Messages in this thread | | | Date | Fri, 3 Feb 2012 19:00:30 +0100 | From | Oleg Nesterov <> | Subject | Re: + kmod-avoid-deadlock-by-recursive-kmod-call.patch added to -mm tree |
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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?
Oleg.
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