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    Subject[PATCH RFC tip/core/rcu 07/13] rcu: Avoid needlessly IPIing CPUs at GP end
    Date
    From: Paul E. McKenney <paul.mckenney@linaro.org>

    If a CPU enters dyntick-idle mode with callbacks pending, it will need
    an IPI at the end of the grace period. However, if it exits dyntick-idle
    mode before the grace period ends, it will be needlessly IPIed at the
    end of the grace period.

    Therefore, this commit clears the per-CPU rcu_awake_at_gp_end flag
    when a CPU determines that it does not need it. This in turn requires
    disabling interrupts across much of rcu_prepare_for_idle() in order to
    avoid having nested interrupts clearing this state out from under us.

    Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---
    kernel/rcutree_plugin.h | 16 ++++++++++++++--
    1 files changed, 14 insertions(+), 2 deletions(-)

    diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
    index 45790bf..c4daf1e 100644
    --- a/kernel/rcutree_plugin.h
    +++ b/kernel/rcutree_plugin.h
    @@ -2027,6 +2027,9 @@ int rcu_needs_cpu(int cpu)
    static void rcu_prepare_for_idle(int cpu)
    {
    int c = 0;
    + unsigned long flags;
    +
    + local_irq_save(flags);

    /*
    * If there are no callbacks on this CPU or if RCU has no further
    @@ -2036,14 +2039,17 @@ static void rcu_prepare_for_idle(int cpu)
    if (!rcu_cpu_has_callbacks(cpu)) {
    per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
    per_cpu(rcu_dyntick_drain, cpu) = 0;
    + per_cpu(rcu_awake_at_gp_end, cpu) = 0;
    + local_irq_restore(flags);
    trace_rcu_prep_idle("No callbacks");
    return;
    }
    if (!rcu_pending(cpu)) {
    - trace_rcu_prep_idle("Dyntick with callbacks");
    per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
    per_cpu(rcu_dyntick_drain, cpu) = 0;
    per_cpu(rcu_awake_at_gp_end, cpu) = 1;
    + local_irq_restore(flags);
    + trace_rcu_prep_idle("Dyntick with callbacks");
    return; /* Nothing to do immediately. */
    }

    @@ -2052,6 +2058,7 @@ static void rcu_prepare_for_idle(int cpu)
    * refrained from disabling the scheduling-clock tick.
    */
    if (per_cpu(rcu_dyntick_holdoff, cpu) == jiffies) {
    + local_irq_restore(flags);
    trace_rcu_prep_idle("In holdoff");
    return;
    }
    @@ -2060,9 +2067,11 @@ static void rcu_prepare_for_idle(int cpu)
    if (per_cpu(rcu_dyntick_drain, cpu) <= 0) {
    /* First time through, initialize the counter. */
    per_cpu(rcu_dyntick_drain, cpu) = RCU_NEEDS_CPU_FLUSHES;
    + per_cpu(rcu_awake_at_gp_end, cpu) = 0;
    } else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
    /* We have hit the limit, so time to give up. */
    per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
    + local_irq_restore(flags);
    trace_rcu_prep_idle("Begin holdoff");
    invoke_rcu_core(); /* Force the CPU out of dyntick-idle. */
    return;
    @@ -2095,10 +2104,13 @@ static void rcu_prepare_for_idle(int cpu)
    * So try forcing the callbacks through the grace period.
    */
    if (c) {
    + local_irq_restore(flags);
    trace_rcu_prep_idle("More callbacks");
    invoke_rcu_core();
    - } else
    + } else {
    + local_irq_restore(flags);
    trace_rcu_prep_idle("Callbacks drained");
    + }
    }

    /*
    --
    1.7.3.2


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