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

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Date:   Tue, 31 May 2022 10:21:03 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>, Tejun Heo <tj@...nel.org>,
        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 5/30/22 09:16, 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.

I am actually thinking about extending the cpuset partition interface 
for isolation. Right now, I have an outstanding patch [1] to add an 
"isolated" state to partition which disable load balancing somewhat 
similar to isolcpus command line option. In the future, we can add 
attribute to the isolation state like "isolated:full" to similar to 
nohz_full currently. If the needs arise, we can even  extend the 
attribute to allow list like "isolated:rcu_nocbs". I don't think it is 
good idea to keep on adding new cpuset control files extensively. I 
would prefer extending the existing ones.

[1] https://lore.kernel.org/lkml/20220510153413.400020-1-longman@redhat.com/

Cheers,
Longman