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

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Message-ID: <b6f88157-cf5e-4c7b-99f3-1944b4e7ebde@redhat.com>
Date:   Wed, 29 Nov 2023 11:01:04 -0500
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
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


On 11/28/23 17:12, Tejun Heo wrote:
> 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.

I may be a bit paranoid about incorrect result due to racing as I had 
been burned before. Just testing a bit in the bitmask may probably be 
OK. I don't think it will be a problem for x86, but I am less certain 
about other more exotic architectures like arm64 or PPC which I am less 
familiar about. I add a seqcount for synchronization just for the peace 
of mind. I can take the seqcount out if you don't it is necessary.

I have also been thinking about an alternative helper that returns the 
whole isolated cpumask since in both cases where cpu_is_isolated() is 
used, we will have to iterate all the possible CPUs anyway, it will be 
more efficient to have the whole cpumask available. In that case, we may 
want to have a seqcount to avoid returning an intermediate result. 
Anyway, this is just a thought for now, I am not planning to do that at 
the moment.

Cheers,
Longman