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

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Date:   Sat, 28 May 2022 16:24:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Waiman Long <longman@...hat.com>,
        Frederic Weisbecker <frederic@...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 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.