Date: Wed, 10 Nov 2004 11:14:52 -0800
From: Andrew Morton <akpm@osdl.org>
To: dcn@sgi.com (Dean Nelson)
Cc: linux-kernel@vger.kernel.org, nickpiggin@yahoo.com.au,
       Christoph Hellwig <hch@lst.de>
Subject: Re: kthread realtime priorities and exporting 
 sys_sched_setscheduler()
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dcn@sgi.com (Dean Nelson) wrote:
>
> I'm trying to push XP[C|NET] out to the community. (For further details:
> http://marc.theaimsgroup.com/?l=linux-ia64&m=109337050919186&w=2 )
> 
> An objection was raised over the exporting and calling of
> sys_sched_setscheduler(), which XPC calls to make its kthreads
> run at realtime priorities. Without this change we found that it
> was possible for user processes to be given a higher effective
> priority than the kthreads used by XPC. The upshot of this was
> that the latencies incurred by XPC increased 300 times in the
> test example given. If XPC's kthreads were given realtime
> priorities this did not happen. (For further details:
> http://marc.theaimsgroup.com/?l=linux-ia64&m=109337503100067&w=2 )

I'd disagree with Christoph on that.  Being able to set the scheduling
policy from a module-based kernel thread is a sensible thing to be able to
do, and you can do it by issuing a direct trap anyway.

EXPORT_SYMBOL_GPL() would be preferred.

Possibly it could be done by adding a variant of kthread_create() which has
a new `policy' argument, but it doesn't really seem worth the fuss.
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