Re: [Intel-gfx] [PATCH] drm/i915: Implement SINGLE_TIMELINE with a syncobj (v2)

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Mar 24, 2021 at 09:28:58AM +0000, Tvrtko Ursulin wrote:
> 
> On 23/03/2021 17:51, Jason Ekstrand wrote:
> > This API is entirely unnecessary and I'd love to get rid of it.  If
> > userspace wants a single timeline across multiple contexts, they can
> > either use implicit synchronization or a syncobj, both of which existed
> > at the time this feature landed.  The justification given at the time
> > was that it would help GL drivers which are inherently single-timeline.
> > However, neither of our GL drivers actually wanted the feature.  i965
> > was already in maintenance mode at the time and iris uses syncobj for
> > everything.
> > 
> > Unfortunately, as much as I'd love to get rid of it, it is used by the
> > media driver so we can't do that.  We can, however, do the next-best
> > thing which is to embed a syncobj in the context and do exactly what
> > we'd expect from userspace internally.  This isn't an entirely identical
> > implementation because it's no longer atomic if userspace races with
> > itself by calling execbuffer2 twice simultaneously from different
> > threads.  It won't crash in that case; it just doesn't guarantee any
> > ordering between those two submits.
> > 
> > Moving SINGLE_TIMELINE to a syncobj emulation has a couple of technical
> > advantages beyond mere annoyance.  One is that intel_timeline is no
> > longer an api-visible object and can remain entirely an implementation
> > detail.  This may be advantageous as we make scheduler changes going
> > forward.  Second is that, together with deleting the CLONE_CONTEXT API,
> > we should now have a 1:1 mapping between intel_context and
> > intel_timeline which may help us reduce locking.
> 
> Much, much better commit message although I still fail to understand where
> do you see implementation details leaking out. So for me this is still
> something I'd like to get to the bottom of.
> 
> I would also mention the difference regarding fence context change.
> 
> And in general I would maintain this patch as part of a series which ends up
> demonstrating the "mays" and "shoulds".

I disagree. The past few years we've merged way too much patches and
features without carefully answering the high level questions of
- do we really need to solve this problem
- and if so, are we really solving this problem in the right place

Now we're quite in a hole, and we're not going to get out of this hole if
we keep applying the same standards that got us here. Anything that does
not clearly and without reservation the above two questions with "yes"
needs to be removed or walled off, just so we can eventually see which
complexity we really need, and what is actually superflous.

