Re: [PATCH RFC] sched/fair: Avoid unnecessary IPIs for ILB

[Ingo Molnar]

* Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:

> > > Fixes: 7fd7a9e0caba ("sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle")
> > 
> > Hurm.. does this really warrant a Fixes tag? Afaict nothing is currently
> > broken -- this is a pure optimization question, no?
> 
> IMHO it is a breakage as it breaks NOHZ -- a lot of times the ILB kicks 
> back the CPU stopping the tick out of idle (effectively breaking NOHZ). 
> The large number of IPIs also wrecks power and it happens only on 6.1 and 
> after. Having the fixes tag means it will also goto all stable kernels >= 
> 6.1. Hope that sounds reasonable and thank you for taking a look!

So it's basically a fix of a NOHZ performance regression, introduced by 
7fd7a9e0caba or so, correct?

As long as the fixes have a good hope of being backported with a low amount 
of overhead, a Fixes: tag for a ~2 years old performance regression is 
unusual but not unprecedented.

We just need to make sure we don't put too much of a burden on the 
shoulders of -stable maintainers ...

Thanks,

	Ingo