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SubjectRe: [PATCH-cgroup 2/2] cgroup/cpuset: Include isolated cpuset CPUs in cpu_is_isolated() check
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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

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