Re: [PATCH v2 tip/core/rcu 01/39] rcu: Maintain special bits at bottom of ->dynticks counter

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Apr 17, 2017 at 04:44:48PM -0700, Paul E. McKenney wrote:
> Currently, IPIs are used to force other CPUs to invalidate their TLBs
> in response to a kernel virtual-memory mapping change.  This works, but
> degrades both battery lifetime (for idle CPUs) and real-time response
> (for nohz_full CPUs), and in addition results in unnecessary IPIs due to
> the fact that CPUs executing in usermode are unaffected by stale kernel
> mappings.  It would be better to cause a CPU executing in usermode to
> wait until it is entering kernel mode to do the flush, first to avoid
> interrupting usemode tasks and second to handle multiple flush requests
> with a single flush in the case of a long-running user task.
> 
> This commit therefore reserves a bit at the bottom of the ->dynticks
> counter, which is checked upon exit from extended quiescent states.
> If it is set, it is cleared and then a new rcu_eqs_special_exit() macro is
> invoked, which, if not supplied, is an empty single-pass do-while loop.
> If this bottom bit is set on -entry- to an extended quiescent state,
> then a WARN_ON_ONCE() triggers.
> 
> This bottom bit may be set using a new rcu_eqs_special_set() function,
> which returns true if the bit was set, or false if the CPU turned
> out to not be in an extended quiescent state.  Please note that this
> function refuses to set the bit for a non-nohz_full CPU when that CPU
> is executing in usermode because usermode execution is tracked by RCU
> as a dyntick-idle extended quiescent state only for nohz_full CPUs.
> 
> Reported-by: Andy Lutomirski <luto@amacapital.net>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Comments below.  With those fixed:
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -290,15 +300,20 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
>  static void rcu_dynticks_eqs_enter(void)
>  {
>  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> -	int special;
> +	int seq;
>  
>  	/*
>  	 * CPUs seeing atomic_inc_return() must see prior RCU read-side
>  	 * critical sections, and we also must force ordering with the
>  	 * next idle sojourn.
>  	 */
> -	special = atomic_inc_return(&rdtp->dynticks);
> -	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && special & 0x1);
> +	seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks);

You changed atomic_inc_return to atomic_add_return here, but the comment
above still says atomic_inc_return.

> @@ -308,15 +323,22 @@ static void rcu_dynticks_eqs_enter(void)
>  static void rcu_dynticks_eqs_exit(void)
>  {
>  	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> -	int special;
> +	int seq;
>  
>  	/*
>  	 * CPUs seeing atomic_inc_return() must see prior idle sojourns,
>  	 * and we also must force ordering with the next RCU read-side
>  	 * critical section.
>  	 */
> -	special = atomic_inc_return(&rdtp->dynticks);
> -	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(special & 0x1));
> +	seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks);

Likewise.

- Josh Triplett