Re: [PATCH -tip 04/32] sched: Core-wide rq->lock

[Balbir Singh]

On Tue, Nov 24, 2020 at 09:16:17AM +0100, Peter Zijlstra wrote:
> On Sun, Nov 22, 2020 at 08:11:52PM +1100, Balbir Singh wrote:
> > On Tue, Nov 17, 2020 at 06:19:34PM -0500, Joel Fernandes (Google) wrote:
> > > From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > 
> > > Introduce the basic infrastructure to have a core wide rq->lock.
> > >
> > 
> > Reading through the patch, it seems like all the CPUs have to be
> > running with sched core enabled/disabled? Is it possible to have some
> > cores with core sched disabled?
> 
> Yep, patch even says so:
> 
>  + * XXX entirely possible to selectively enable cores, don't bother for now.

Yes, it does in the comments, I looked at just the changelog :)

> 
> > I don't see a strong use case for it,
> > but I am wondering if the design will fall apart if that assumption is
> > broken?
> 
> The use-case I have is not using stop-machine. That is, stopping a whole
> core at a time, instead of the whole sodding machine. It's on the todo
> list *somewhere*....
> 
>

Good to know, I guess that would need a transition of the entire core to
idle and maintenance of a mask that prevents tasks that need core
scheduling from getting off of CPUs that enable core scheduling.

Balbir Singh.