[Intel-gfx] [PATCH] drm/i915: Prevent bonded requests from overtaking each other on preemption

[Bloomfield, Jon]

> -----Original Message-----
> From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of Tvrtko
> Ursulin
> Sent: Friday, September 20, 2019 8:12 AM
> To: Chris Wilson <chris at chris-wilson.co.uk>; intel-gfx at lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH] drm/i915: Prevent bonded requests from
> overtaking each other on preemption
> 
> 
> On 20/09/2019 15:57, Chris Wilson wrote:
> > Quoting Chris Wilson (2019-09-20 09:36:24)
> >> Force bonded requests to run on distinct engines so that they cannot be
> >> shuffled onto the same engine where timeslicing will reverse the order.
> >> A bonded request will often wait on a semaphore signaled by its master,
> >> creating an implicit dependency -- if we ignore that implicit dependency
> >> and allow the bonded request to run on the same engine and before its
> >> master, we will cause a GPU hang.
> >
> > Thinking more, it should not directly cause a GPU hang, as the stuck request
> > should be timesliced away, and each preemption should be enough to keep
> > hangcheck at bay (though we have evidence it may not). So at best it runs
> > at half-speed, at worst a third (if my model is correct).
> 
> But I think it is still correct to do since we don't have the coupling
> information on re-submit. Hm.. but don't we need to prevent slave from
> changing engines as well?

Unless I'm missing something, the proposal here is to set the engines in stone at first submission, and never change them?
If so, that does sound overly restrictive, and will prevent any kind of rebalancing as workloads (of varying slave counts) come and go.
During the original design it was called out that the workloads should be pre-empted atomically. That allows the entire bonding mask to be re-evaluated at every context switch and so we can then rebalance. Still not easy to achieve I agree :-(
 
> 
> Regards,
> 
> Tvrtko
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