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    SubjectRe: + kmod-avoid-deadlock-by-recursive-kmod-call.patch added to -mm tree
    Hello,

    On Mon, Jan 30, 2012 at 02:03:35PM +0100, Oleg Nesterov wrote:
    > Perhaps we can use another system_wq, but afaics WQ_UNBOUND makes sense
    > in this case. I mean, there is no reason to bind this work to any CPU.
    > See also below.

    I've been trying to nudge people away from using special wqs or flags
    unless really necessary. Other than non-reentrancy and strict
    ordering, all behaviors are mostly for optimization and using them
    incorrectly / spuriously usually doesn't cause any visible failure,
    making it very easy to get them wrong and if you have enough of wrong
    / unnecessary usages in tree, the whole thing gets really confusing
    and difficult to update in the future.

    > > Is it expected consume large
    > > amount of CPU cycles?
    >
    > Currently __call_usermodehelper() does kernel_thread(), this is almost
    > all. But it can block waiting for kernel_execve().

    Blocking is completely fine on any workqueue. The only reason to
    require the use of unbound_wq is if work items would burn a lot of CPU
    cycles. In such cases, we want to let the scheduler have full
    jurisdiction instead of wq regulating concurrency.

    > Not sure this really makes sense, but if we kill khelper_wq perhaps we
    > can simplify this code a bit. We can change __call_usermodehelper()
    >
    > if (wait == UMH_WAIT_PROC)
    > - pid = kernel_thread(wait_for_helper, sub_info,
    > - CLONE_FS | CLONE_FILES | SIGCHLD);
    > + wait_for_helper(...);
    > else
    >
    > IOW, the worker thread itself can do the UMH_WAIT_PROC work. This makes
    > this work really "long running", but then we can kill sub_info->complete
    > and use flush_work().

    Again, long-running in the sense that the work item spending a lot of
    time sleeping should be fine on system_wq or any other wq with default
    attributes. AFAICS, the things to consider here are...

    * If work items are expected to consume large amount of CPU cycles (as
    in crypto work items), consider using system_unbound_wq / WQ_UNBOUND.

    * If per-domain concurrency limit is necessary (ie. the number of
    concurrent work items doing this particular task should be limited
    rather than consuming global system_wq limit), a dedicated workqueue
    would be better.

    Thanks.

    --
    tejun


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