Re: [PATCH v2] context_tracking: Add comments on interface and internals

[Ingo Molnar <mingo@kernel.org>]

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> This subsystem lacks many explanations on its purpose and
> design. Add these missing comments.
> 
> v2: Address comments from Andrew
> 
> Reported-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Gilad Ben-Yossef <gilad@benyossef.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Li Zhong <zhong@linux.vnet.ibm.com>
> ---
>  kernel/context_tracking.c |   73 ++++++++++++++++++++++++++++++++++++++------
>  1 files changed, 63 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index e0e07fd..9f6c38f 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -1,3 +1,19 @@
> +/*
> + * Context tracking: Probe on high level context boundaries such as kernel
> + * and userspace. This includes syscalls and exceptions entry/exit.
> + *
> + * This is used by RCU to remove its dependency on the timer tick while a CPU
> + * runs in userspace.
> + *
> + *  Started by Frederic Weisbecker:
> + *
> + * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com>
> + *
> + * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
> + * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
> + *
> + */
> +
>  #include <linux/context_tracking.h>
>  #include <linux/rcupdate.h>
>  #include <linux/sched.h>
> @@ -6,8 +22,8 @@
>  
>  struct context_tracking {
>  	/*
> -	 * When active is false, hooks are not set to
> -	 * minimize overhead: TIF flags are cleared
> +	 * When active is false, hooks are unset in order
> +	 * to minimize overhead: TIF flags are cleared
>  	 * and calls to user_enter/exit are ignored. This
>  	 * may be further optimized using static keys.
>  	 */
> @@ -24,6 +40,15 @@ static DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
>  #endif
>  };
>  
> +/**
> + * user_enter - Inform the context tracking that the CPU is going to
> + *              enter userspace mode.
> + *
> + * This function must be called right before we switch from the kernel
> + * to userspace, when it's guaranteed the remaining kernel instructions
> + * to execute won't use any RCU read side critical section because this
> + * function sets RCU in extended quiescent state.
> + */
>  void user_enter(void)
>  {
>  	unsigned long flags;
> @@ -39,40 +64,68 @@ void user_enter(void)
>  	if (in_interrupt())
>  		return;
>  
> +	/* Kernel threads aren't supposed to go to userspace */
>  	WARN_ON_ONCE(!current->mm);
>  
>  	local_irq_save(flags);
>  	if (__this_cpu_read(context_tracking.active) &&
>  	    __this_cpu_read(context_tracking.state) != IN_USER) {
>  		__this_cpu_write(context_tracking.state, IN_USER);
> +		/*
> +		 * At this stage, only low level arch entry code remains and
> +		 * then we'll run in userspace. We can assume there won't be
> +		 * any RCU read-side critical section until the next call to
> +		 * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
> +		 * on the tick.
> +		 */
>  		rcu_user_enter();
>  	}
>  	local_irq_restore(flags);
>  }
>  
> +
> +/**
> + * user_exit - Inform the context tracking that the CPU is
> + *             exiting userspace mode and entering the kernel.
> + *
> + * This function must be called after we entered the kernel from userspace
> + * before any use of RCU read side critical section. This potentially include
> + * any high level kernel code like syscalls, exceptions, signal handling, etc...
> + *
> + * This call supports re-entrancy. This way it can be called from any exception
> + * handler without needing to know if we came from userspace or not.
> + */
>  void user_exit(void)
>  {
>  	unsigned long flags;
>  
> -	/*
> -	 * Some contexts may involve an exception occuring in an irq,
> -	 * leading to that nesting:
> -	 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
> -	 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
> -	 * helpers are enough to protect RCU uses inside the exception. So
> -	 * just return immediately if we detect we are in an IRQ.
> -	 */
>  	if (in_interrupt())
>  		return;
>  
>  	local_irq_save(flags);
>  	if (__this_cpu_read(context_tracking.state) == IN_USER) {
>  		__this_cpu_write(context_tracking.state, IN_KERNEL);
> +		/*
> +		 * We are going to run code that may use RCU. Inform
> +		 * RCU core about that (ie: we may need the tick again).
> +		 */
>  		rcu_user_exit();
>  	}
>  	local_irq_restore(flags);
>  }
>  
> +
> +/**
> + * context_tracking_task_switch - context switch the syscall hooks
> + *
> + * The context tracking uses the syscall slow path to implement its user-kernel
> + * boundaries hooks on syscalls. This way it doesn't impact the syscall fast
> + * path on CPUs that don't do context tracking.
> + *
> + * But we need to clear the flag on the previous task because it may later
> + * migrate to some CPU that doesn't do the context tracking. As such the TIF
> + * flag may not be desired there.

If possible: s/hooks/callbacks

'hook' gives me the visual of a box match. YMMV.

Thanks,

	Ingo