[Intel-gfx] [PATCH 06/21] drm/i915: Implement SINGLE_TIMELINE with a syncobj (v3)
Tvrtko Ursulin
tvrtko.ursulin at linux.intel.com
Thu Apr 29 08:06:47 UTC 2021
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.
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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