Re: [Intel-gfx] [PATCH 03/10] drm/syncobj: add new drm_syncobj_add_point interface v2

Daniel Vetter
2018-12-12T11:15:01Z

On Wed, Dec 12, 2018 at 12:08 PM Koenig, Christian
<Christian.Koenig@amd.com> wrote:
>
> Am 12.12.18 um 11:49 schrieb Daniel Vetter:
> > On Fri, Dec 07, 2018 at 11:54:15PM +0800, Chunming Zhou wrote:
> >> From: Christian König <ckoenig.leichtzumerken@gmail.com>
> >>
> >> Use the dma_fence_chain object to create a timeline of fence objects
> >> instead of just replacing the existing fence.
> >>
> >> v2: rebase and cleanup
> >>
> >> Signed-off-by: Christian König <christian.koenig@amd.com>
> > Somewhat jumping back into this. Not sure we discussed this already or
> > not. I'm a bit unclear on why we have to chain the fences in the timeline:
> >
> > - The timeline stuff is modelled after the WDDM2 monitored fences. Which
> > really are just u64 counters in memory somewhere (I think could be
> > system ram or vram). Because WDDM2 has the memory management entirely
> > separated from rendering synchronization it totally allows userspace to
> > create loops and deadlocks and everything else nasty using this - the
> > memory manager won't deadlock because these monitored fences never leak
> > into the buffer manager. And if CS deadlock, gpu reset takes care of the
> > mess.
> >
> > - This has a few consequences, as in they seem to indeed work like a
> > memory location: Userspace incrementing out-of-order (because they run
> > batches updating the same fence on different engines) is totally fine,
> > as is doing anything else "stupid".
> >
> > - Now on linux we can't allow anything, because we need to make sure that
> > deadlocks don't leak into the memory manager. But as long as we block
> > until the underlying dma_fence has materialized, nothing userspace can
> > do will lead to such a deadlock. Even if userspace ends up submitting
> > jobs without enough built-in synchronization, leading to out-of-order
> > signalling of fences on that "timeline". And I don't think that would
> > pose a problem for us.
> >
> > Essentially I think we can look at timeline syncobj as a dma_fence
> > container indexed through an integer, and there's no need to enforce that
> > the timline works like a real dma_fence timeline, with all it's
> > guarantees. It's just a pile of (possibly, if userspace is stupid)
> > unrelated dma_fences. You could implement the entire thing in userspace
> > after all, except for the "we want to share these timeline objects between
> > processes" problem.
> >
> > tldr; I think we can drop the dma_fence_chain complexity completely. Or at
> > least I'm not really understanding why it's needed.
> >
> > Of course that means drivers cannot treat a drm_syncobj timeline as a
> > dma_fence timeline. But given the future fences stuff and all that, that's
> > already out of the window anyway.
> >
> > What am I missing?
>
> Good question, since that was exactly my initial idea as well.
>
> Key point is that our Vulcan guys came back and said that this wouldn't
> be sufficient, but I honestly don't fully understand why.

Hm, sounds like we really need those testscases (vk cts on top of
mesa, igt) so we can talk about the exact corner cases we care about
and why.

I guess one thing that might happen is that userspace leaves out a
number and never sets that fence, relying on the >= semantics of the
monitored fence to unblock that thread. E.g. when skipping a frame in
one of the auxiliary workloads. For that case we'd need to make sure
we don't just wait for the given fence to materialize, but also any
fences later in the timeline.

But we can't decide that without understanding the actual use-case
that needs to be supported at the other end of the stack, and how all
the bits in between should look like.

I guess we're back to "uapi design without userspace doesn't make sense" ...

> Anyway that's why David came up with using the fence array to wait for
> all previously added fences, which I then later on extended into this
> chain container.
>
> I have to admit that it is way more defensive implemented this way. E.g.
> there is much fewer things userspace can do wrong.
>
> The principal idea is that when they mess things up they are always
> going to wait more than necessary, but never less.

