Re: nohz_full left a periodic tick cpu issue

[Frederic Weisbecker]

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@xxxxxxxxxx>
> > 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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