[Mesa-dev] [RFC 2/2] drm/doc/rfc: i915 new parallel submission uAPI plan

Thu May 20 11:11:59 UTC 2021

Am 19.05.21 um 18:51 schrieb Matthew Brost:
> On Wed, May 19, 2021 at 01:45:39PM +0200, Christian König wrote:
>> Oh, yeah we call that gang submit on the AMD side.
>>
>> Had already some internal discussions how to implement this, but so far
>> couldn't figure out how to cleanly introduce that into the DRM scheduler.
>>
>> Can you briefly describe in a few words how that is supposed to work on the
>> Intel side?
>>
> Sure, I've done a quick PoC internally and have been able to hook this
> into the DRM scheduler.
>
> Basically each BB still maps to a single job as each job is somewhat
> unique (e.g. each job has its own ring, lrc, seqno, etc...). However all
> the jobs configured to run in parallel map to a single sched_entity
> which maintains the order each job was generated from the execbuf IOCTL
> (1 - N). When the backend receives jobs 1 to N - 1 it basically just
> updates some internal state. When the backend sees job N (last job) it
> actually does the submit for jobs 1 - N which with GuC submission is a
> simple command moving the LRC tail of the N jobs.
>
> Daniel has suggested that we create a single job for the NN BBs but that
> would be huge rework to the internals of the i915 and likely won't
> happen by the time this code first lands.
>
> Also worth noting one way a job isn't really a treated individually is
> the excl slot with dma-resv. In that case we create a composite fence of
> all jobs (dma_fence_array).

Yeah, that's something we have discussed as well.

How do you prevent the scheduler from over committing to a single ring 
buffer in this scenario?

Christian.

