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

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Date:   Mon, 30 May 2022 16:13:42 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        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 30/05/22 15:16, 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.

All features on/off knob + house-keeping sub-partition/mask seem to be
what isolation users I could reach so far (OCP mostly) would indeed like
to have in the future.