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    SubjectRe: [PATCH tip/core/rcu 14/47] rcu: Limit lazy-callback duration
    On Fri, Feb 03, 2012 at 05:44:49PM -0800, Paul E. McKenney wrote:
    > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
    >
    > Currently, a given CPU is permitted to remain in dyntick-idle mode
    > indefinitely if it has only lazy RCU callbacks queued. This is vulnerable
    > to corner cases in NUMA systems, so limit the time to six seconds by
    > default. (Currently controlled by a cpp macro.)
    >
    > 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 | 12 +++++++++++-
    > 1 files changed, 11 insertions(+), 1 deletions(-)
    >
    > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
    > index eeb2cc6..8041310 100644
    > --- a/kernel/rcutree_plugin.h
    > +++ b/kernel/rcutree_plugin.h
    > @@ -2050,6 +2050,9 @@ static void rcu_prepare_for_idle(int cpu)
    > * number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
    > * system. And if you are -that- concerned about energy efficiency,
    > * just power the system down and be done with it!
    > + * RCU_IDLE_LAZY_GP_DELAY gives the number of jiffies that a CPU is
    > + * permitted to sleep in dyntick-idle mode with only lazy RCU
    > + * callbacks pending. Setting this too high can OOM your system.
    > *
    > * The values below work well in practice. If future workloads require
    > * adjustment, they can be converted into kernel config parameters, though
    > @@ -2058,11 +2061,13 @@ static void rcu_prepare_for_idle(int cpu)
    > #define RCU_IDLE_FLUSHES 5 /* Number of dyntick-idle tries. */
    > #define RCU_IDLE_OPT_FLUSHES 3 /* Optional dyntick-idle tries. */
    > #define RCU_IDLE_GP_DELAY 6 /* Roughly one grace period. */
    > +#define RCU_IDLE_LAZY_GP_DELAY (6 * HZ) /* Roughly six seconds. */
    >
    > static DEFINE_PER_CPU(int, rcu_dyntick_drain);
    > static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
    > static DEFINE_PER_CPU(struct hrtimer, rcu_idle_gp_timer);
    > -static ktime_t rcu_idle_gp_wait;
    > +static ktime_t rcu_idle_gp_wait; /* If some non-lazy callbacks. */
    > +static ktime_t rcu_idle_lazy_gp_wait; /* If only lazy callbacks. */
    >
    > /*
    > * Allow the CPU to enter dyntick-idle mode if either: (1) There are no
    > @@ -2151,6 +2156,8 @@ static void rcu_prepare_for_idle_init(int cpu)
    > unsigned int upj = jiffies_to_usecs(RCU_IDLE_GP_DELAY);
    >
    > rcu_idle_gp_wait = ns_to_ktime(upj * (u64)1000);
    > + upj = jiffies_to_usecs(6 * HZ);

    I think you meant to use RCU_IDLE_LAZY_GP_DELAY here?

    > + rcu_idle_lazy_gp_wait = ns_to_ktime(upj * (u64)1000);
    > firsttime = 0;
    > }
    > }
    > @@ -2225,6 +2232,9 @@ static void rcu_prepare_for_idle(int cpu)
    > if (rcu_cpu_has_nonlazy_callbacks(cpu))
    > hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
    > rcu_idle_gp_wait, HRTIMER_MODE_REL);
    > + else
    > + hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
    > + rcu_idle_lazy_gp_wait, HRTIMER_MODE_REL);

    Might be a good idea to cancel it from rcu_cleanup_after_idle() to avoid
    needless hrtimer interrupt (if something else woke up from idle before it
    triggered).

    > return; /* Nothing more to do immediately. */
    > } else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
    > /* We have hit the limit, so time to give up. */
    > --
    > 1.7.8
    >


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