linux-kernel - Re: nohz problem with idle time on old hardware

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Date:	Wed, 13 Nov 2013 10:57:37 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Matthew Whitehead <tedheadster@...il.com>,
	john.stultz@...aro.org, linux-kernel@...r.kernel.org,
	mwhitehe@...hat.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: nohz problem with idle time on old hardware

On Wed, 13 Nov 2013 10:31:34 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> The trace does indeed show that a tick is happening, as the config has
> HZ=250 (4ms) and we see a tick happen every 4ms. But for some reason,
> we don't update the the idle time correctly when nohz is enabled.
> 
> When I say nohz is enabled, I mean that we don't have nohz=off in the
> command line. There seems to be some difference between having nohz=off
> and having nohz disabled at runtime.

Looking at the differences between nohz=off from the command line, and
disabled at run time seems to be the variable "tick_nohz_enabled". I
don't see where it gets set to zero except for nohz=off.

$ git grep tick_nohz_enabled
kernel/rcutree.h:       int tick_nohz_enabled_snap; /* Previously seen
value from sysfs. */ kernel/rcutree_plugin.h:extern int
tick_nohz_enabled; kernel/rcutree_plugin.h:        tne =
ACCESS_ONCE(tick_nohz_enabled); kernel/rcutree_plugin.h:        if
(tne != rdtp->tick_nohz_enabled_snap)
{ kernel/rcutree_plugin.h:                rdtp->tick_nohz_enabled_snap
= tne; kernel/rcutree_plugin.h:
rdtp->tick_nohz_enabled_snap ? '.' : 'D'); kernel/time/tick-sched.c:int
tick_nohz_enabled __read_mostly  = 1;
kernel/time/tick-sched.c:               tick_nohz_enabled = 0;
kernel/time/tick-sched.c:               tick_nohz_enabled = 1;
kernel/time/tick-sched.c:       if (!tick_nohz_enabled)
kernel/time/tick-sched.c:       if (!tick_nohz_enabled)
kernel/time/tick-sched.c:       if (!tick_nohz_enabled)
kernel/time/tick-sched.c:       if (tick_nohz_enabled)

What's even stranger is that the RCU code in rcutree_plugin.h does an
ACCESS_ONCE(tick_nohz_enabled) as if it can change.

That said, looking at the fs/proc/stat.c get_idle_time() it does an
idle_time = get_cpu_idle_time_us(cpu, NULL) which has:

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)
		return -1;

	now = ktime_get();
	if (last_update_time) {
		update_ts_time_stats(cpu, ts, now, last_update_time);
		idle = ts->idle_sleeptime;
	} else {
		if (ts->idle_active && !nr_iowait_cpu(cpu)) {
			ktime_t delta = ktime_sub(now, ts->idle_entrytime);
			idle = ktime_add(ts->idle_sleeptime, delta);
		} else {
			idle = ts->idle_sleeptime;
		}
	}

	return ktime_to_us(idle);

}

This is one of the differences between nohz=off and nohz=on with jiffy
accounting. When we have nohz=off, this returns -1 and the calling
function calculates the idle time differently.

-- Steve
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