Re: [PATCH v4] time/tick-sched: idle load balancing when nohz_full cpu becomes idle.

[Peter Zijlstra]

On Thu, May 16, 2024 at 05:02:51PM +0200, Frederic Weisbecker wrote:

> > I'm confused, none of that makes sense. If you're part of a
> > load-balancer, you're part of a load-balancer, no ifs buts or other
> > nonsense.
> > 
> > idle load balancer is no different from regular load balancing.
> > 
> > Fundamentally, you can't disable the tick if you're part of a
> > load-balance group, the load-balancer needs the tick.
> > 
> > The only possible way to use nohz_full is to not be part of a
> > load-balancer, and the only way that is so is by having (lots of) single
> > CPU partitions.
> 
> So you're suggesting that nohz_full should just be part of the whole
> ilb machinery by default (that is, not fiddle with ilb internals) and
> then it's up to CPU partitioning (through cpuset or isolcpus) to disable
> ilb naturally. Right?

Yes, but stronger, as long as the CPU is part of a load-balance domain,
it must not disable the tick while running anything.

that is, NOHZ_FULL must not become active unless it's running on a
single CPU partition.