[PATCH RFC 10/18] drm/scheduler: Add can_run_job callback

[Christian König]

Am 08.03.23 um 20:05 schrieb Asahi Lina:
> [SNIP]
>> Well it's not the better way, it's the only way that works.
>>
>> I have to admit that my bet on your intentions was wrong, but even that
>> use case doesn't work correctly.
>>
>> See when your callback returns false it is perfectly possible that all
>> hw fences are signaled between returning that information and processing it.
>>
>> The result would be that the scheduler goes to sleep and never wakes up
>> again.
> That can't happen, because it will just go into another iteration of the
> drm_sched main loop since there is an entity available still.
>
> Rather there is probably the opposite bug in this patch: the can_run_job
> logic should be moved into the wait_event_interruptible() condition
> check, otherwise I think it can end up busy-looping since the condition
> itself can be true even when the can_run_job check blocks it.
>
> But there is no risk of it going to sleep and never waking up because
> job completions will wake up the waitqueue by definition, and that
> happens after the driver-side queues are popped. If this problem could
> happen, then the existing hw_submission_limit logic would be broken in
> the same way. It is logically equivalent in how it works.
>
> Basically, if properly done in wait_event_interruptible, it is exactly
> the logic of that macro that prevents this race condition and makes
> everything work at all. Without it, drm_sched would be completely broken.
>
>> As I said we exercised those ideas before and yes this approach here
>> came up before as well and no it doesn't work.
> It can never deadlock with this patch as it stands (though it could busy
> loop), and if properly moved into the wait_event_interruptible(), it
> would also never busy loop and work entirely as intended. The actual API
> change is sound.
>
> I don't know why you're trying so hard to convince everyone that this
> approach is fundamentally broken... It might be a bad idea for other
> reasons, it might encourage incorrect usage, it might not be the best
> option, there are plenty of arguments you can make... but you just keep
> trying to make an argument that it just can't work at all for some
> reason. Why? I already said I'm happy dropping it in favor of the fences...

Well because it is broken.

When you move the check into the wait_event_interruptible condition then 
who is going to call wait_event_interruptible when the condition changes?

As I said this idea came up before and was rejected multiple times.

Regards,
Christian.

