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

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Date:	Wed, 13 Nov 2013 08:17:19 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Matthew Whitehead <tedheadster@...il.com>,
	john.stultz@...aro.org, LKML <linux-kernel@...r.kernel.org>,
	mwhitehe@...hat.com
Subject: Re: nohz problem with idle time on old hardware

On Wed, Nov 13, 2013 at 04:50:20PM +0100, Thomas Gleixner wrote:
> On Wed, 13 Nov 2013, Steven Rostedt wrote:
> > > I'm not saying that we are actually getting into nohz, but something
> > > with the nohz code is messing with cpu accounting.
> > 
> > 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.
> 
> Right that affects tick_nohz_enabled
> 
> Two files use this variable:
> kernel/rcu/tree_plugin.h
> kernel/time/tick-sched.c
> 
> The only accounting related stuff is in tick-sched.c:
> 
> get_cpu_idle_time_us() and get_cpu_iowait_time_us()
> 
> Both functions bail out if (!tick_nohz_enabled).
> 
> The users of get_cpu_idle_time_us() are cpufreq and fs/proc/stat.c!
> 
> Now the simplest fix is to let those functions check whether we
> actually switched into NOHZ mode. Should work for the RCU tree stuff
> as well.

RCU's use of tick_nohz_enabled is for the RCU_FAST_NO_HZ stuff.  If
it sees !tick_nohz_enabled, it skips trying to get RCU out of the way
of disabling the scheduling-clock tick.  If RCU detects a change
in the value of tick_nohz_enabled, it does a raise_softirq() to
force re-evaluation of the situation.

							Thanx, Paul

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