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

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Message-ID: <YpR8PUlIraYE2+5L@worktop.programming.kicks-ass.net>
Date:   Mon, 30 May 2022 10:11:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>, Tejun Heo <tj@...nel.org>,
        Waiman Long <longman@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Phil Auld <pauld@...hat.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Daniel Bristot de Oliveira <bristot@...nel.org>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        rcu@...r.kernel.org
Subject: Re: [RFC PATCH 4/4] cpuset: Support RCU-NOCB toggle on v2 root
 partitions

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.