Re: [PATCH tip/core/rcu 03/26] rcu: Make RCU_FAST_NO_HZ handle adaptive ticks

Josh Triplett
2012-08-31T23:40:32Z

On Thu, Aug 30, 2012 at 02:05:20PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@linaro.org>
>
> The current implementation of RCU_FAST_NO_HZ tries reasonably hard to rid
> the current CPU of RCU callbacks. This is appropriate when the CPU is
> entering idle, where it doesn't have much useful to do anyway, but is most
> definitely not what you want when transitioning to user-mode execution.
> This commit therefore detects the adaptive-tick case, and refrains from
> burning CPU time getting rid of RCU callbacks in that case.

With the OOM handler from your other patch series, I don't know that it
makes as much sense in the idle case, either; perhaps it would make more
sense to wait and batch up more callbacks as long as you have memory,
and then run them in one big burst.

> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> kernel/rcutree_plugin.h | 20 ++++++++++++++++++++
> 1 files changed, 20 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 7f3244c..b0f09d6 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -1997,6 +1997,26 @@ static void rcu_prepare_for_idle(int cpu)
> if (!tne)
> return;
>
> + /* Adaptive-tick mode, where usermode execution is idle to RCU. */
> + if (!is_idle_task(current)) {
> + rdtp->dyntick_holdoff = jiffies - 1;
> + if (rcu_cpu_has_nonlazy_callbacks(cpu)) {
> + trace_rcu_prep_idle("User dyntick with callbacks");
> + rdtp->idle_gp_timer_expires =
> + round_up(jiffies + RCU_IDLE_GP_DELAY,
> + RCU_IDLE_GP_DELAY);
> + } else if (rcu_cpu_has_callbacks(cpu)) {
> + rdtp->idle_gp_timer_expires =
> + round_jiffies(jiffies + RCU_IDLE_LAZY_GP_DELAY);
> + trace_rcu_prep_idle("User dyntick with lazy callbacks");
> + } else {
> + return;
> + }
> + tp = &rdtp->idle_gp_timer;
> + mod_timer_pinned(tp, rdtp->idle_gp_timer_expires);
> + return;
> + }
> +
> /*
> * If this is an idle re-entry, for example, due to use of
> * RCU_NONIDLE() or the new idle-loop tracing API within the idle
> --
> 1.7.8
>