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Message-ID: <20131113214942.GA31722@mwhitehe.csb>
Date: Wed, 13 Nov 2013 16:49:42 -0500
From: Matthew Whitehead <tedheadster@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
john.stultz@...aro.org, linux-kernel@...r.kernel.org,
mwhitehe@...hat.com
Subject: Re: nohz problem with idle time on old hardware
On Wed, Nov 13, 2013 at 03:07:45PM -0500, Steven Rostedt wrote:
> On Wed, 13 Nov 2013 21:01:57 +0100 (CET)
> Thomas Gleixner <tglx@...utronix.de> wrote:
>
>
> > ---------------->
> > Subject: NOHZ: Check for nohz active instead of nohz enabled
> >
> > RCU and the fine grained idle time accounting functions check
> > tick_nohz_enabled. But that variable is merily telling that NOHZ has
> > been enabled in the config and not been disabled on the command line.
> >
> > But it does not tell anything about nohz being active. That's what all
> > this should check for.
> >
> > Matthew reported, that the idle accounting on his old P1 machine
> > showed bogus values, when he enabled NOHZ in the config and did not
> > disable it on the kernel command line. The reason is that his machine
> > uses (refined) jiffies as a clocksource which explains why the "fine"
> > grained accounting went into lala land, because it depends on when the
> > system goes and leaves idle relative to the jiffies increment.
> >
> > Provide a tick_nohz_active indicator and let RCU and the accounting
> > code use this instead of tick_nohz_enable.
> >
> > Reported-by: Matthew Whitehead <tedheadster@...il.com>
>
> Matthew, can you test this patch to make sure it causes the idle issue
> to go away (remember to remove nohz=off from your command line).
>
Early testing looks good:
# Verify I haven't overridden nohz as a kernel parameter
root@...t:~# grep nohz /proc/cmdline
# See how my kernel chose its clocksource
root@...t:~# dmesg | egrep "TSC|clocksource"
[ 0.000000] tsc: Fast TSC calibration using PIT
[ 2.492468] Switched to clocksource refined-jiffies
[ 24.197356] Switched to clocksource tsc
[ 26.211058] Switched to clocksource refined-jiffies
# Check idle time since the reboot
root@...t:~# sar -s 16:28:00 -P ALL
Linux 3.12.0-rc6+ (host) 11/13/2013 _i586_ (2 CPU)
04:28:26 PM LINUX RESTART
04:32:02 PM CPU %user %nice %system %iowait %steal %idle
04:34:02 PM all 4.98 0.00 1.03 0.04 0.00 93.96
04:34:02 PM 0 4.07 0.00 0.69 0.03 0.00 95.21
04:34:02 PM 1 5.88 0.00 1.37 0.06 0.00 92.70
04:36:01 PM all 4.12 0.00 1.13 0.02 0.00 94.73
04:36:01 PM 0 1.98 0.00 1.34 0.02 0.00 96.66
04:36:01 PM 1 6.28 0.00 0.91 0.03 0.00 92.79
04:38:01 PM all 0.39 0.00 0.65 0.05 0.00 98.90
04:38:01 PM 0 0.33 0.00 0.54 0.03 0.00 99.09
04:38:01 PM 1 0.45 0.00 0.75 0.07 0.00 98.73
04:40:01 PM all 0.14 0.00 0.40 0.02 0.00 99.44
04:40:01 PM 0 0.12 0.00 0.26 0.01 0.00 99.62
04:40:01 PM 1 0.16 0.00 0.55 0.02 0.00 99.27
04:42:01 PM all 0.13 0.00 0.28 0.01 0.00 99.58
04:42:01 PM 0 0.09 0.00 0.14 0.01 0.00 99.76
04:42:01 PM 1 0.16 0.00 0.43 0.02 0.00 99.40
Average: all 1.95 0.00 0.70 0.03 0.00 97.33
Average: 0 1.32 0.00 0.59 0.02 0.00 98.07
Average: 1 2.58 0.00 0.80 0.04 0.00 96.58
I will continue to test but I think this is good.
Tested-by: Matthew Whitehead <tedheadster@...il.com>
- Matthew W
> Reviewed-by: Steven Rostedt <rostedt@...dmis.org>
>
> -- Steve
>
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > ---
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 3822ac0..da6e6de 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -1632,7 +1632,7 @@ module_param(rcu_idle_gp_delay, int, 0644);
> > static int rcu_idle_lazy_gp_delay = RCU_IDLE_LAZY_GP_DELAY;
> > module_param(rcu_idle_lazy_gp_delay, int, 0644);
> >
> > -extern int tick_nohz_enabled;
> > +extern int tick_nohz_active;
> >
> > /*
> > * Try to advance callbacks for all flavors of RCU on the current CPU, but
> > @@ -1729,7 +1729,7 @@ static void rcu_prepare_for_idle(int cpu)
> > int tne;
> >
> > /* Handle nohz enablement switches conservatively. */
> > - tne = ACCESS_ONCE(tick_nohz_enabled);
> > + tne = ACCESS_ONCE(tick_nohz_active);
> > if (tne != rdtp->tick_nohz_enabled_snap) {
> > if (rcu_cpu_has_callbacks(cpu, NULL))
> > invoke_rcu_core(); /* force nohz to see update. */
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 3612fc7..a12df5a 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -361,8 +361,8 @@ void __init tick_nohz_init(void)
> > /*
> > * NO HZ enabled ?
> > */
> > -int tick_nohz_enabled __read_mostly = 1;
> > -
> > +static int tick_nohz_enabled __read_mostly = 1;
> > +int tick_nohz_active __read_mostly;
> > /*
> > * Enable / Disable tickless mode
> > */
> > @@ -465,7 +465,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
> > struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> > ktime_t now, idle;
> >
> > - if (!tick_nohz_enabled)
> > + if (!tick_nohz_active)
> > return -1;
> >
> > now = ktime_get();
> > @@ -506,7 +506,7 @@ u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
> > struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> > ktime_t now, iowait;
> >
> > - if (!tick_nohz_enabled)
> > + if (!tick_nohz_active)
> > return -1;
> >
> > now = ktime_get();
> > @@ -799,11 +799,6 @@ void tick_nohz_idle_enter(void)
> > local_irq_disable();
> >
> > ts = &__get_cpu_var(tick_cpu_sched);
> > - /*
> > - * set ts->inidle unconditionally. even if the system did not
> > - * switch to nohz mode the cpu frequency governers rely on the
> > - * update of the idle time accounting in tick_nohz_start_idle().
> > - */
> > ts->inidle = 1;
> > __tick_nohz_idle_enter(ts);
> >
> > @@ -973,7 +968,7 @@ static void tick_nohz_switch_to_nohz(void)
> > struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
> > ktime_t next;
> >
> > - if (!tick_nohz_enabled)
> > + if (!tick_nohz_active)
> > return;
> >
> > local_irq_disable();
> > @@ -981,7 +976,7 @@ static void tick_nohz_switch_to_nohz(void)
> > local_irq_enable();
> > return;
> > }
> > -
> > + tick_nohz_active = 1;
> > ts->nohz_mode = NOHZ_MODE_LOWRES;
> >
> > /*
> > @@ -1139,8 +1134,10 @@ void tick_setup_sched_timer(void)
> > }
> >
> > #ifdef CONFIG_NO_HZ_COMMON
> > - if (tick_nohz_enabled)
> > + if (tick_nohz_enabled) {
> > ts->nohz_mode = NOHZ_MODE_HIGHRES;
> > + tick_nohz_active = 1;
> > + }
> > #endif
> > }
> > #endif /* HIGH_RES_TIMERS */
>
--
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