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

[Waiman Long]

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