Re: [External] Re: [PATCH] sched/fair: fix broken bandwidth control with nohz_full

[Steven Rostedt]

On Wed, 30 Mar 2022 20:23:27 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> > Chengming brought up VMs. That's a case to want to control the bandwidth,
> > but also not interrupt them with timer interrupts when they are running as
> > the top priority task on a CPU.  
> 
> It's CFS, there is nothing top priority about that.

If there's only one task running on a CPU, even with CFS, why do we need a
tick?

Look, we have a host that is doing nothing but running VMs. Could be a
single VM. The host doesn't even have disk, it just runs on from initrd,
and the user is actually just logging into a guest.

Why should the host be triggering a tick when running this VM, which is the
only thing that is running on the host? Yeah, it's CFS, or does CFS have to
have a tick? even when there's nothing else on the CPU?

-- Steve