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

[Peter Zijlstra]

On Mon, Mar 28, 2022 at 12:44:54PM -0400, Steven Rostedt wrote:
> On Mon, 28 Mar 2022 17:56:07 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > > echo $$ > test/cgroup.procs
> > > taskset -c 1 bash -c "while true; do let i++; done"  --> will be throttled  
> > 
> > Ofcourse.. I'm arguing that bandiwdth control and NOHZ_FULL are somewhat
> > mutually exclusive, use-case wise. So I really don't get why you'd want
> > them both.
> 
> Is it?
> 
> One use case I can see for having both is for having a deadline task that
> needs to get something done in a tight deadline. NOHZ_FULL means "do not
> interrupt this task when it is the top priority task on the CPU and is
> running in user space".

This is absolute batshit.. It means no such thing. We'll happily wake
another task to this CPU and re-enable the tick any instant.

Worse; the use-case at hand pertains to cfs bandwidth control, which
pretty much guarantees there *will* be an interrupt.

> Why is it mutually exclusive to have a deadline task that does not want to
> be interrupted by timer interrupts?

This has absolutely nothing to do with deadline tasks, nada, noppes.

> Just because the biggest pushers of NOHZ_FULL is for those that are running
> RT tasks completely in user space and event want to fault if it ever goes
> into the kernel, doesn't mean that's the only use case.

Because there's costs associated with the whole thing. system entry/exit
get far more expensive. It just doesn't make much sense to use NOHZ_FULL
if you're not absoultely limiting system entry.

> 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.