Re: [Query]: tick-sched: why don't we stop tick when we are running idle task?

From: Viresh Kumar <viresh.kumar@linaro.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Lists linaro-kernel <linaro-kernel@lists.linaro.org>
Subject: Re: [Query]: tick-sched: why don't we stop tick when we are running idle task?
Date: Mon, 14 Apr 2014 17:12:08 +0530	[thread overview]
Message-ID: <CAKohpo==1=K0=gZz_Z_Y0zO2PtyvA-Q67_+VKTJN7kbQgbiHTg@mail.gmail.com> (raw)
In-Reply-To: <20140414110245.GG11096@twins.programming.kicks-ass.net>

On 14 April 2014 16:32, Peter Zijlstra <peterz@infradead.org> wrote:
> I'm still not sure _what_ you're trying to solve here. What are you
> doing and why?

Hi Peter,

We are working building ARM Networking machines. Networking Data
plane is handled completely at user space. At run time we may fix
any number of CPUs for data plane activities. There will be a single
user space thread per CPU for these data plane packet processing.
Due to timing constraints these cores can't allow any interruption
from kernel. These include interruption from:

- other tasks: Fixed with cpusets
- timers/hrtimers: Implemented cpuset.quiesce as you suggested:
Waiting for reviews
- workqueues: Probably would be fixed by Frederic's work.
- Tick: Even with NO_HZ_FULL we get a tick every second. This is
what I am trying to address here. Frederic earlier suggested to
offload this accounting to other CPUs and so was my initial proposal.

Please let me know what's the right way to get this fixed and I will
try it that way.

Thanks for your inputs.