Re: [PATCH] sched/idle: disable tick in idle=poll idle entry

Marcelo Tosatti
2023-06-09T19:44:04Z

On Thu, Jun 08, 2023 at 10:34:00AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 07, 2023 at 10:10:46PM -0300, Marcelo Tosatti wrote:
> >
> > Commit a5183862e76fdc25f36b39c2489b816a5c66e2e5
> > ("tick/nohz: Conditionally restart tick on idle exit") allows
> > a nohz_full CPU to enter idle and return from it with the
> > scheduler tick disabled (since the tick might be undesired noise).
> >
> > The idle=poll case still unconditionally restarts the tick when entering
> > idle.
> >
> > To reduce the noise for that case as well, stop the tick when entering
> > idle, for the idle=poll case.
> >
> > tick_nohz_idle_exit (called if the NEED_RESCHED bit is set)
> > is responsible for re-enabling the tick when necessary.
>
> Who cares and why?

Systems that use isolcpus/nohz_full, and use idle=poll to avoid sleeping.
For example:

https://github.com/intel/FlexRAN/blob/master/README.md

More specifically, the case which HLTs (via nanosleep usually) is:

https://www.isa.org/intech-home/2022/february-2022/departments/what-you-may-not-know-about-softplcs

Can add this to the next version of the patch (unless you have any other
comments).

Thanks.