Re: [PATCH v2 00/25] AMDKFD kernel driver

[Oded Gabbay]

On 22/07/14 10:28, Daniel Vetter wrote:
> On Mon, Jul 21, 2014 at 03:03:07PM -0400, Jerome Glisse wrote:
> > On Mon, Jul 21, 2014 at 09:41:29PM +0300, Oded Gabbay wrote:
> > > On 21/07/14 21:22, Daniel Vetter wrote:
> > > > On Mon, Jul 21, 2014 at 7:28 PM, Oded Gabbay <oded.gabbay@xxxxxxx> wrote:
> > > > > > I'm not sure whether we can do the same trick with the hw scheduler. But
> > > > > > then unpinning hw contexts will drain the pipeline anyway, so I guess we
> > > > > > can just stop feeding the hw scheduler until it runs dry. And then unpin
> > > > > > and evict.
> > > > > So, I'm afraid but we can't do this for AMD Kaveri because:
> > > >
> > > > Well as long as you can drain the hw scheduler queue (and you can do
> > > > that, worst case you have to unmap all the doorbells and other stuff
> > > > to intercept further submission from userspace) you can evict stuff.
> > >
> > > I can't drain the hw scheduler queue, as I can't do mid-wave preemption.
> > > Moreover, if I use the dequeue request register to preempt a queue
> > > during a dispatch it may be that some waves (wave groups actually) of
> > > the dispatch have not yet been created, and when I reactivate the mqd,
> > > they should be created but are not. However, this works fine if you use
> > > the HIQ. the CP ucode correctly saves and restores the state of an
> > > outstanding dispatch. I don't think we have access to the state from
> > > software at all, so it's not a bug, it is "as designed".
> >
> > I think here Daniel is suggesting to unmapp the doorbell page, and track
> > each write made by userspace to it and while unmapped wait for the gpu to
> > drain or use some kind of fence on a special queue. Once GPU is drain we
> > can move pinned buffer, then remap the doorbell and update it to the last
> > value written by userspace which will resume execution to the next job.
>
> Exactly, just prevent userspace from submitting more. And if you have
> misbehaving userspace that submits too much, reset the gpu and tell it
> that you're sorry but won't schedule any more work.

I'm not sure how you intend to know if a userspace misbehaves or not. Can you 
elaborate ?

	Oded
> We have this already in i915 (since like all other gpus we're not
> preempting right now) and it works. There's some code floating around to
> even restrict the reset to _just_ the offending submission context, with
> nothing else getting corrupted.
>
> You can do all this with the doorbells and unmapping them, but it's a
> pain. Much easier if you have a real ioctl, and I haven't seen anyone with
> perf data indicating that an ioctl would be too much overhead on linux.
> Neither in this thread nor internally here at intel.
> -Daniel

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