>
> It's intended to mirror the hw_submission_limit logic. If you think this
> is broken, then that's broken too. They are equivalent mechanisms.
>
>>> This particular issue aside, fairness in global resource allocation is a
>>> conversation I'd love to have! Right now the driver doesn't try to
>>> ensure that, a queue can easily monopolize certain hardware resources
>>> (though one queue can only monopolize one of each, so you'd need
>>> something like 63 queues with 63 distinct VMs all submitting
>>> free-running jobs back to back in order to starve other queues of
>>> resources forever). For starters, one thing I'm thinking of doing is
>>> reserving certain subsets of hardware resources for queues with a given
>>> priority, so you can at least guarantee forward progress of
>>> higher-priority queues when faced with misbehaving lower-priority
>>> queues. But if we want to guarantee proper fairness, I think I'll have
>>> to start doing things like switching to a CPU-roundtrip submission model
>>> when resources become scarce (to guarantee that queues actually release
>>> the resources once in a while) and then figure out how to add fairness
>>> to the allocation code...
>>>
>>> But let's have that conversation when we talk about the driver (or maybe
>>> on IRC or something?), right now I'm more interested in getting the
>>> abstractions reviewed ^^
>> Well that stuff is highly problematic as well. The fairness aside you
>> risk starvation which in turn breaks the guarantee of forward progress.
>>
>> In this particular case you can catch this with a timeout for the hw
>> operation, but you should consider blocking that from the sw side as well.
> In the current state I actually think it's not really that problematic,
> because the resources are acquired directly in the ioctl path. So that
> can block if starved, but if that can cause overall forward progress to
> stop because some fence doesn't get signaled, then so can just not doing
> the ioctl in the first place, so there's not much point (userspace can
> always misbehave with its fence usage...). By the time anything gets
> submitted to drm_sched, the resources are already guaranteed to be
> acquired, we never block in the run callback.
>
> It needs to be fixed of course, but if the threat model is a malicious
> GPU process, well, there are many other ways to DoS your system... and I
> don't think it's very likely that 63+ queues (which usually means 63+
> processes with OpenGL) will end up accidentally starving the GPU in a
> tight loop at the same time. I'd love to hear about real-world scenarios
> where this kind of thing has been a real problem and not just a
> theoretical one though... maybe I'm missing something?
>
> Basically my priorities with the driver are:
>
> 1. Make sure it never crashes
> 2. Make sure it works well for real users
> 3. Make it work smoothly for real users under reasonable load
> (priorities, CPU scheduler interactions, etc.)
> 4. Make it handle accidental problems more gracefully (OOMs etc, I need
> to look into private GEM BO accounting to processes so the OOM killer
> has better data to work with)
> 5. Make it more robust against deliberate abuse/starvation (this should
> matter more once we have some kind of paravirtualization solution...)
>
> And right now we're somewhere between 2 and 3. So if there are cases
> where this resource acquisition stuff can cause a problem for real
> users, I'll want to fix it earlier. But if this is more theoretical than
> anything (with the resource limits of AGX GPUs), I'd rather focus on
> things like memory accounting and shrinker support first.
>
>>> We don't even have a shrinker yet, and I'm sure that's going to be a lot
>>> of fun when we add it too... but yes, if we can't do any memory
>>> allocations in some of these callbacks (is this documented anywhere?),
>>> that's going to be interesting...
>> Yes, that is all part of the dma_fence documentation. It's just
>> absolutely not obvious what all this means.
> I mean is there any documentation on how this interacts with drm_sched?
> Like, am I not allowed to allocate memory in prepare()? What about
> run()? What about GPU interrupt work? (not a raw IRQ handler context, I
> mean the execution path from GPU IRQ to drm_sched run() fences getting
> signaled)
>
>>> It's not all bad news though! All memory allocations are fallible in
>>> kernel Rust (and therefore explicit, and also failures have to be
>>> explicitly handled or propagated), so it's pretty easy to point out
>>> where they are, and there are already discussions of higher-level
>>> tooling to enforce rules like that (and things like wait contexts).
>>> Also, Rust makes it a lot easier to refactor code in general and not be
>>> scared that you're going to regress everything, so I'm not really
>>> worried if I need to turn a chunk of the driver on its head to solve
>>> some of these problems in the future ^^ (I already did that when I
>>> switched it from the "demo" synchronous submission model to the proper
>>> explicit sync + fences one.)
>> Yeah, well the problem isn't that you run into memory allocation failure.
> What I mean is that the mandatory failure handling means it's relatively
> easy to audit where memory allocations can actually happen.
>
>> The problem is rather something like this:
>> 1. You try to allocate memory to signal your fence.
>> 2. This memory allocation can't be fulfilled and goes to sleep to wait
>> for reclaim.
>> 3. On another CPU reclaim is running and through the general purpose
>> shrinker, page fault or MMU notifier ends up wait for your dma_fence.
>>
>> You don't even need to implement the shrinker for this to go boom
>> extremely easy.
> Hmm, can you actually get something waiting on a dma_fence like that
> today with this driver? We don't have a shrinker, we don't have
> synchronous page faults or MMU notifications for the GPU, and this is
> explicit sync so all in/out fences cross over into userspace so surely
> they can't be trusted anyway?
>
> I'm definitely not familiar with the intricacies of DMA fences and how
> they interact with everything else yet, but it's starting to sound like
> either this isn't quite broken for our simple driver yet, or it must be
> broken pretty much everywhere in some way...
>
>> So everything involved with signaling the fence can allocate memory only
>> with GFP_ATOMIC and only if you absolutely have to.
> I don't think we even have a good story for passing around gfp_flags in
> Rust code so that will be interesting... though I need to actually audit
> the code paths and see how many allocations we really do. I know I alloc
> some vectors for holding completed commands and stuff like that, but I'm
> pretty sure I can fix that one with some reworking, and I'm not sure how
> many other random things there really are...? Obviously most allocations
> happen at command creation time, on completion you mostly get a lot of
> freeing, so maybe I can just eliminate all allocs and not worry about
> GFP_ATOMIC.
>
> ~~ Lina