Re: [PATCH 1/9] drm: execution context for GEM buffers v3

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On 2023-02-28 03:33, Christian König wrote:
> This adds the infrastructure for an execution context for GEM buffers
> which is similar to the existinc TTMs execbuf util and intended to replace
> it in the long term.
> 
> The basic functionality is that we abstracts the necessary loop to lock
> many different GEM buffers with automated deadlock and duplicate handling.
> 
> v2: drop xarray and use dynamic resized array instead, the locking
>     overhead is unecessary and measureable.
> v3: drop duplicate tracking, radeon is really the only one needing that.
> 
> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> ---
>  Documentation/gpu/drm-mm.rst |  12 ++
>  drivers/gpu/drm/Kconfig      |   6 +
>  drivers/gpu/drm/Makefile     |   2 +
>  drivers/gpu/drm/drm_exec.c   | 249 +++++++++++++++++++++++++++++++++++
>  include/drm/drm_exec.h       | 115 ++++++++++++++++
>  5 files changed, 384 insertions(+)
>  create mode 100644 drivers/gpu/drm/drm_exec.c
>  create mode 100644 include/drm/drm_exec.h
> 
> diff --git a/Documentation/gpu/drm-mm.rst b/Documentation/gpu/drm-mm.rst
> index a79fd3549ff8..a52e6f4117d6 100644
> --- a/Documentation/gpu/drm-mm.rst
> +++ b/Documentation/gpu/drm-mm.rst
> @@ -493,6 +493,18 @@ DRM Sync Objects
>  .. kernel-doc:: drivers/gpu/drm/drm_syncobj.c
>     :export:
>  
> +DRM Execution context
> +=====================
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_exec.c
> +   :doc: Overview
> +
> +.. kernel-doc:: include/drm/drm_exec.h
> +   :internal:
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_exec.c
> +   :export:
> +
>  GPU Scheduler
>  =============
>  
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index 17d252dc25e2..84a5fc28c48d 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -200,6 +200,12 @@ config DRM_TTM
>  	  GPU memory types. Will be enabled automatically if a device driver
>  	  uses it.
>  
> +config DRM_EXEC
> +	tristate
> +	depends on DRM
> +	help
> +	  Execution context for command submissions
> +
>  config DRM_BUDDY
>  	tristate
>  	depends on DRM
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index ab4460fcd63f..d40defbb0347 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -78,6 +78,8 @@ obj-$(CONFIG_DRM_PANEL_ORIENTATION_QUIRKS) += drm_panel_orientation_quirks.o
>  #
>  # Memory-management helpers
>  #
> +#
> +obj-$(CONFIG_DRM_EXEC) += drm_exec.o
>  
>  obj-$(CONFIG_DRM_BUDDY) += drm_buddy.o
>  
> diff --git a/drivers/gpu/drm/drm_exec.c b/drivers/gpu/drm/drm_exec.c
> new file mode 100644
> index 000000000000..df546cc5a227
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_exec.c
> @@ -0,0 +1,249 @@
> +/* SPDX-License-Identifier: GPL-2.0 OR MIT */
> +
> +#include <drm/drm_exec.h>
> +#include <drm/drm_gem.h>
> +#include <linux/dma-resv.h>
> +
> +/**
> + * DOC: Overview
> + *
> + * This component mainly abstracts the retry loop necessary for locking
> + * multiple GEM objects while preparing hardware operations (e.g. command
> + * submissions, page table updates etc..).
> + *
> + * If a contention is detected while locking a GEM object the cleanup procedure
> + * unlocks all previously locked GEM objects and locks the contended one first
> + * before locking any further objects.
> + *
> + * After an object is locked fences slots can optionally be reserved on the
> + * dma_resv object inside the GEM object.
> + *
> + * A typical usage pattern should look like this::
> + *
> + *	struct drm_gem_object *obj;
> + *	struct drm_exec exec;
> + *	unsigned long index;
> + *	int ret;
> + *
> + *	drm_exec_init(&exec, true);
> + *	drm_exec_while_not_all_locked(&exec) {
> + *		ret = drm_exec_prepare_obj(&exec, boA, 1);
> + *		drm_exec_continue_on_contention(&exec);
> + *		if (ret)
> + *			goto error;
> + *
> + *		ret = drm_exec_lock(&exec, boB, 1);
> + *		drm_exec_continue_on_contention(&exec);
> + *		if (ret)
> + *			goto error;
> + *	}
> + *
> + *	drm_exec_for_each_locked_object(&exec, index, obj) {
> + *		dma_resv_add_fence(obj->resv, fence, DMA_RESV_USAGE_READ);
> + *		...
> + *	}
> + *	drm_exec_fini(&exec);

Maybe add the error: label here to show how recovery is to be had.

