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

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Message-ID: <20140414114744.GI11096@twins.programming.kicks-ass.net>
Date:	Mon, 14 Apr 2014 13:47:44 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>
Subject: Re: [Query]: tick-sched: why don't we stop tick when we are running
 idle task?

On Mon, Apr 14, 2014 at 05:12:08PM +0530, Viresh Kumar wrote:
> On 14 April 2014 16:32, Peter Zijlstra <peterz@...radead.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.

Ok.

> - 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.

What causes this tick? I was under the impression that once there's a
single task (not doing any syscalls) and the above issues are sorted, no
more tick would happen.



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