Messages in this thread | | | Date | Tue, 28 Nov 2023 13:32:53 -0500 | Subject | Re: [PATCH-cgroup 2/2] cgroup/cpuset: Include isolated cpuset CPUs in cpu_is_isolated() check | From | Waiman Long <> |
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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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