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    SubjectRe: nohz_full left a periodic tick cpu issue
    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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