linux-kernel - Re: [External] Re: [PATCH] sched/fair: fix broken bandwidth control with nohz

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Message-ID: <20220330143748.37f4804c@gandalf.local.home>
Date:   Wed, 30 Mar 2022 14:37:48 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Chengming Zhou <zhouchengming@...edance.com>, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org,
        duanxiongchun@...edance.com, songmuchun@...edance.com,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [External] Re: [PATCH] sched/fair: fix broken bandwidth control
 with nohz_full

On Wed, 30 Mar 2022 20:23:27 +0200
Peter Zijlstra <peterz@...radead.org> 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