Re: + kmod-avoid-deadlock-by-recursive-kmod-call.patch added to-mm tree
From: Oleg Nesterov
Date: Fri Feb 03 2012 - 13:07:46 EST
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
> > > 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
> * 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?
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/