> + *
> + * See struct dma_exec for more details.
> + */
> +
> +/* Dummy value used to initially enter the retry loop */
> +#define DRM_EXEC_DUMMY (void*)~0
> +
> +/* Unlock all objects and drop references */
> +static void drm_exec_unlock_all(struct drm_exec *exec)
> +{
> +	struct drm_gem_object *obj;
> +	unsigned long index;
> +
> +	drm_exec_for_each_locked_object(exec, index, obj) {
> +		dma_resv_unlock(obj->resv);
> +		drm_gem_object_put(obj);
> +	}
> +
> +	if (exec->prelocked) {
> +		dma_resv_unlock(exec->prelocked->resv);
> +		drm_gem_object_put(exec->prelocked);
> +		exec->prelocked = NULL;
> +	}
> +}
> +
> +/**
> + * drm_exec_init - initialize a drm_exec object
> + * @exec: the drm_exec object to initialize
> + * @interruptible: if locks should be acquired interruptible
> + *
> + * Initialize the object and make sure that we can track locked and duplicate
> + * objects.
> + */
> +void drm_exec_init(struct drm_exec *exec, bool interruptible)
> +{
> +	exec->interruptible = interruptible;
> +	exec->objects = kmalloc(PAGE_SIZE, GFP_KERNEL);
> +
> +	/* If allocation here fails, just delay that till the first use */
> +	exec->max_objects = exec->objects ? PAGE_SIZE / sizeof(void *) : 0;

I didn't see a subsequent allocation for objects. Does it make
sense to continue here if we weren't able to allocate? It seems
that the "first use" is most likely immediately after drm_exec_init(),
and if kmalloc() failed, perhaps things are only about to get worse.

(Pet peeve :-) "till" --> "until".)

