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


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