linux-kernel - Re: [PATCH 4/6] watchdog: Boot-disable by default on full dynticks

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Date:	Fri, 14 Jun 2013 09:47:31 +0530
From:	anish singh <anish198519851985@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Don Zickus <dzickus@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: Re: [PATCH 4/6] watchdog: Boot-disable by default on full dynticks

On Thu, Jun 13, 2013 at 10:46 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Thu, 2013-06-13 at 21:51 +0530, anish singh wrote:
>
>> > The concept behind full dynamic ticks is very easy. When you set a given
>> > CPU(s) to dynamic tick, when it only has a single task scheduled on that
>> > CPU, it disables the periodic tick. This removes essentially *all*
>>
>> Documentation/timers/highres.txt states that
>> hrtimer_stop_sched_tick() is called when a CPU goes into idle state.Would
>> you mind elaborating "single task scheduled on that CPU"?
>> I am bit new to this so please excuse me if the question is too basic.
>
> That's the old CONFIG_NO_HZ, which only disables the tick on idle. What
> we are working on is to also disable the tick when there's only one task
> running on a given CPU. Why have as schedule tick when there's nothing
> to schedule?
>
> 3.10 now has new config options:
>
> CONFIG_NO_HZ_PERIODIC - which is NO_HZ disabled
>    (the old # CONFIG_NO_HZ not set)
>
> CONFIG_NO_HZ_IDLE - which is what CONFIG_NO_HZ use to be.
>
> Note, CONFIG_NO_HZ still exists and if set, will make CONFIG_NO_HZ_IDLE
> the default. This was to keep the same configuration for people who
> update their kernel and run make oldconfig.
>
> Then there's the new:
>
> CONFIG_NO_HZ_FULL - this enables CONFIG_NO_HZ_IDLE plus adds the new
> feature with disabling the tick when only one task is running on a given
> CPU.

Thanks and some more explanation in below documents.
Documentation/timers/NO_HZ.txt
Documentation/timers/highres.txt
>
>
>> > latency from the kernel! That is, if the task is doing some complex
>> > calculations, it wont be interrupted for kernel maintenance. A lot of
>> > Red Hat customers would love to have this feature. It allows for
>> > extremely low latency actions even without a real-time kernel. Heck, it
>> > works without even kernel preemption.
>> >
>> > Now removing the periodic tick is not a trivial task, and this is where
>>
>> You mean getting rid of period ticks in the kernel code is not easy as there
>> are many features such as perf is dependent on it right and that is why
>> instead of completely removing periodic ticks we just set the periodic tick
>> to happen at 1HZ instead of CONFIG_HZ value?
>
> IIRC, the reason for moving to 1 HZ is so that the scheduler doesn't get
> confused with overflows. It still needs to handle time keeping for
"overflows" meaning the tick happening at 1HZ?
However as I see here in this patch http://lwn.net/Articles/549592/ -
you have plans to move it to 0Hz then how does scheduler cope
with this?Does it not need this information to schedule a different
task when the current task on "adaptive-ticks CPU" is done?

Anyway doesn't "future works" should be part of No-hz.txt document?
> managing how to schedule tasks according to CFS.
>
> Everything else shouldn't depend on the tick... period.
>
> -- Steve
>
>> > all our issues come from. In fact, we can not even completely remove the
>> > tick yet, we just move it to 1 HZ instead of whatever the CONFIG_HZ is
>> > set to. We have to handle everything that depends on that tick, which
>> > includes perf, among other things.
>> >
>
>
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