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    SubjectRe: cpusets and kthreads, inconsistent behaviour
    On Tue, 10 Jun 2008, Max Krasnyansky wrote:

    > Basically the issue is that current behaviour of the cpusets is inconsistent
    > with regards to kthreads. Kthreads inherit cpuset from a parent properly but
    > they simply ignore cpuset.cpus when their cpu affinity is set/updated.
    > I think the behaviour must be consistent across the board. cpuset.cpus must
    > apply to _all_ the tasks in the set, not just some of the tasks. If kthread
    > must run on the cpus other than current_cpuset.cpus then it should detach from
    > the cpuset.
    >

    I disagree that a cpuset's set of allowable cpus should apply to all tasks
    attached to that cpuset. It's certainly beneficial to be able to further
    constrict the set of allowed cpus for a task using sched_setaffinity().

    It makes more sense to argue that for each task p, p->cpus_allowed is a
    subset of task_cs(p)->cpus_allowed.

    > To give you an example kthreads like scsi_eh, kswapd, kacpid, pdflush,
    > kseriod, etc are all started with cpus_allows=ALL_CPUS even though they
    > inherit a cpuset from kthreadd. Yes they can moved manually (with
    > sched_setaffinity) but the behaviour is not consistent, and for no good
    > reason. kthreads can be stopped/started at any time (module load for example)
    > which means that the user will have to keep moving them.
    >

    This doesn't seem to be purely a kthread issue. Tasks can be moved to a
    disjoint set of cpus by any caller to set_cpus_allowed_ptr() in the
    kernel.

    David


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