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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Apr 14, 2014 at 05:12:08PM +0530, Viresh Kumar wrote:
> 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.

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.