linux-kernel - Re: [PATCH-cgroup 2/2] cgroup/cpuset: Include isolated cpuset CPUs in cpu_is

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Message-ID: <ZWZl0uvqeZ-fR1O9@slm.duckdns.org>
Date:   Tue, 28 Nov 2023 12:12:34 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mrunal Patel <mpatel@...hat.com>,
        Ryan Phillips <rphillips@...hat.com>,
        Brent Rowsell <browsell@...hat.com>,
        Peter Hunt <pehunt@...hat.com>
Subject: Re: [PATCH-cgroup 2/2] cgroup/cpuset: Include isolated cpuset CPUs
 in cpu_is_isolated() check

Hello,

On Tue, Nov 28, 2023 at 01:32:53PM -0500, Waiman Long wrote:
> On 11/28/23 11:56, Tejun Heo wrote:
> > Hello,
> > 
> > On Sun, Nov 26, 2023 at 11:19:56PM -0500, Waiman Long wrote:
> > > +bool cpuset_cpu_is_isolated(int cpu)
> > > +{
> > > +	unsigned int seq;
> > > +	bool ret;
> > > +
> > > +	do {
> > > +		seq = read_seqcount_begin(&isolcpus_seq);
> > > +		ret = cpumask_test_cpu(cpu, isolated_cpus);
> > > +	} while (read_seqcount_retry(&isolcpus_seq, seq));
> > > +	return ret;
> > > +}
> > > +EXPORT_SYMBOL_GPL(cpuset_cpu_is_isolated);
> > We're testing a bit in a bitmask. I don't think we need to worry about value
> > integrity from RMW updates being broken up. ie. We can just test the bit
> > without seqlock and drop the first patch?
> 
> My concern is that if we have an isolated partition with a set of isolated
> CPUs (say 2-4), I don't want any addition, deletion of changes made to
> another isolated partition affects the test of the pre-existing one. Testing
> result of the partition being change is fair game.
> 
> Depending on how the cpumask operators are implemented, we may not have a
> guarantee that testing CPU 2, for instance, will always return true. That is

Can you please elaborate this part a bit? I'm having a difficult time
imagining the sequence of operations where this would matter but that could
easily be me not being familiar with the details.

Thanks.

-- 
tejun