Re: nohz_full left a periodic tick cpu issue

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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?

							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
>