>
> Matt
>
>> Thanks,
>> Christian.
>>
>> Am 19.05.21 um 01:58 schrieb Matthew Brost:
>>> Add entry fpr i915 new parallel submission uAPI plan.
>>>
>>> v2:
>>>    (Daniel Vetter):
>>>     - Expand logical order explaination
>>>     - Add dummy header
>>>     - Only allow N BBs in execbuf IOCTL
>>>     - Configure parallel submission per slot not per gem context
>>>
>>> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>> Cc: Tony Ye <tony.ye at intel.com>
>>> CC: Carl Zhang <carl.zhang at intel.com>
>>> Cc: Daniel Vetter <daniel.vetter at intel.com>
>>> Cc: Jason Ekstrand <jason at jlekstrand.net>
>>> Signed-off-by: Matthew Brost <matthew.brost at intel.com>
>>> ---
>>>    Documentation/gpu/rfc/i915_parallel_execbuf.h | 144 ++++++++++++++++++
>>>    Documentation/gpu/rfc/i915_scheduler.rst      |  53 ++++++-
>>>    2 files changed, 196 insertions(+), 1 deletion(-)
>>>    create mode 100644 Documentation/gpu/rfc/i915_parallel_execbuf.h
>>>
>>> diff --git a/Documentation/gpu/rfc/i915_parallel_execbuf.h b/Documentation/gpu/rfc/i915_parallel_execbuf.h
>>> new file mode 100644
>>> index 000000000000..8c64b983ccad
>>> --- /dev/null
>>> +++ b/Documentation/gpu/rfc/i915_parallel_execbuf.h
>>> @@ -0,0 +1,144 @@
>>> +#define I915_CONTEXT_ENGINES_EXT_PARALLEL_SUBMIT 2 /* see i915_context_engines_parallel_submit */
>>> +
>>> +/*
>>> + * i915_context_engines_parallel_submit:
>>> + *
>>> + * Setup a slot to allow multiple BBs to be submitted in a single execbuf IOCTL.
>>> + * Those BBs will then be scheduled to run on the GPU in parallel. Multiple
>>> + * hardware contexts are created internally in the i915 run these BBs. Once a
>>> + * slot is configured for N BBs only N BBs can be submitted in each execbuf
>>> + * IOCTL and this is implict behavior (e.g. the user doesn't tell the execbuf
>>> + * IOCTL there are N BBs, the execbuf IOCTL know how many BBs there are based on
>>> + * the slots configuration).
>>> + *
>>> + * Their are two currently defined ways to control the placement of the
>>> + * hardware contexts on physical engines: default behavior (no flags) and
>>> + * I915_PARALLEL_IMPLICT_BONDS (a flag). More flags may be added the in the
>>> + * future as new hardware / use cases arise. Details of how to use this
>>> + * interface below above the flags.
>>> + *
>>> + * Returns -EINVAL if hardware context placement configuration invalid or if the
>>> + * placement configuration isn't supported on the platform / submission
>>> + * interface.
>>> + * Returns -ENODEV if extension isn't supported on the platform / submission
>>> + * inteface.
>>> + */
>>> +struct i915_context_engines_parallel_submit {
>>> +	struct i915_user_extension base;
>>> +
>>> +	__u16 engine_index;	/* slot for parallel engine */
>>> +	__u16 width;		/* number of contexts per parallel engine */
>>> +	__u16 num_siblings;	/* number of siblings per context */
>>> +	__u16 mbz16;
>>> +/*
>>> + * Default placement behvavior (currently unsupported):
>>> + *
>>> + * Rather than restricting parallel submission to a single class with a
>>> + * logically contiguous placement (I915_PARALLEL_IMPLICT_BONDS), add a mode that
>>> + * enables parallel submission across multiple engine classes. In this case each
>>> + * context's logical engine mask indicates where that context can placed. It is
>>> + * implied in this mode that all contexts have mutual exclusive placement (e.g.
>>> + * if one context is running CS0 no other contexts can run on CS0).
>>> + *
>>> + * Example 1 pseudo code:
>>> + * CSX[Y] = engine class X, logical instance Y
>>> + * INVALID = I915_ENGINE_CLASS_INVALID, I915_ENGINE_CLASS_INVALID_NONE
>>> + * set_engines(INVALID)
>>> + * set_parallel(engine_index=0, width=2, num_siblings=2,
>>> + *		engines=CS0[0],CS0[1],CS1[0],CS1[1])
>>> + *
>>> + * Results in the following valid placements:
>>> + * CS0[0], CS1[0]
>>> + * CS0[0], CS1[1]
>>> + * CS0[1], CS1[0]
>>> + * CS0[1], CS1[1]
>>> + *
>>> + * This can also be though of as 2 virtual engines:
>>> + * VE[0] = CS0[0], CS0[1]
>>> + * VE[1] = CS1[0], CS1[1]
>>> + *
>>> + * Example 2 pseudo code:
>>> + * CS[X] = generic engine of same class, logical instance X
>>> + * INVALID = I915_ENGINE_CLASS_INVALID, I915_ENGINE_CLASS_INVALID_NONE
>>> + * set_engines(INVALID)
>>> + * set_parallel(engine_index=0, width=2, num_siblings=3,
>>> + *		engines=CS[0],CS[1],CS[2],CS[0],CS[1],CS[2])
>>> + *
>>> + * Results in the following valid placements:
>>> + * CS[0], CS[1]
>>> + * CS[0], CS[2]
>>> + * CS[1], CS[0]
>>> + * CS[1], CS[2]
>>> + * CS[2], CS[0]
>>> + * CS[2], CS[1]
>>> + *
>>> + *
>>> + * This can also be though of as 2 virtual engines:
>>> + * VE[0] = CS[0], CS[1], CS[2]
>>> + * VE[1] = CS[0], CS[1], CS[2]
>>> +
>>> + * This enables a use case where all engines are created equally, we don't care
>>> + * where they are scheduled, we just want a certain number of resources, for
>>> + * those resources to be scheduled in parallel, and possibly across multiple
>>> + * engine classes.
>>> + */
>>> +
>>> +/*
>>> + * I915_PARALLEL_IMPLICT_BONDS - Create implict bonds between each context.
>>> + * Each context must have the same number sibling and bonds are implictly create
>>> + * of the siblings.
>>> + *
>>> + * All of the below examples are in logical space.
>>> + *
>>> + * Example 1 pseudo code:
>>> + * CS[X] = generic engine of same class, logical instance X
>>> + * INVALID = I915_ENGINE_CLASS_INVALID, I915_ENGINE_CLASS_INVALID_NONE
>>> + * set_engines(INVALID)
>>> + * set_parallel(engine_index=0, width=2, num_siblings=1,
>>> + *		engines=CS[0],CS[1], flags=I915_PARALLEL_IMPLICT_BONDS)
>>> + *
>>> + * Results in the following valid placements:
>>> + * CS[0], CS[1]
>>> + *
>>> + * Example 2 pseudo code:
>>> + * CS[X] = generic engine of same class, logical instance X
>>> + * INVALID = I915_ENGINE_CLASS_INVALID, I915_ENGINE_CLASS_INVALID_NONE
>>> + * set_engines(INVALID)
>>> + * set_parallel(engine_index=0, width=2, num_siblings=2,
