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

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Date:	Thu, 16 Jan 2014 10:46:22 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Viresh Kumar <viresh.kumar@...aro.org>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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 Thu, 16 Jan 2014, Viresh Kumar wrote:

> On 15 January 2014 22:47, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > Are you sure about that? NO_HZ_FULL shouldn't touch much hrtimers.
> > Those are independant from the tick.
> >
> > Although some of them seem to rely on the softirq, but that seem to
> > concern the tick hrtimer only.
> 
> To make it clear I was talking about the hrtimer used by tick_sched_timer.
> I have crossed checked which timers are active on isolated CPU from
> /proc/timer_list and it gave on tick_sched_timer's hrtimer.
> 
> In the attached trace (dft.txt), see these locations:
> - Line 252: Time 302.573881: we scheduled the hrtimer for 300 seconds
> ahead of current time.
> - Line 254, 258, 262, 330, 334: We got interruptions continuously after
> ~90 seconds and this looked to be a case of timer's counter overflow.
> Isn't it? (I have removed some lines towards the end of this file to make
> it shorter, though dft.dat is untouched)

Just do the math.

     max reload value / timer freq = max time span

So:

     0x7fffffff / 24MHz = 89.478485 sec

Nothing to do here except to get rid of the requirement to arm the
timer at all.

Thanks,

	tglx
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