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

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Date:	Wed, 13 Nov 2013 16:50:20 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Matthew Whitehead <tedheadster@...il.com>, john.stultz@...aro.org,
	LKML <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, 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.

Thanks,

	tglx



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