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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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