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

    On 01/30, Tejun Heo wrote:
    > 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.

    You know, I am a bit suprized. To me, it is the !WQ_UNBOUND case is
    "special". IOW, I think we need some reason to bind the work to the
    specific CPU.

    > > > 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.

    I understand. But, the blocked worker "consumes" nr_active/worker.

    > 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.

    I am starting to think I do not understand this code at all. OK,
    perhaps unbound_wq should be used for cpu-intensive works only.

    But why do you think that we should use a !WQ_UNBOUND workque
    instead of khelper_wq? And why "a lot of CPU" is the only reason
    for WQ_UNBOUND?

    > * 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.

    So I don't understand whether you like the idea to kill khelper_wq
    and use some system_ wq or not (and fix the bug).

    I do not really like the current patch. If nothing else, what if
    UMH_WAIT_EXEC request actually needs another UMH_WAIT_EXEC/PROC
    request to succeed?

    Tetsuo, we spent a lot of time discussing other problems. What
    do you think about s/khelper/system/ instead of this patch?


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