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

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Date:	Tue, 28 Jan 2014 08:11:25 -0800
From:	Kevin Hilman <khilman@...aro.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	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 Tue, Jan 28, 2014 at 5:23 AM, Frederic Weisbecker <fweisbec@...il.com> wrote:
> On Fri, Jan 24, 2014 at 10:51:14AM +0530, Viresh Kumar wrote:
>> On 23 January 2014 20:28, Frederic Weisbecker <fweisbec@...il.com> wrote:
>> > On Tue, Jan 21, 2014 at 04:03:53PM +0530, Viresh Kumar wrote:
>>
>> >> 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?
>> >
>> > So it's scheduled away 300 seconds later. It might be a pending timer_list. Enabling the
>> > timer tracepoints may give you some clues.
>>
>> Trace was done with that enabled. /proc/timer_list confirms that a hrtimer
>> is queued for 300 seconds later for tick_sched_timer. And so I assumed
>> this is part of the current NO_HZ_FULL implementation.
>>
>> Just to confirm, when we decide that a CPU is running a single task and so
>> can enter tickless mode, do we queue this tick_sched_timer for 300 seconds
>> ahead of time? If not, then who is doing this :)
>
> No, when a single task is running on a full dynticks CPU, the tick is supposed to run
> every seconds. I'm actually suprised it doesn't happen in your traces, did you tweak
> something specific?

I think Viresh is using my patch/hack to configure/disable the 1Hz
residual tick.

Kevin
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