Messages in this thread | | | Date | Sat, 4 Feb 2012 06:30:22 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 14/47] rcu: Limit lazy-callback duration |
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On Sat, Feb 04, 2012 at 02:54:58PM +0100, Frederic Weisbecker wrote: > 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?
Good catch!!! Indeed I do, fixed.
> > > + 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).
Very much so! And rcu_cleanup_after_idle() is in fact defined as:
static void rcu_cleanup_after_idle(int cpu) { hrtimer_cancel(&per_cpu(rcu_idle_gp_timer, cpu)); }
The reason this does not appear in this patch is that it is already in mainline.
> > 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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