Re: dynticks: Remove atomic ops for dynticks_idle

[Paul E. McKenney]

On Thu, Sep 04, 2014 at 10:18:31AM -0500, Christoph Lameter wrote:
> This is on top of the first patch (but still not against your tree). I
> wonder what you think about this one. No special operations.
> 
> 
> Since dynticks_idle is only ever modified by the local cpu we do
> not need to use atomics there.
> 
> Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>

Sorry, as discussed earlier, this change is non-trivial.  In addition,
you have not justified the change.

> Index: linux/kernel/rcu/tree.c
> ===================================================================
> --- linux.orig/kernel/rcu/tree.c
> +++ linux/kernel/rcu/tree.c
> @@ -213,7 +213,7 @@ static DEFINE_PER_CPU(struct rcu_dyntick
>  	.dynticks = ATOMIC_INIT(1),
>  #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
>  	.dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE,
> -	.dynticks_idle = ATOMIC_INIT(1),
> +	.dynticks_idle = 1,
>  #endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
>  };
> 
> Index: linux/kernel/rcu/tree.h
> ===================================================================
> --- linux.orig/kernel/rcu/tree.h
> +++ linux/kernel/rcu/tree.h
> @@ -91,7 +91,7 @@ struct rcu_dynticks {
>  #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
>  	long long dynticks_idle_nesting;
>  				    /* irq/process nesting level from idle. */
> -	atomic_t dynticks_idle;	    /* Even value for idle, else odd. */
> +	long dynticks_idle;	    /* Even value for idle, else odd. */
>  				    /*  "Idle" excludes userspace execution. */
>  	unsigned long dynticks_idle_jiffies;
>  				    /* End of last non-NMI non-idle period. */
> Index: linux/kernel/rcu/tree_plugin.h
> ===================================================================
> --- linux.orig/kernel/rcu/tree_plugin.h
> +++ linux/kernel/rcu/tree_plugin.h
> @@ -2644,9 +2644,9 @@ static void rcu_sysidle_enter(int irq)
>  	j = jiffies;
>  	ACCESS_ONCE(rdtp->dynticks_idle_jiffies) = j;
>  	smp_mb__before_atomic();
> -	atomic_inc(&rdtp->dynticks_idle);
> +	rdtp->dynticks_idle++;
>  	smp_mb__after_atomic();

And this is completely broken.

> -	WARN_ON_ONCE(atomic_read(&rdtp->dynticks_idle) & 0x1);
> +	WARN_ON_ONCE(rdtp->dynticks_idle & 0x1);
>  }
> 
>  /*
> @@ -2712,9 +2712,9 @@ static void rcu_sysidle_exit(int irq)
> 
>  	/* Record end of idle period. */
>  	smp_mb__before_atomic();
> -	atomic_inc(&rdtp->dynticks_idle);
> +	rdtp->dynticks_idle++;
>  	smp_mb__after_atomic();

As is this.

> -	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks_idle) & 0x1));
> +	WARN_ON_ONCE(!(rdtp->dynticks_idle & 0x1));
> 
>  	/*
>  	 * If we are the timekeeping CPU, we are permitted to be non-idle
> @@ -2755,7 +2755,7 @@ static void rcu_sysidle_check_cpu(struct
>  		WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);
> 
>  	/* Pick up current idle and NMI-nesting counter and check. */
> -	cur = atomic_read(&rdtp->dynticks_idle);
> +	cur = rdtp->dynticks_idle;

And this as well.

							Thanx, Paul

>  	if (cur & 0x1) {
>  		*isidle = false; /* We are not idle! */
>  		return;
> 

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