Messages in this thread | | | Date | Wed, 11 Dec 2013 16:31:17 +0100 | From | Frederic Weisbecker <> | Subject | Re: nohz_full left a periodic tick cpu issue |
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On Wed, Dec 11, 2013 at 07:25:38AM -0800, Paul E. McKenney wrote: > On Wed, Dec 11, 2013 at 07:00:13PM +0800, Alex Shi wrote: > > On 12/11/2013 02:28 PM, Alex Shi wrote: > > > Hi Frederic, > > > > > > Sorry for idiot of nohz_full. When we using this feature on my mobile > > > devices, we found this feature keep cpu0 in periodic tick mode. then the > > > timer interrupt on cpu0 is very higher than normal nohz mode. > > > that cause high power consuming cost. > > > > > > I found you have mention this on commit: a382bf934449 > > > nohz: Assign timekeeping duty to a CPU outside the full dynticks range > > > > > > In fact, if all full dynticks cpu are in idle, cpu0 should be safe to > > > get into idle too. Do you have some plan or idea to implement this? > > > otherwise, power cost is too high to enable nohz_full in mobile platform. > > > > CC to more experts: Paul and zhong. > > > > In fact, I try to figure out a simple solution for this: > > We can use a global variable to store full dyntick cpus number, when the number goes to 0, the cpu0 can be get into nohz mode as normal nohz. > > Then before any cpu get into full dyntick, it can send out a need resched to wake up the cpu0. > > > > but I am stalled for a week in making the nr_nohz_busy_cpu balance on a 2 cpus system - pandaboard es. the variable may increases 2 or 3 times continuously without a decrease. so nr_nohz_busy_cpus increased to 28 in 7200 seconds. > > > > Anyone like point out what I missed in this simple patch? > > OK, I'll bite... Why not just use CONFIG_NO_HZ_FULL_SYSIDLE=y with > CONFIG_NO_HZ_FULL_SYSIDLE_SMALL set to at least the number of CPUs on > your system? The default for CONFIG_NO_HZ_FULL_SYSIDLE_SMALL is eight. > This stuff is in mainline, though Frederic's exploitation of it is > still in the works. > > On smaller systems, with Frederic's latest work, this should allow CPU 0 > to go idle quickly. > > What are you needing that this would not provide?
Yeah I think that's what he needs, but we haven't much advertized that feature yet. I mean there are good reasons, it's not yet working :o)
But yeah Alex, I'm working on plugging the full system idle detection (CONFIG_NO_HZ_FULL_SYSIDLE=y) to shutdown the timekeeping CPUs adaptively.
I have the patches ready, I'm just polishing a few things, writing changelogs, then I'll send to LKML with you Cc'ed :)
Thanks!
> > Thanx, Paul > > > For more cpus system you can just assign one cpu as full dyntick cpu to debug this patch. > > > > > > >From 14afeca87d8d74cb65c1d0f2bdd270466d4c7083 Mon Sep 17 00:00:00 2001 > > From: Alex Shi <alex.shi@linaro.org> > > Date: Wed, 11 Dec 2013 18:32:52 +0800 > > Subject: [PATCH 4/5] nohz_full: tracking the full dynticks cpu number > > > > This patch try to track the full dynticks cpu number with a global > > varible nr_nohz_busy_cpus. > > > > When ts->tick_stopped and !ts->inidle, it is time for full dyntick. > > To achieve this, the ts->inidle = 0 was delayed in tick_nohz_idle_exit > > safely. > > > > But the patch failed, it may increases 2 or 3 times continuously without > > a decrease. so nr_nohz_busy_cpus increased to 28 in 7200 seconds. :( > > > > --- > > include/linux/tick.h | 3 +++ > > kernel/time/tick-sched.c | 19 +++++++++++++++++-- > > 2 files changed, 20 insertions(+), 2 deletions(-) > > > > diff --git a/include/linux/tick.h b/include/linux/tick.h > > index 5128d33..3784f9f 100644 > > --- a/include/linux/tick.h > > +++ b/include/linux/tick.h > > @@ -161,6 +161,7 @@ static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; } > > > > #ifdef CONFIG_NO_HZ_FULL > > extern bool tick_nohz_full_running; > > +extern atomic_t nr_nohz_busy_cpus; > > extern cpumask_var_t tick_nohz_full_mask; > > > > static inline bool tick_nohz_full_enabled(void) > > @@ -184,6 +185,7 @@ extern void __tick_nohz_full_check(void); > > extern void tick_nohz_full_kick(void); > > extern void tick_nohz_full_kick_all(void); > > extern void __tick_nohz_task_switch(struct task_struct *tsk); > > +extern bool has_nohz_busy_cpus(void); > > #else > > static inline void tick_nohz_init(void) { } > > static inline bool tick_nohz_full_enabled(void) { return false; } > > @@ -192,6 +194,7 @@ static inline void __tick_nohz_full_check(void) { } > > static inline void tick_nohz_full_kick(void) { } > > static inline void tick_nohz_full_kick_all(void) { } > > static inline void __tick_nohz_task_switch(struct task_struct *tsk) { } > > +static inline bool has_nohz_busy_cpus(void) { return true; } > > #endif > > > > static inline void tick_nohz_full_check(void) > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > > index b55d1c0..37f2784 100644 > > --- a/kernel/time/tick-sched.c > > +++ b/kernel/time/tick-sched.c > > @@ -148,9 +148,13 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs) > > profile_tick(CPU_PROFILING); > > } > > > > +atomic_t nr_nohz_busy_cpus = ATOMIC_INIT(0); > > #ifdef CONFIG_NO_HZ_FULL > > cpumask_var_t tick_nohz_full_mask; > > bool tick_nohz_full_running; > > +inline bool has_nohz_busy_cpus(void) { > > + return !!atomic_read(&nr_nohz_busy_cpus); > > +} > > > > static bool can_stop_full_tick(void) > > { > > @@ -642,6 +646,11 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts, > > ts->last_tick = hrtimer_get_expires(&ts->sched_timer); > > ts->tick_stopped = 1; > > trace_tick_stop(1, " "); > > + if (!ts->inidle) { > > + if (atomic_inc_return(&nr_nohz_busy_cpus) != 1) { > > + printk(KERN_ERR "inc cpu %d as %d\n", cpu, test); > > + } > > + } > > } > > > > /* > > @@ -881,6 +890,12 @@ static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now) > > ts->tick_stopped = 0; > > ts->idle_exittime = now; > > > > + if (!ts->inidle) { > > + if (atomic_dec_return(&nr_nohz_busy_cpus)) { > > + printk(KERN_ERR "dec cpu %d to %d\n", > > + smp_processor_id(), atomic_read(&nr_nohz_busy_cpus)); > > + } > > + } > > tick_nohz_restart(ts, now); > > } > > > > @@ -922,8 +937,6 @@ void tick_nohz_idle_exit(void) > > > > WARN_ON_ONCE(!ts->inidle); > > > > - ts->inidle = 0; > > - > > if (ts->idle_active || ts->tick_stopped) > > now = ktime_get(); > > > > @@ -935,6 +948,8 @@ void tick_nohz_idle_exit(void) > > tick_nohz_account_idle_ticks(ts); > > } > > > > + ts->inidle = 0; > > + > > local_irq_enable(); > > } > > EXPORT_SYMBOL_GPL(tick_nohz_idle_exit); > > -- > > 1.8.1.2 > > > > -- > > Thanks > > Alex > > >
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