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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 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 why I am adding some synchronization primitive to prevent
racing. My original plan was to take the callback_lock. However, that
can be somewhat costly if this API is used rather frequently, especially
on systems with large # of CPUs. So I change it to use seqcount for read
protection which has a much lower cost.

Regarding patch 1 on converting callback_lock to raw_spinlock_t, I can
drop it if you have concern about that change. I just need to surround
the write_seqcount_begin()/write_seqcount_end() calls with
preempt_disabled()/preempt_enabled().

Cheers,
Longman

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