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

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Date:	Wed, 13 Nov 2013 17:21:53 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	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, Thomas Gleixner wrote:

> On Wed, 13 Nov 2013, Steven Rostedt wrote:
> 
> > 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.
> 
> Right. It's telling you if NOHZ is enabled. It's not telling you that
> NOHZ is active.

And its even worse:

tick_nohz_idle_enter()

        /*
         * 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;

So there is code relying on that accounting stuff even if nohz is not
active.

Frozen shark time ....






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