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Subject: Re: [RFC][PATCH] lru_add_drain_all() don't use
 schedule_on_each_cpu()
From: Peter Zijlstra <peterz@infradead.org>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
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Date: Mon, 27 Oct 2008 08:56:30 +0100
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On Mon, 2008-10-27 at 12:14 +0900, KOSAKI Motohiro wrote:
> > Right, and would be about 4k+sizeof(task_struct), some people might be
> > bothered, but most won't care.
> > 
> > > Perhaps, I misunderstand your intension. so can you point your
> > > previous discussion url?
> > 
> > my google skillz fail me, but once in a while people complain that we
> > have too many kernel threads.
> > 
> > Anyway, if we can re-use this per-cpu workqueue for more goals, I guess
> > there is even less of an objection.
> 
> In general, you are right.
> but this is special case. mmap_sem is really widely used various subsystem and drivers.
> (because page fault via copy_user introduce to depend on mmap_sem)
> 
> Then, any work-queue reu-sing can cause similar dead-lock easily.

Yeah, I know, and the cpu-hotplug discussion needed another thread due
to yet another locking incident. I was hoping these two could go
together.

Neither are general-purpose workqueues, both need to stay away from the
normal eventd due to deadlocks.

ego, does you extra thread ever use mmap_sem?