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Date:   Mon, 30 May 2022 23:35:56 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Juri Lelli <juri.lelli@redhat.com>, Tejun Heo <tj@kernel.org>,
        Waiman Long <longman@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Phil Auld <pauld@redhat.com>,
        Zefan Li <lizefan.x@bytedance.com>,
        Daniel Bristot de Oliveira <bristot@kernel.org>,
        Nicolas Saenz Julienne <nsaenz@kernel.org>,
        rcu@vger.kernel.org
Subject: Re: [RFC PATCH 4/4] cpuset: Support RCU-NOCB toggle on v2 root
 partitions
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On Mon, May 30, 2022 at 03:16:27PM +0200, Peter Zijlstra wrote:
> On Mon, May 30, 2022 at 12:56:50PM +0200, Frederic Weisbecker wrote:
> 
> > > 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
> 
> Well clearly that doesn't make sense. I was more thinking along the
> lines of cgroup.subtree_control, where instead all features are enabled
> by default.
> 
> But only if there's a real usecase, otherwise there's no point in
> providing such knobs.

That makes sense. So there would be a simple cpuset.isolation that can
be either 1 or 0 where 1 has all possible isolation stuff on. Then
if the need arises we can provide more tuning through a new specific
cgroup controller, right?

If so that sounds good to me.