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    SubjectRe: [PATCH v2 0/7] CPU hotplug, cpusets: Fix issues with cpusets handling upon CPU hotplug
    On 05/05/2012 12:54 AM, Peter Zijlstra wrote:

    >
    >> Documentation/cgroups/cpusets.txt | 43 +++--
    >> include/linux/cpuset.h | 4
    >> kernel/cpuset.c | 317 ++++++++++++++++++++++++++++---------
    >> kernel/sched/core.c | 4
    >> 4 files changed, 274 insertions(+), 94 deletions(-)
    >
    > Bah, I really hate this complexity you've created for a problem that
    > really doesn't exist.
    >


    Doesn't exist? Well, I believe we do have a problem and a serious one
    at that too!

    The heart of the problem can be summarized in 2 sentences:

    o During a CPU hotplug, tasks can move between cpusets, and never
    come back to their original cpuset.
    o Tasks might get pinned to lesser number of cpus, unreasonably.

    Both these are undesirable from a system-admin point of view.
    Moreover, having workarounds for this from userspace is way too messy and
    ugly, if not impossible.

    > So why not fix the active mask crap?


    Because I doubt if that is the right way to approach this problem.

    An updated cpu_active_mask not being the necessary and sufficient condition
    for all scheduler related activities, is a different problem altogether, IMHO.

    (Btw, Ingo had also suggested reworking this whole cpuset thing, while
    reviewing the previous version of this fix.
    http://thread.gmane.org/gmane.linux.kernel/1250097/focus=1252133)

    Also, we need to fix this problem at the CPU Hotplug level itself, and
    not just for the suspend/resume case. Because, we have had numerous bug
    reports and people complaining about this issue, in various scenarios,
    including those that didn't involve suspend/resume.

    I am sure some of the people in Cc will have more to add to this, but in
    general, when the CPU hotplug (maybe even cpu offline + online) and the
    cpuset administration are done asynchronously, it leads to nasty surprises.
    In fact, there have been reports where people spent inordinate amounts of
    time before they figured out that a long-forgotten cpu hotplug operation
    which was performed, was the root-cause of a low-performing workload!.

    All these only suggest that it is time that we cleaned this up thoroughly,
    and at the root cause level itself.

    Btw, though there are 7 patches in this series, I don't think this patchset
    increases the complexity of the code.. In fact, it makes many things simpler
    and saner/cleaner, IMHO.

    Regards,
    Srivatsa S. Bhat



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