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Date: Wed, 11 Dec 2013 16:31:17 +0100
From: Frederic Weisbecker <fweisbec@...il.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Alex Shi <alex.shi@...aro.org>, Ingo Molnar <mingo@...nel.org>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Linaro Kernel <linaro-kernel@...ts.linaro.org>,
Li Zhong <zhong@...ux.vnet.ibm.com>
Subject: Re: 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@...aro.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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