linux-kernel - Re: [QUERY]: Is using CPU hotplug right for isolating CPUs?

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Message-ID: <20140120155145.GB9436@localhost.localdomain>
Date:	Mon, 20 Jan 2014 16:51:46 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linaro Networking <linaro-networking@...aro.org>
Subject: Re: [QUERY]: Is using CPU hotplug right for isolating CPUs?

On Mon, Jan 20, 2014 at 05:00:20PM +0530, Viresh Kumar wrote:
> On 16 January 2014 15:16, Thomas Gleixner <tglx@...utronix.de> wrote:
> > Just do the math.
> >
> >      max reload value / timer freq = max time span
> 
> Thanks.
> 
> > So:
> >
> >      0x7fffffff / 24MHz = 89.478485 sec
> >
> > Nothing to do here except to get rid of the requirement to arm the
> > timer at all.
> 
> @Frederic: Any inputs on how to get rid of this timer here?

I fear you can't. If you schedule a timer in 4 seconds away and your clockdevice
can only count up to 2 seconds, you can't help much the interrupt in the middle to
cope with the overflow.

So you need to act on the source of the timer:

* identify what cause this timer
* try to turn that feature off
* if you can't then move the timer to the housekeeping CPU

I'll have a look into the latter point to affine global timers to the
housekeeping CPU. Per cpu timers need more inspection though. Either we rework
them to be possibly handled by remote/housekeeping CPUs, or we let the associate feature
to be turned off. All in one it's a case by case work.
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