Especially when the kernel patch is this simple.
-Daniel

> 
> > 
> > v2 (Jason Ekstrand):
> >   - Update the comment on i915_gem_context::syncobj to mention that it's
> >     an emulation and the possible race if userspace calls execbuffer2
> >     twice on the same context concurrently.
> >   - Wrap the checks for eb.gem_context->syncobj in unlikely()
> >   - Drop the dma_fence reference
> >   - Improved commit message
> > 
> > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Matthew Brost <matthew.brost@intel.com>
> > ---
> >   drivers/gpu/drm/i915/gem/i915_gem_context.c   | 47 ++++---------------
> >   .../gpu/drm/i915/gem/i915_gem_context_types.h | 14 +++++-
> >   .../gpu/drm/i915/gem/i915_gem_execbuffer.c    | 16 +++++++
> >   3 files changed, 39 insertions(+), 38 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > index f88bac19333ec..e094f4a1ca4cd 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > @@ -67,6 +67,8 @@
> >   #include <linux/log2.h>
> >   #include <linux/nospec.h>
> > +#include <drm/drm_syncobj.h>
> > +
> >   #include "gt/gen6_ppgtt.h"
> >   #include "gt/intel_context.h"
> >   #include "gt/intel_engine_heartbeat.h"
> > @@ -224,10 +226,6 @@ static void intel_context_set_gem(struct intel_context *ce,
> >   		ce->vm = vm;
> >   	}
> > -	GEM_BUG_ON(ce->timeline);
> > -	if (ctx->timeline)
> > -		ce->timeline = intel_timeline_get(ctx->timeline);
> > -
> >   	if (ctx->sched.priority >= I915_PRIORITY_NORMAL &&
> >   	    intel_engine_has_timeslices(ce->engine))
> >   		__set_bit(CONTEXT_USE_SEMAPHORES, &ce->flags);
> > @@ -344,8 +342,8 @@ void i915_gem_context_release(struct kref *ref)
> >   	mutex_destroy(&ctx->engines_mutex);
> >   	mutex_destroy(&ctx->lut_mutex);
> > -	if (ctx->timeline)
> > -		intel_timeline_put(ctx->timeline);
> > +	if (ctx->syncobj)
> > +		drm_syncobj_put(ctx->syncobj);
> >   	put_pid(ctx->pid);
> >   	mutex_destroy(&ctx->mutex);
> > @@ -790,33 +788,11 @@ static void __assign_ppgtt(struct i915_gem_context *ctx,
> >   		i915_vm_close(vm);
> >   }
> > -static void __set_timeline(struct intel_timeline **dst,
> > -			   struct intel_timeline *src)
> > -{
> > -	struct intel_timeline *old = *dst;
> > -
> > -	*dst = src ? intel_timeline_get(src) : NULL;
> > -
> > -	if (old)
> > -		intel_timeline_put(old);
> > -}
> > -
> > -static void __apply_timeline(struct intel_context *ce, void *timeline)
> > -{
> > -	__set_timeline(&ce->timeline, timeline);
> > -}
> > -
> > -static void __assign_timeline(struct i915_gem_context *ctx,
> > -			      struct intel_timeline *timeline)
> > -{
> > -	__set_timeline(&ctx->timeline, timeline);
> > -	context_apply_all(ctx, __apply_timeline, timeline);
> > -}
> > -
> >   static struct i915_gem_context *
> >   i915_gem_create_context(struct drm_i915_private *i915, unsigned int flags)
> >   {
> >   	struct i915_gem_context *ctx;
> > +	int ret;
> >   	if (flags & I915_CONTEXT_CREATE_FLAGS_SINGLE_TIMELINE &&
> >   	    !HAS_EXECLISTS(i915))
> > @@ -845,16 +821,13 @@ i915_gem_create_context(struct drm_i915_private *i915, unsigned int flags)
> >   	}
> >   	if (flags & I915_CONTEXT_CREATE_FLAGS_SINGLE_TIMELINE) {
> > -		struct intel_timeline *timeline;
> > -
> > -		timeline = intel_timeline_create(&i915->gt);
> > -		if (IS_ERR(timeline)) {
> > +		ret = drm_syncobj_create(&ctx->syncobj,
> > +					 DRM_SYNCOBJ_CREATE_SIGNALED,
> > +					 NULL);
> > +		if (ret) {
> >   			context_close(ctx);
> > -			return ERR_CAST(timeline);
> > +			return ERR_PTR(ret);
> >   		}
> > -
> > -		__assign_timeline(ctx, timeline);
> > -		intel_timeline_put(timeline);
> >   	}
> >   	trace_i915_context_create(ctx);
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > index 676592e27e7d2..df76767f0c41b 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context_types.h
> > @@ -83,7 +83,19 @@ struct i915_gem_context {
> >   	struct i915_gem_engines __rcu *engines;
> >   	struct mutex engines_mutex; /* guards writes to engines */
> > -	struct intel_timeline *timeline;
> > +	/**
> > +	 * @syncobj: Shared timeline syncobj
> > +	 *
> > +	 * When the SHARED_TIMELINE flag is set on context creation, we
> > +	 * emulate a single timeline across all engines using this syncobj.
> > +	 * For every execbuffer2 call, this syncobj is used as both an in-
> > +	 * and out-fence.  Unlike the real intel_timeline, this doesn't
> > +	 * provide perfect atomic in-order guarantees if the client races
> > +	 * with itself by calling execbuffer2 twice concurrently.  However,
> > +	 * if userspace races with itself, that's not likely to yield well-
> > +	 * defined results anyway so we choose to not care.
> > +	 */
> > +	struct drm_syncobj *syncobj;
> >   	/**
> >   	 * @vm: unique address space (GTT)
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > index 96403130a373d..2e9748c1edddf 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > @@ -3295,6 +3295,16 @@ i915_gem_do_execbuffer(struct drm_device *dev,
> >   		goto err_vma;
> >   	}
> > +	if (unlikely(eb.gem_context->syncobj)) {
> > +		struct dma_fence *fence;
> > +
> > +		fence = drm_syncobj_fence_get(eb.gem_context->syncobj);
> > +		err = i915_request_await_dma_fence(eb.request, fence);
> > +		if (err)
> > +			goto err_ext;
> > +		dma_fence_put(fence);
> 
> I think put goes before the error bail.
> 
> > +	}
> > +
> >   	if (in_fence) {
> >   		if (args->flags & I915_EXEC_FENCE_SUBMIT)
> >   			err = i915_request_await_execution(eb.request,
> > @@ -3351,6 +3361,12 @@ i915_gem_do_execbuffer(struct drm_device *dev,
> >   			fput(out_fence->file);
> >   		}
> >   	}
> > +
> > +	if (unlikely(eb.gem_context->syncobj)) {
> > +		drm_syncobj_replace_fence(eb.gem_context->syncobj,
> > +					  &eb.request->fence);
> > +	}
> > +
> >   	i915_request_put(eb.request);
> >   err_vma:
> > 
> 
> Regards,
> 
> Tvrtko
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> Intel-gfx@lists.freedesktop.org
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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