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    SubjectRe: [PATCH 2/3] sched/deadline: fix bandwidth check/update when migrating tasks between exclusive cpusets
    On Fri, Sep 19, 2014 at 10:22:40AM +0100, Juri Lelli wrote:
    > Exclusive cpusets are the only way users can restrict SCHED_DEADLINE tasks
    > affinity (performing what is commonly called clustered scheduling).
    > Unfortunately, such thing is currently broken for two reasons:
    >
    > - No check is performed when the user tries to attach a task to
    > an exlusive cpuset (recall that exclusive cpusets have an
    > associated maximum allowed bandwidth).
    >
    > - Bandwidths of source and destination cpusets are not correctly
    > updated after a task is migrated between them.
    >
    > This patch fixes both things at once, as they are opposite faces
    > of the same coin.
    >
    > The check is performed in cpuset_can_attach(), as there aren't any
    > points of failure after that function. The updated is split in two
    > halves. We first reserve bandwidth in the destination cpuset, after
    > we pass the check in cpuset_can_attach(). And we then release
    > bandwidth from the source cpuset when the task's affinity is
    > actually changed. Even if there can be time windows when sched_setattr()
    > may erroneously fail in the source cpuset, we are fine with it, as
    > we can't perfom an atomic update of both cpusets at once.

    The thing I cannot find is if we correctly deal with updates to the
    cpuset. Say we first setup 2 (exclusive) sets A:cpu0 B:cpu1-3. Then
    assign tasks and then update the cpu masks like: B:cpu2,3, A:cpu1,2.




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