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

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Message-ID: <CAKohpom8av7A1wwcdOr5+LpHz--P+s9m_gR_9B+Aa4fpwkN7uQ@mail.gmail.com>
Date:	Tue, 21 Jan 2014 16:03:53 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
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 20 January 2014 21:21, Frederic Weisbecker <fweisbec@...il.com> wrote:
> 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

So, the main problem in my case was caused by this:

           <...>-2147  [001] d..2   302.573881: hrtimer_start:
hrtimer=c172aa50 function=tick_sched_timer expires=602075000000
softexpires=602075000000

I have mentioned this earlier when I sent you attachments. I think
this is somehow
tied with the NO_HZ_FULL stuff? As the timer is queued for 300 seconds after
current time.

How to get this out?

> 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.

Which CPUs are housekeeping CPUs? How do we declare them?
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