Re: [PATCH tip/core/rcu 4/9] nohz_full: Add rcu_dyntick data for scalable detection of all-idle state

Josh Triplett
2013-08-18T03:02:44Z

On Sat, Aug 17, 2013 at 06:49:39PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
>
> This commit adds fields to the rcu_dyntick structure that are used to
> detect idle CPUs. These new fields differ from the existing ones in
> that the existing ones consider a CPU executing in user mode to be idle,
> where the new ones consider CPUs executing in user mode to be busy.
> The handling of these new fields is otherwise quite similar to that for
> the exiting fields. This commit also adds the initialization required
> for these fields.
>
> So, why is usermode execution treated differently, with RCU considering
> it a quiescent state equivalent to idle, while in contrast the new
> full-system idle state detection considers usermode execution to be
> non-idle?
>
> It turns out that although one of RCU's quiescent states is usermode
> execution, it is not a full-system idle state. This is because the
> purpose of the full-system idle state is not RCU, but rather determining
> when accurate timekeeping can safely be disabled. Whenever accurate
> timekeeping is required in a CONFIG_NO_HZ_FULL kernel, at least one
> CPU must keep the scheduling-clock tick going. If even one CPU is
> executing in user mode, accurate timekeeping is requires, particularly for
> architectures where gettimeofday() and friends do not enter the kernel.
> Only when all CPUs are really and truly idle can accurate timekeeping be
> disabled, allowing all CPUs to turn off the scheduling clock interrupt,
> thus greatly improving energy efficiency.
>
> This naturally raises the question "Why is this code in RCU rather than in
> timekeeping?", and the answer is that RCU has the data and infrastructure
> to efficiently make this determination.
>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>

One comment below. With that change:
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> +#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> +
> +/*
> + * Initialize dynticks sysidle state for CPUs coming online.
> + */
> +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> +{
> + rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE;
> +}
> +
> +#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> +
> +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> +{
> +}
> +
> +#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */

Just move the ifdef around the function body:

static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
{
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE;
#endif /* CONFIG_NO_HZ_FULL_SYSIDLE */
}

- Josh Triplett