Re: [PATCH 03/13] rcu: Exclude all potential timekeepers from sysidle detection

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

On Tue, Dec 17, 2013 at 11:51:22PM +0100, Frederic Weisbecker wrote:
> The purpose of the full system idle detection is to notify the CPU
> handling the timekeeping when the rest of the system is idle so that it
> can sleep when nobody needs the jiffies nor GTOD to be maintained.
> 
> Now this machinery excludes CPU 0 itself from the range of the idle
> detection because if CPU 0 has any non-idle task to execute, it is going
> to restart its own tick since it's guaranteed to be outside the full
> dynticks range. And as it is the only eligible timekeeper when
> CONFIG_NO_HZ_FULL=y anyway, it can handle the timekeeping duty for
> and by itself.
> 
> Still we also plan to extend the timekeepers affinity and allow every CPU
> outside the full dynticks range to handle the timekeeping duty, not just
> CPU 0.
> 
> So once we reach that step, we can state that all CPUs that are not
> full dynticks can be excluded from the full system idle detection,
> simply because those CPUs share the same property than CPU 0 today. When
> a non full dynticks CPU needs to run some busy task, it restarts its
> tick and handles the timekeeping duty for its own needs as is currently
> done under CONFIG_NO_HZ_IDLE=y.
> 
> To prepare for this support in the sysidle detection, we can use the
> tick_timekeeping_cpu() API which checks if a CPU is allowed to handle
> timekeeping duty. If so we can conclude that it's not full dynticks and
> it can maintain timekeeping by itself and as such it can be excluded
> from the sysidle detection.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: John Stultz <john.stultz@linaro.org>
> Cc: Alex Shi <alex.shi@linaro.org>
> Cc: Kevin Hilman <khilman@linaro.org>

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

A few comments below as well.

> ---
>  kernel/rcu/tree_plugin.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 6abb03d..08004da 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h

The rcu_sysidle_force_exit() function uses tick_do_timer_cpu, but
presumably needs to continue doing so in order to whack the right
CPU over the head.  I am happy to defer worrying about the interaction
with multiple timekeeping CPUs for the moment.  ;-)

> @@ -2539,7 +2539,7 @@ static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq)
>  	 * invoke rcu_sysidle_force_exit() directly if it does anything
>  	 * more than take a scheduling-clock interrupt.
>  	 */
> -	if (smp_processor_id() == tick_do_timer_cpu)
> +	if (tick_timekeeping_cpu(smp_processor_id()))
>  		return;
> 
>  	/* Update system-idle state: We are clearly no longer fully idle! */
> @@ -2563,10 +2563,10 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
>  	 * is an offline or the timekeeping CPU, nothing to do.
>  	 */
>  	if (!*isidle || rdp->rsp != rcu_sysidle_state ||
> -	    cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)
> +	    cpu_is_offline(rdp->cpu) || tick_timekeeping_cpu(rdp->cpu))
>  		return;
>  	if (rcu_gp_in_progress(rdp->rsp))
> -		WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);
> +		WARN_ON_ONCE(!tick_timekeeping_cpu(smp_processor_id()));
> 
>  	/* Pick up current idle and NMI-nesting counter and check. */
>  	cur = atomic_read(&rdtp->dynticks_idle);

The rcu_bind_gp_kthread() uses tick_do_timer_cpu to figure out where
to run.  Is there some CPU mask that it should use instead once there
can be multiple timekeeping CPUs?

> @@ -2729,7 +2729,7 @@ bool rcu_sys_is_idle(void)
>  	static struct rcu_sysidle_head rsh;
>  	int rss = ACCESS_ONCE(full_sysidle_state);
> 
> -	if (WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu))
> +	if (WARN_ON_ONCE(!tick_timekeeping_cpu(smp_processor_id())))
>  		return false;
> 
>  	/* Handle small-system case by doing a full scan of CPUs. */
> -- 
> 1.8.3.1
>