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

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,
> + 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.



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