Am 01.12.21 um 13:16 schrieb Thomas Hellström (Intel):
On 12/1/21 12:25, Christian König wrote:
Am 01.12.21 um 12:04 schrieb Thomas Hellström (Intel):
On 12/1/21 11:32, Christian König wrote:
Am 01.12.21 um 11:15 schrieb Thomas Hellström (Intel):
[SNIP]
What we could do is to avoid all this by not calling the callback
with the lock held in the first place.
If that's possible that might be a good idea, pls also see below.
The problem with that is
dma_fence_signal_locked()/dma_fence_signal_timestamp_locked(). If
we could avoid using that or at least allow it to drop the lock
then we could call the callback without holding it.
Somebody would need to audit the drivers and see if holding the
lock is really necessary anywhere.
/Thomas
Oh, and a follow up question:
If there was a way to break the recursion on final put()
(using the same basic approach as patch 2 in this series uses
to break recursion in enable_signaling()), so that none of
these containers did require any special treatment, would it
be worth pursuing? I guess it might be possible by having the
callbacks drop the references rather than the loop in the
final put. + a couple of changes in code iterating over the
fence pointers.
That won't really help, you just move the recursion from the
final put into the callback.
How do we recurse from the callback? The introduced fence_put()
of individual fence pointers
doesn't recurse anymore (at most 1 level), and any callback
recursion is broken by the irq_work?
Yeah, but then you would need to take another lock to avoid
racing with dma_fence_array_signaled().
I figure the big amount of work would be to adjust code that
iterates over the individual fence pointers to recognize that
they are rcu protected.
Could be that we could solve this with RCU, but that sounds like
a lot of churn for no gain at all.
In other words even with the problems solved I think it would be
a really bad idea to allow chaining of dma_fence_array objects.
Yes, that was really the question, Is it worth pursuing this? I'm
not really suggesting we should allow this as an intentional
feature. I'm worried, however, that if we allow these containers
to start floating around cross-driver (or even internally)
disguised as ordinary dma_fences, they would require a lot of
driver special casing, or else completely unexpeced WARN_ON()s and
lockdep splats would start to turn up, scaring people off from
using them. And that would be a breeding ground for hairy
driver-private constructs.
Well the question is why we would want to do it?
If it's to avoid inter driver lock dependencies by avoiding to call
the callback with the spinlock held, then yes please. We had tons
of problems with that, resulting in irq_work and work_item
delegation all over the place.
Yes, that sounds like something desirable, but in these containers,
what's causing the lock dependencies is the enable_signaling()
callback that is typically called locked.
If it's to allow nesting of dma_fence_array instances, then it's
most likely a really bad idea even if we fix all the locking order
problems.
Well I think my use-case where I hit a dead end may illustrate what
worries me here:
1) We use a dma-fence-array to coalesce all dependencies for ttm
object migration.
2) We use a dma-fence-chain to order the resulting dm_fence into a
timeline because the TTM resource manager code requires that.
Initially seemingly harmless to me.
But after a sequence evict->alloc->clear, the dma-fence-chain feeds
into the dma-fence-array for the clearing operation. Code still
works fine, and no deep recursion, no warnings. But if I were to add
another driver to the system that instead feeds a dma-fence-array
into a dma-fence-chain, this would give me a lockdep splat.
So then if somebody were to come up with the splendid idea of using
a dma-fence-chain to initially coalesce fences, I'd hit the same
problem or risk illegaly joining two dma-fence-chains together.
To fix this, I would need to look at the incoming fences and iterate
over any dma-fence-array or dma-fence-chain that is fed into the
dma-fence-array to flatten out the input. In fact all
dma-fence-array users would need to do that, and even
dma-fence-chain users watching out for not joining chains together
or accidently add an array that perhaps came as a disguised
dma-fence from antother driver.
So the purpose to me would be to allow these containers as input to
eachother without a lot of in-driver special-casing, be it by
breaking recursion on built-in flattening to avoid
a) Hitting issues in the future or with existing interoperating
drivers.
b) Avoid driver-private containers that also might break the
interoperability. (For example the i915 currently driver-private
dma_fence_work avoid all these problems, but we're attempting to
address issues in common code rather than re-inventing stuff
internally).
I don't think that a dma_fence_array or dma_fence_chain is the right
thing to begin with in those use cases.
When you want to coalesce the dependencies for a job you could either
use an xarray like Daniel did for the scheduler or some hashtable
like we use in amdgpu. But I don't see the need for exposing the
dma_fence interface for those.
This is because the interface to our migration code takes just a
single dma-fence as dependency. Now this is of course something we
need to look at to mitigate this, but see below.
Yeah, that's actually fine.
And why do you use dma_fence_chain to generate a timeline for TTM?
That should come naturally because all the moves must be ordered.
Oh, in this case because we're looking at adding stuff at the end of
migration (like coalescing object shared fences and / or async unbind
fences), which may not complete in order.
Well that's ok as well. My question is why does this single dma_fence
then shows up in the dma_fence_chain representing the whole migration?
That somehow doesn't seem to make sense because each individual step of
the migration needs to wait for those dependencies as well even when it
runs in parallel.
But that's not really the point, the point was that an (at least to
me) seemingly harmless usage pattern, be it real or fictious, ends up
giving you severe internal- or cross-driver headaches.
Yeah, we probably should document that better. But in general I don't
see much reason to allow mixing containers. The dma_fence_array and
dma_fence_chain objects have some distinct use cases and and using them
to build up larger dependency structures sounds really questionable.
Christian.
/Thomas
Regards,
Christian.
