Re: [RFC] [PATCH] DRM TTM Memory Manager patch

Jerome Glisse
2007-05-04T08:49:08Z

On 5/4/07, Thomas Hellström <thomas@tungstengraphics.com> wrote:
> Keith Packard wrote:
> > On Thu, 2007-05-03 at 01:01 +0200, Thomas Hellström wrote:
> >
> >
> >> It might be possible to find schemes that work around this. One way
> >> could possibly be to have a buffer mapping -and validate order for
> >> shared buffers.
> >>
> >
> > If mapping never blocks on anything other than the fence, then there
> > isn't any dead lock possibility. What this says is that ordering of
> > rendering between clients is *not DRMs problem*. I think that's a good
> > solution though; I want to let multiple apps work on DRM-able memory
> > with their own CPU without contention.
> >
> > I don't recall if Eric layed out the proposed rules, but:
> >
> > 1) Map never blocks on map. Clients interested in dealing with this
> > are on their own.
> >
> > 2) Submit blocks on map. You must unmap all buffers before submitting
> > them. Doing the relocations in the kernel makes this all possible.
> >
> > 3) Map blocks on the fence from submit. We can play with pending the
> > flush until the app asks for the buffer back, or we can play with
> > figuring out when flushes are useful automatically. Doesn't matter
> > if the policy is in the kernel.
> >
> > I'm interested in making deadlock avoidence trivial and eliminating any
> > map-map contention.
> >
> >
> It's rare to have two clients access the same buffer at the same time.
> In what situation will this occur?
>
> If we think of map / unmap and validation / fence as taking a buffer
> mutex either for the CPU or for the GPU, that's the way implementation
> is done today. The CPU side of the mutex should IIRC be per-client
> recursive. OTOH, the TTM implementation won't stop the CPU from
> accessing the buffer when it is unmapped, but then you're on your own.
> "Mutexes" need to be taken in the correct order, otherwise a deadlock
> will occur, and GL will, as outlined in Eric's illustration, more or
> less encourage us to take buffers in the "incorrect" order.
>
> In essence what you propose is to eliminate the deadlock problem by just
> avoiding taking the buffer mutex unless we know the GPU has it. I see
> two problems with this:
>
> * It will encourage different DRI clients to simultaneously access
> the same buffer.
> * Inter-client and GPU data coherence can be guaranteed if we issue
> a mb() / write-combining flush with the unmap operation (which,
> BTW, I'm not sure is done today). Otherwise it is up to the
> clients, and very easy to forget.
>
> I'm a bit afraid we might in the future regret taking the easy way out?
>
> OTOH, letting DRM resolve the deadlock by unmapping and remapping shared
> buffers in the correct order might not be the best one either. It will
> certainly mean some CPU overhead and what if we have to do the same with
> buffer validation? (Yes for some operations with thousands and thousands
> of relocations, the user space validation might need to stay).
>
> Personally, I'm slightly biased towards having DRM resolve the deadlock,
> but I think any solution will do as long as the implications and why we
> choose a certain solution are totally clear.
>
> For item 3) above the kernel must have a way to issue a flush when
> needed for buffer eviction.
> The current implementation also requires the buffer to be completely
> flushed before mapping.
> Other than that the flushing policy is currently completely up to the
> DRM drivers.
>
> /Thomas

I might say stupid things as i don't think i fully understand all
the input to this problem. Anyway here is my thought on all this:

1) First client map never block (as in Keith layout) except on
fence from drm side (point 3 in Keith layout)
2) Client should always unmap buffer before submitting (as in Keith layout)
3) In drm side you always acquire buffer lock in a give order for
instance each buffer got and id and you lock from smaller id to bigger
one (with a clever implementation the cost for that will be small)
4) We got 2 gpu queue:
- one with pending apps ask in which we do all stuff necessary
to be done before submitting (locking buffer, validation, ...)
for instance we might wait here for each buffer that are still
mapped by some other apps in user space
- one run queue in which we add each apps ask that are now
ready to be submited to the gpu

Of course in this scheme we keep the fencing stuff so user space can
know when it safe to use previously submited buffer again. The outcome
of having two seperate queue in drm is that if two apps lockup each other
other apps can still use the gpu so only the apps fighting for a buffer will
suffer.

And for user space synchronization i believe it a user space problem i.e.
it's up to user space to add proper synch. For instance as map doesn't
block for any client in user space two apps can mess with same buffer
it's up to user to have a policy to exclude each other (i believe this will
be a dri or xorg problem to synch btw consumer).

I believe in this scheme you can only have deadlock btw two buggy
apps (like one forgeting to unmap a buffer) or any apps that forgot
to unmap buffer before submitting won't cause damage to other
apps. I hope i properly explain my thought (hey my brain is in a sleepy
state right now).

best,
Jerome Glisse