That seems against the spirit of vulkan, which is very much about "you
get all the pieces". It also might dig us a hole in the future, if we
ever get around to moving towards a WDDM2 style memory management
model. For future proving I think it would make sense if we implement
the minimal uapi we need for vk timelines, not the strictest
guarantees we can get away with (without performance impact) with
current drivers.
-Daniel

> Christian.
>
> > -Daniel
> >
> >> ---
> >> drivers/gpu/drm/drm_syncobj.c | 37 +++++++++++++++++++++++++++++++++++
> >> include/drm/drm_syncobj.h | 5 +++++
> >> 2 files changed, 42 insertions(+)
> >>
> >> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> >> index e19525af0cce..51f798e2194f 100644
> >> --- a/drivers/gpu/drm/drm_syncobj.c
> >> +++ b/drivers/gpu/drm/drm_syncobj.c
> >> @@ -122,6 +122,43 @@ static void drm_syncobj_remove_wait(struct drm_syncobj *syncobj,
> >> spin_unlock(&syncobj->lock);
> >> }
> >>
> >> +/**
> >> + * drm_syncobj_add_point - add new timeline point to the syncobj
> >> + * @syncobj: sync object to add timeline point do
> >> + * @chain: chain node to use to add the point
> >> + * @fence: fence to encapsulate in the chain node
> >> + * @point: sequence number to use for the point
> >> + *
> >> + * Add the chain node as new timeline point to the syncobj.
> >> + */
> >> +void drm_syncobj_add_point(struct drm_syncobj *syncobj,
> >> + struct dma_fence_chain *chain,
> >> + struct dma_fence *fence,
> >> + uint64_t point)
> >> +{
> >> + struct syncobj_wait_entry *cur, *tmp;
> >> + struct dma_fence *prev;
> >> +
> >> + dma_fence_get(fence);
> >> +
> >> + spin_lock(&syncobj->lock);
> >> +
> >> + prev = rcu_dereference_protected(syncobj->fence,
> >> + lockdep_is_held(&syncobj->lock));
> >> + dma_fence_chain_init(chain, prev, fence, point);
> >> + rcu_assign_pointer(syncobj->fence, &chain->base);
> >> +
> >> + list_for_each_entry_safe(cur, tmp, &syncobj->cb_list, node) {
> >> + list_del_init(&cur->node);
> >> + syncobj_wait_syncobj_func(syncobj, cur);
> >> + }
> >> + spin_unlock(&syncobj->lock);
> >> +
> >> + /* Walk the chain once to trigger garbage collection */
> >> + dma_fence_chain_for_each(prev, fence);
> >> +}
> >> +EXPORT_SYMBOL(drm_syncobj_add_point);
> >> +
> >> /**
> >> * drm_syncobj_replace_fence - replace fence in a sync object.
> >> * @syncobj: Sync object to replace fence in
> >> diff --git a/include/drm/drm_syncobj.h b/include/drm/drm_syncobj.h
> >> index 7c6ed845c70d..8acb4ae4f311 100644
> >> --- a/include/drm/drm_syncobj.h
> >> +++ b/include/drm/drm_syncobj.h
> >> @@ -27,6 +27,7 @@
> >> #define __DRM_SYNCOBJ_H__
> >>
> >> #include "linux/dma-fence.h"
> >> +#include "linux/dma-fence-chain.h"
> >>
> >> /**
> >> * struct drm_syncobj - sync object.
> >> @@ -110,6 +111,10 @@ drm_syncobj_fence_get(struct drm_syncobj *syncobj)
> >>
> >> struct drm_syncobj *drm_syncobj_find(struct drm_file *file_private,
> >> u32 handle);
> >> +void drm_syncobj_add_point(struct drm_syncobj *syncobj,
> >> + struct dma_fence_chain *chain,
> >> + struct dma_fence *fence,
> >> + uint64_t point);
> >> void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,
> >> struct dma_fence *fence);
> >> int drm_syncobj_find_fence(struct drm_file *file_private,
> >> --
> >> 2.17.1
> >>
> >> _______________________________________________
> >> Intel-gfx mailing list
> >> Intel-gfx@lists.freedesktop.org
> >> https://lists.freedesktop.org/mailman/listinfo/intel-gfx
>

--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
_______________________________________________
amd-gfx mailing list
amd-gfx@lists.freedesktop.org