter userspace mode.
> diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
> index 70f27e8..747bbc7 100644
> --- a/kernel/time/Kconfig
> +++ b/kernel/time/Kconfig
> @@ -105,7 +105,6 @@ config NO_HZ_FULL
>  	select RCU_USER_QS
>  	select RCU_NOCB_CPU
>  	select VIRT_CPU_ACCOUNTING_GEN
> -	select CONTEXT_TRACKING_FORCE

OK, I'm confused.

-- Steve

>  	select IRQ_WORK
>  	help
>  	 Adaptively try to shutdown the tick whenever possible, even when
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 088c411..062759e 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -23,6 +23,7 @@
>  #include <linux/irq_work.h>
>  #include <linux/posix-timers.h>
>  #include <linux/perf_event.h>
> +#include <linux/context_tracking.h>
>  
>  #include <asm/irq_regs.h>
>  
> @@ -344,11 +345,16 @@ static int tick_nohz_init_all(void)
>  
>  void __init tick_nohz_init(void)
>  {
> +	int cpu;
> +
>  	if (!have_nohz_full_mask) {
>  		if (tick_nohz_init_all() < 0)
>  			return;
>  	}
>  
> +	for_each_cpu(cpu, nohz_full_mask)
> +		context_tracking_cpu_set(cpu);
> +
>  	cpu_notifier(tick_nohz_cpu_down_callback, 0);
>  	cpulist_scnprintf(nohz_full_buf, sizeof(nohz_full_buf), nohz_full_mask);
>  	pr_info("NO_HZ: Full dynticks CPUs: %s.\n", nohz_full_buf);


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