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
