Messages in this thread | | | Date | Mon, 30 Jan 2012 09:28:51 -0800 | From | Tejun Heo <> | Subject | Re: + kmod-avoid-deadlock-by-recursive-kmod-call.patch added to -mm tree |
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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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