linux-kernel - Re: [BUG] completely bonkers use of set_need_resched + VM_FAULT

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Date:	Thu, 12 Sep 2013 22:37:59 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Chris Wilson <chris@...is-wilson.co.uk>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Dave Airlie <airlied@...ux.ie>,
	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Thomas Hellstrom <thellstrom@...are.com>,
	intel-gfx <intel-gfx@...ts.freedesktop.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [BUG] completely bonkers use of set_need_resched +
 VM_FAULT_NOPAGE

On Thu, 12 Sep 2013, Peter Zijlstra wrote:

> On Thu, Sep 12, 2013 at 05:35:43PM +0100, Chris Wilson wrote:
> > Not quite, as it would be possible for the evil userspace to trigger a
> > GPU hang that would cause the sane userspace to spin indefinitely 
> > waiting for the error recovery to kick in.
> 
> So with FIFOn+1 preempting FIFOn its a live-lock because the faulting
> thread will forever keep yielding to itself since its the highest
> priority task around, therefore the set_need_resched() is an absolute
> NOP in that case.
> 
> For OTHER it might run another task with set_need_resched(), without
> set_need_resched() it'll simply spin on the fault until it runs out of
> time and gets force preempted and another task gets to run.
> 
> So for either case, the set_need_resched() doesn't make an appreciable
> difference.
> 
> Removing it will not make evil userspace much worse -- at worst it will
> cause slightly more wasted cycles.

Well, yield() is a completely doomed concept by definition no matter
whether you add set_need_resched() or not.

We really should put a

	schedule_timeout_uninterruptible(1hour);

into the yield() implementation to get finally rid of it.

Thanks,

	tglx


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