[Intel-gfx] [PATCH 06/21] drm/i915: Implement SINGLE_TIMELINE with a syncobj (v3)
Daniel Vetter
daniel at ffwll.ch
Thu Apr 29 12:08:54 UTC 2021
On Thu, Apr 29, 2021 at 09:06:47AM +0100, Tvrtko Ursulin wrote:
>
> On 28/04/2021 18:26, Jason Ekstrand wrote:
> > On Wed, Apr 28, 2021 at 10:49 AM Tvrtko Ursulin
> > <tvrtko.ursulin at linux.intel.com> wrote:
> > >
> > >
> > > On 23/04/2021 23:31, 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.
> > >
> > > 1)
> > >
> > > Please also mention the difference in context/timeline name when
> > > observed via the sync file API.
> > >
> > > 2)
> > >
> > > I don't remember what we have concluded in terms of observable effects
> > > in sync_file_merge?
> >
> > I don't see how either of these are observable since this syncobj is
> > never exposed to userspace in any way. Please help me understand what
> > I'm missing here.
>
> Single timeline context - two execbufs - return two out fences.
>
> Before the patch those two had the same fence context, with the patch they
> have different ones.
>
> Fence context is visible to userspace via sync file info (timeline name at
> least) and rules in sync_file_merge.
Good point worth mentioninig in the commit message.
media-driver doesn't use any of this in combination with single_timeline,
so we just dont care.
-Daniel
>
> Regards,
>
> Tvrtko
>
> >
> > --Jason
> >
> >
> > > Regards,
> > >
> > > Tvrtko
> > >
> > > > 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.
> > > >
> > > > 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
> > > >
> > > > v3 (Jason Ekstrand):
> > > > - Move the dma_fence_put() to before the error exit
> > > >
> > > > Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> > > > Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > > > Cc: Matthew Brost <matthew.brost at intel.com>
> > > > ---
> > > > drivers/gpu/drm/i915/gem/i915_gem_context.c | 49 +++++--------------
> > > > .../gpu/drm/i915/gem/i915_gem_context_types.h | 14 +++++-
> > > > .../gpu/drm/i915/gem/i915_gem_execbuffer.c | 16 ++++++
> > > > 3 files changed, 40 insertions(+), 39 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > > > index 2c2fefa912805..a72c9b256723b 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_context_param.h"
> > > > @@ -225,10 +227,6 @@ 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);
> > > > @@ -351,9 +349,6 @@ 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);
> > > > -
> > > > put_pid(ctx->pid);
> > > > mutex_destroy(&ctx->mutex);
> > > >
> > > > @@ -570,6 +565,9 @@ static void context_close(struct i915_gem_context *ctx)
> > > > if (vm)
> > > > i915_vm_close(vm);
> > > >
> > > > + if (ctx->syncobj)
> > > > + drm_syncobj_put(ctx->syncobj);
> > > > +
> > > > ctx->file_priv = ERR_PTR(-EBADF);
> > > >
> > > > /*
> > > > @@ -765,33 +763,11 @@ 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))
> > > > @@ -820,16 +796,13 @@ 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 b812f313422a9..d640bba6ad9ab 100644
> > > > --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > > > +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> > > > @@ -3460,6 +3460,16 @@ 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);
> > > > + dma_fence_put(fence);
> > > > + if (err)
> > > > + goto err_ext;
> > > > + }
> > > > +
> > > > if (in_fence) {
> > > > if (args->flags & I915_EXEC_FENCE_SUBMIT)
> > > > err = i915_request_await_execution(eb.request,
> > > > @@ -3517,6 +3527,12 @@ 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:
> > > >
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--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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