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    SubjectRe: [PATCH v2]mm/oom-kill: direct hardware access processes should get bonus
    On Sun, 14 Nov 2010, KOSAKI Motohiro wrote:

    > > So the question that needs to be answered is: why do these threads deserve
    > > to use 3% more memory (not >4%) than others without getting killed? If
    > > there was some evidence that these threads have a certain quantity of
    > > memory they require as a fundamental attribute of CAP_SYS_RAWIO, then I
    > > have no objection, but that's going to be expressed in a memory quantity
    > > not a percentage as you have here.
    >
    > 3% is choosed by you :-/
    >

    No, 3% was chosen in __vm_enough_memory() for LSMs as the comment in the
    oom killer shows:

    /*
    * Root processes get 3% bonus, just like the __vm_enough_memory()
    * implementation used by LSMs.
    */

    and is described in Documentation/filesystems/proc.txt.

    I think in cases of heuristics like this where we obviously want to give
    some bonus to CAP_SYS_ADMIN that there is consistency with other bonuses
    given elsewhere in the kernel.

    > Old background is very simple and cleaner.
    >

    The old heuristic divided the arbitrary badness score by 4 with
    CAP_SYS_RESOURCE. The new heuristic doesn't consider it.

    How is that more clean?

    > CAP_SYS_RESOURCE mean the process has a privilege of using more resource.
    > then, oom-killer gave it additonal bonus.
    >

    As a side-effect of being given more resources to allocate, those
    applications are relatively unbounded in terms of memory consumption to
    other tasks. Thus, it's possible that these applications are using a
    massive amount of memory (say, 75%) and now with the proposed change a
    task using 25% of memory would be killed instead. This increases the
    liklihood that the CAP_SYS_RESOURCE thread will have to be killed
    eventually, anyway, and the goal is to kill as few tasks as possible to
    free sufficient amount of memory.

    Since threads having CAP_SYS_RESOURCE have full control over their
    oom_score_adj, they can take the additional precautions to protect
    themselves if necessary. It doesn't need to be a part of the heuristic to
    bias these tasks which will lead to the undesired result described above
    by default rather than intentionally from userspace.

    > CAP_SYS_RAWIO mean the process has a direct hardware access privilege
    > (eg X.org, RDB). and then, killing it might makes system crash.
    >

    Then you would want to explicitly filter these tasks from oom kill just as
    OOM_SCORE_ADJ_MIN works rather than giving them a memory quantity bonus.


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