Re: [RFC PATCH 4/4] cpuset: Support RCU-NOCB toggle on v2 root partitions

[Frederic Weisbecker <frederic@kernel.org>]

On Mon, May 30, 2022 at 10:11:41AM +0200, Peter Zijlstra wrote:
> On Mon, May 30, 2022 at 02:40:49AM +0200, Frederic Weisbecker wrote:
> > On Sat, May 28, 2022 at 04:24:50PM +0200, Peter Zijlstra wrote:
> > > On Fri, May 27, 2022 at 10:30:18AM +0200, Juri Lelli wrote:
> > > > Hi,
> > > > 
> > > > On 26/05/22 14:37, Tejun Heo wrote:
> > > > > On Thu, May 26, 2022 at 08:28:43PM -0400, Waiman Long wrote:
> > > > > > I am thinking along the line that it will not be hierarchical. However,
> > > > > > cpuset can be useful if we want to have multiple isolated partitions
> > > > > > underneath the top cpuset with different isolation attributes, but no more
> > > > > > sub-isolated partition with sub-attributes underneath them. IOW, we can only
> > > > > > set them at the first level under top_cpuset. Will that be useful?
> > > > > 
> > > > > At that point, I'd just prefer to have it under /proc or /sys.
> > > > 
> > > > FWIW, I was under the impression that this would nicely fit along the
> > > > side of other feaures towards implenting dynamic isolation of CPUs (say
> > > > https://lore.kernel.org/lkml/20220510153413.400020-1-longman@redhat.com/
> > > > for example). Wouldn't be awkward to have to poke different places to
> > > > achieve isolation at runtime?
> > > 
> > > This, that's what I was thinking.
> > > 
> > > My main objection to the whole thing is that it's an RCU_NOCB specific
> > > interface. *That* I think is daft.
> > > 
> > > I was thinking a partition would be able to designate a house-keeping
> > > sub-partition/mask, but who cares about all the various different
> > > housekeeping parties.
> > 
> > It's time for the isolation users to step up here! I very rarely hear from them
> > and I just can't figure out by myself all the variants of uses for each of the
> > isolation features. May be some people are only interested in nocb for some
> > specific uses, or may be it never makes sense without nohz full and all the rest
> > of the isolation features. So for now I take the very cautious path to split the
> > interface.
> 
> This is ABI, you can't walk back on it. I would suggest starting with an
> 'all feature' isolation. Only if there's real demand for something more
> fine-grained add that on top. Simple first etc.

That's actually my worry. If we start with an all in one ABI, how do we later
mix that up with more finegrained features? Like what will be the behaviour of:

cpuset.isolation.rcu_nocb = 0
cpuset.isolation.all = 1