>>> + *		engines=CS[0],CS[2],CS[1],CS[3], flags=I915_PARALLEL_IMPLICT_BONDS)
>>> + *
>>> + * Results in the following valid placements:
>>> + * CS[0], CS[1]
>>> + * CS[2], CS[3]
>>> + *
>>> + * This can also be though of as 2 virtual engines:
>>> + * VE[0] = CS[0], CS[2]
>>> + * VE[1] = CS[1], CS[3]
>>> + *
>>> + * This enables a use case where all engines are not equal and certain placement
>>> + * rules are required (i.e. split-frame requires all contexts to be placed in a
>>> + * logically contiguous order on the VCS engines on gen11+ platforms). This use
>>> + * case (logically contiguous placement, within a single engine class) is
>>> + * supported when using GuC submission. Execlist mode could support all possible
>>> + * bonding configurations but currently doesn't support this extension.
>>> + */
>>> +#define I915_PARALLEL_IMPLICT_BONDS			(1<<0)
>>> +/*
>>> + * Do not allow BBs to be preempted mid BB rather insert coordinated preemption
>>> + * points on all hardware contexts between each set of BBs. An example use case
>>> + * of this feature is split-frame on gen11+ hardware. When using this feature a
>>> + * BB must be submitted on each hardware context in the parallel gem context.
>>> + * The execbuf2 IOCTL enforces the user adheres to policy.
>>> + */
>>> +#define I915_PARALLEL_NO_PREEMPT_MID_BATCH		(1<<1)
>>> +#define __I915_PARALLEL_UNKNOWN_FLAGS	(-(I915_PARALLEL_NO_PREEMPT_MID_BATCH << 1))
>>> +	__u64 flags;		/* all undefined flags must be zero */
>>> +	__u64 mbz64[3];		/* reserved for future use; must be zero */
>>> +
>>> +	/*
>>> +	 * width (i) * num_siblings (j) in length
>>> +	 * index = j + i * num_siblings
>>> +	 */
>>> +	struct i915_engine_class_instance engines[0];
>>> +} __attribute__ ((packed));
>>> +
>>> diff --git a/Documentation/gpu/rfc/i915_scheduler.rst b/Documentation/gpu/rfc/i915_scheduler.rst
>>> index 7faa46cde088..64c539486ee4 100644
>>> --- a/Documentation/gpu/rfc/i915_scheduler.rst
>>> +++ b/Documentation/gpu/rfc/i915_scheduler.rst
>>> @@ -82,4 +82,55 @@ https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fspec.oneapi.com%2Flevel-zero%2Flatest%2Fcore%2Fapi.html%23ze-command-queue-priorit&data=04%7C01%7Cchristian.koenig%40amd.com%7C49a7557f4e494090755608d91ae758a6%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637570403202969375%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=dn3PVdxYQpkpWIru5eAXHgbfuLDkppAA5daV5sHQF7s%3D&reserved=0
>>>    New parallel submission uAPI
>>>    ============================
>>> -Details to come in a following patch.
>>> +The existing bonding uAPI is completely broken with GuC submission because
>>> +whether a submission is a single context submit or parallel submit isn't known
>>> +until execbuf time activated via the I915_SUBMIT_FENCE. To submit multiple
>>> +contexts in parallel with the GuC the context must be explictly registered with
>>> +N contexts and all N contexts must be submitted in a single command to the GuC.
>>> +These interfaces doesn't support dynamically changing between N contexts as the
>>> +bonding uAPI does. Hence the need for a new parallel submission interface. Also
>>> +the legacy bonding uAPI is quite confusing and not intuitive at all.
>>> +
>>> +The new parallel submission uAPI consists of 3 parts:
>>> +
>>> +* Export engines logical mapping
>>> +* A 'set_parallel' extension to configure contexts for parallel
>>> +  submission
>>> +* Extend execbuf2 IOCTL to support submitting N BBs in a single IOCTL
>>> +
>>> +Export engines logical mapping
>>> +------------------------------
>>> +Certain use cases require BBs to be placed on engine instances in logical order
>>> +(e.g. split-frame on gen11+). The logical mapping of engine instances can change
>>> +based on fusing. Rather than making UMDs be aware of fusing, simply expose the
>>> +logical mapping with the existing query engine info IOCTL. Also the GuC
>>> +submission interface currently only supports submitting multiple contexts to
>>> +engines in logical order which is a new requirement compared to execlists.
>>> +Lastly, all current platforms have at most 2 instances and the logical order is
>>> +the same a uABI order. This will change on platforms with more than 2 instances.
>>> +
>>> +A single bit will be added to drm_i915_engine_info.flags indicating that the
>>> +logical instance has been returned and a new field,
>>> +drm_i915_engine_info.logical_instance, returns the logical instance.
>>> +
>>> +A 'set_parallel' extension to configure contexts for parallel submission
>>> +------------------------------------------------------------------------
>>> +The 'set_parallel' extension configures a slot for parallel submission of N BBs.
>>> +It is setup step that should be called before using any of the contexts. See
>>> +I915_CONTEXT_ENGINES_EXT_LOAD_BALANCE or I915_CONTEXT_ENGINES_EXT_BOND for
>>> +similar existing examples. Once a slot is configured for parallel submission the
>>> +execbuf2 IOCTL can be called submiting N BBs in a single IOCTL. Initially only
>>> +support GuC submission. Execlist support can be added later if needed.
>>> +
>>> +Add I915_CONTEXT_ENGINES_EXT_PARALLEL_SUBMIT and
>>> +i915_context_engines_parallel_submit to the uAPI to implement this extension.
>>> +
>>> +Extend execbuf2 IOCTL to support submitting N BBs in a single IOCTL
>>> +-------------------------------------------------------------------
>>> +Contexts that have been configured with the 'set_parallel' extension are allowed
>>> +to submit N BBs in a single execbuf2 IOCTL. The BBs are either the last N
>>> +objects in the drm_i915_gem_exec_object2 list or the first N if
>>> +I915_EXEC_BATCH_FIRST is set. The number of BBs is implict based on the slot
>>> +submitted and how it has been configured by 'set_parallel' or other extensions.
>>> +No uAPI changes in the execbuf IOCTL but worth mentioning the new behavior of
>>> +the IOCTL.