> +	exec->num_objects = 0;
> +	exec->contended = DRM_EXEC_DUMMY;
> +	exec->prelocked = NULL;
> +}
> +EXPORT_SYMBOL(drm_exec_init);
> +
> +/**
> + * drm_exec_fini - finalize a drm_exec object
> + * @exec: the drm_exec object to finilize
> + *
> + * Unlock all locked objects, drop the references to objects and free all memory
> + * used for tracking the state.
> + */
> +void drm_exec_fini(struct drm_exec *exec)
> +{
> +	drm_exec_unlock_all(exec);
> +	kvfree(exec->objects);
> +	if (exec->contended != DRM_EXEC_DUMMY) {
> +		drm_gem_object_put(exec->contended);
> +		ww_acquire_fini(&exec->ticket);
> +	}
> +}
> +EXPORT_SYMBOL(drm_exec_fini);
> +
> +/**
> + * drm_exec_cleanup - cleanup when contention is detected
> + * @exec: the drm_exec object to cleanup
> + *
> + * Cleanup the current state and return true if we should stay inside the retry
> + * loop, false if there wasn't any contention detected and we can keep the
> + * objects locked.
> + */
> +bool drm_exec_cleanup(struct drm_exec *exec)
> +{
> +	if (likely(!exec->contended)) {
> +		ww_acquire_done(&exec->ticket);
> +		return false;
> +	}
> +
> +	if (likely(exec->contended == DRM_EXEC_DUMMY)) {
> +		exec->contended = NULL;
> +		ww_acquire_init(&exec->ticket, &reservation_ww_class);
> +		return true;
> +	}
> +
> +	drm_exec_unlock_all(exec);
> +	exec->num_objects = 0;
> +	return true;
> +}
> +EXPORT_SYMBOL(drm_exec_cleanup);
> +
> +/* Track the locked object in the xa and reserve fences */
> +static int drm_exec_obj_locked(struct drm_exec *exec,
> +			       struct drm_gem_object *obj)
> +{
> +	if (unlikely(exec->num_objects == exec->max_objects)) {
> +		size_t size = exec->max_objects * sizeof(void *);
> +		void *tmp;
> +
> +		tmp = kvrealloc(exec->objects, size, size + PAGE_SIZE,
> +				GFP_KERNEL);
> +		if (!tmp)
> +			return -ENOMEM;
> +
> +		exec->objects = tmp;
> +		exec->max_objects += PAGE_SIZE / sizeof(void *);
> +	}
> +	drm_gem_object_get(obj);
> +	exec->objects[exec->num_objects++] = obj;
> +
> +	return 0;
> +}
> +
> +/* Make sure the contended object is locked first */
> +static int drm_exec_lock_contended(struct drm_exec *exec)
> +{
> +	struct drm_gem_object *obj = exec->contended;
> +	int ret;
> +
> +	if (likely(!obj))
> +		return 0;
> +
> +	if (exec->interruptible) {
> +		ret = dma_resv_lock_slow_interruptible(obj->resv,
> +						       &exec->ticket);
> +		if (unlikely(ret))
> +			goto error_dropref;
> +	} else {
> +		dma_resv_lock_slow(obj->resv, &exec->ticket);
> +	}
> +
> +	ret = drm_exec_obj_locked(exec, obj);
> +	if (unlikely(ret)) {
> +		dma_resv_unlock(obj->resv);
> +		goto error_dropref;
> +	}
> +
> +	swap(exec->prelocked, obj);
> +
> +error_dropref:
> +	/* Always cleanup the contention so that error handling can kick in */
> +	drm_gem_object_put(obj);
> +	exec->contended = NULL;
> +	return ret;
> +}
> +
> +/**
> + * drm_exec_prepare_obj - prepare a GEM object for use
> + * @exec: the drm_exec object with the state
> + * @obj: the GEM object to prepare
> + * @num_fences: how many fences to reserve
> + *
> + * Prepare a GEM object for use by locking it and reserving fence slots. All
> + * succesfully locked objects are put into the locked container. Duplicates
> + * detected as well and automatically moved into the duplicates container.
> + *
> + * Returns: -EDEADLK if a contention is detected, -ENOMEM when memory
> + * allocation failed and zero for success.
> + */
> +int drm_exec_prepare_obj(struct drm_exec *exec, struct drm_gem_object *obj,
> +			 unsigned int num_fences)
> +{
> +	int ret;
> +
> +	ret = drm_exec_lock_contended(exec);
> +	if (unlikely(ret))
> +		return ret;
> +
> +	if (exec->prelocked == obj) {
> +		drm_gem_object_put(exec->prelocked);
> +		exec->prelocked = NULL;
> +
> +		return dma_resv_reserve_fences(obj->resv, num_fences);
> +	}
> +
> +	if (exec->interruptible)
> +		ret = dma_resv_lock_interruptible(obj->resv, &exec->ticket);
> +	else
> +		ret = dma_resv_lock(obj->resv, &exec->ticket);
> +
> +	if (unlikely(ret == -EDEADLK)) {
> +		drm_gem_object_get(obj);
> +		exec->contended = obj;
> +		return -EDEADLK;
> +	}
> +
> +	if (unlikely(ret))
> +		return ret;
> +
> +	ret = drm_exec_obj_locked(exec, obj);
> +	if (ret)
> +		goto error_unlock;
> +
> +	/* Keep locked when reserving fences fails */
> +	return dma_resv_reserve_fences(obj->resv, num_fences);
> +
> +error_unlock:
> +	dma_resv_unlock(obj->resv);
> +	return ret;
> +}
> +EXPORT_SYMBOL(drm_exec_prepare_obj);
> +
> +MODULE_DESCRIPTION("DRM execution context");
> +MODULE_LICENSE("Dual MIT/GPL");
> diff --git a/include/drm/drm_exec.h b/include/drm/drm_exec.h
> new file mode 100644
> index 000000000000..65e518c01db3
> --- /dev/null
> +++ b/include/drm/drm_exec.h
> @@ -0,0 +1,115 @@
> +/* SPDX-License-Identifier: GPL-2.0 OR MIT */
> +
> +#ifndef __DRM_EXEC_H__
> +#define __DRM_EXEC_H__
> +
> +#include <linux/ww_mutex.h>
> +
> +struct drm_gem_object;
> +
> +/**
> + * struct drm_exec - Execution context
> + */
> +struct drm_exec {
> +	/**
> +	 * @interruptible: If locks should be taken interruptible
> +	 */
> +	bool			interruptible;
> +
> +	/**
> +	 * @ticket: WW ticket used for acquiring locks
> +	 */
> +	struct ww_acquire_ctx	ticket;
> +
> +	/**
> +	 * @num_objects: number of objects locked
> +	 */
> +	unsigned int		num_objects;

"num_objects" may be confused with the size of "objects" array.
Maybe "num_locked" would be clearer?

> +
> +	/**
> +	 * @max_objects: maximum objects in array
> +	 */
> +	unsigned int		max_objects;
> +
> +	/**
> +	 * @objects: array of the locked objects
> +	 */
> +	struct drm_gem_object	**objects;
> +
> +	/**
> +	 * @contended: contended GEM object we backet of for
> +	 */
> +	struct drm_gem_object	*contended;

Perhaps "backed off for" in the comment?

> +
> +	/**
> +	 * @prelocked: already locked GEM object because of contention
> +	 */
> +	struct drm_gem_object *prelocked;

Could this be a subset of the objects array as opposed to a single object?
-- 
Regards,
Luben




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