On Wed, May 04, 2022 at 09:47:32AM +0200, 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.
>
> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> ---
> Documentation/gpu/drm-mm.rst | 12 ++
> drivers/gpu/drm/Kconfig | 7 +
> drivers/gpu/drm/Makefile | 2 +
> drivers/gpu/drm/drm_exec.c | 295 +++++++++++++++++++++++++++++++++++
> include/drm/drm_exec.h | 144 +++++++++++++++++
> 5 files changed, 460 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 f32ccce5722d..bf7dd2a78e9b 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 e88c497fa010..1b35c10df263 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -179,6 +179,12 @@ 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
> @@ -252,6 +258,7 @@ config DRM_AMDGPU
> select DRM_SCHED
> select DRM_TTM
> select DRM_TTM_HELPER
> + select DRM_EXEC
> select POWER_SUPPLY
> select HWMON
> select BACKLIGHT_CLASS_DEVICE
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index 15fe3163f822..ee8573b683f3 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -37,6 +37,8 @@ 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..ed2106c22786
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_exec.c
> @@ -0,0 +1,295 @@
> +/* 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);
> + *
> + * See struct dma_exec for more details.
> + */
> +
> +/* Dummy value used to initially enter the retry loop */
> +#define DRM_EXEC_DUMMY (void*)~0
> +
> +/* Initialize the drm_exec_objects container */
> +static void drm_exec_objects_init(struct drm_exec_objects *container)
> +{
> + container->objects = kmalloc(PAGE_SIZE, GFP_KERNEL);
> +
> + /* If allocation here fails, just delay that till the first use */
> + container->max_objects = container->objects ?
> + PAGE_SIZE / sizeof(void *) : 0;
> + container->num_objects = 0;
> +}
> +
> +/* Cleanup the drm_exec_objects container */
> +static void drm_exec_objects_fini(struct drm_exec_objects *container)
> +{
> + kvfree(container->objects);
> +}
> +
> +/* Make sure we have enough room and add an object the container */
> +static int drm_exec_objects_add(struct drm_exec_objects *container,
> + struct drm_gem_object *obj)
> +{
> + if (unlikely(container->num_objects == container->max_objects)) {
> + size_t size = container->max_objects * sizeof(void *);
> + void *tmp;
> +
> + tmp = kvrealloc(container->objects, size, size + PAGE_SIZE,
> + GFP_KERNEL);
> + if (!tmp)
> + return -ENOMEM;
> +
> + container->objects = tmp;
> + container->max_objects += PAGE_SIZE / sizeof(void *);
Might be worth it to inquire the actual allocation size here, since if
it's kmalloc the generic buckets only cover doubling of sizes, so once
it's big it goes up a lot quicker than PAGE_SIZE.
But also krealloc checks this internally already so maybe better to not
break the abstraction.
> + }
> + drm_gem_object_get(obj);
> + container->objects[container->num_objects++] = obj;
> + return 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_duplicate_object(exec, index, obj)
> + drm_gem_object_put(obj);
> +
> + drm_exec_for_each_locked_object(exec, index, obj) {
> + dma_resv_unlock(obj->resv);
> + drm_gem_object_put(obj);
> + }
> +}
> +
> +/**
> + * 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;
> + drm_exec_objects_init(&exec->locked);
> + drm_exec_objects_init(&exec->duplicates);
> + exec->contended = DRM_EXEC_DUMMY;
> +}
> +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);
> + drm_exec_objects_fini(&exec->locked);
> + drm_exec_objects_fini(&exec->duplicates);
> + 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);
Not sure why this is here instead of in _init()? I thought you're playing
some really dangerous tricks with re-initting the acquire ctx, which would
at least be questionable, but does not look like that.
> + return true;
> + }
> +
> + drm_exec_unlock_all(exec);
> + exec->locked.num_objects = 0;
> + exec->duplicates.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_objects *container,
> + struct drm_gem_object *obj,
> + unsigned int num_fences)
> +{
> + int ret;
> +
> + if (container) {
> + ret = drm_exec_objects_add(container, obj);
> + if (ret)
> + return ret;
> + }
> +
> + if (num_fences) {
> + ret = dma_resv_reserve_fences(obj->resv, num_fences);
> + if (ret)
> + goto error_erase;
> + }
> +
> + return 0;
> +
> +error_erase:
> + if (container) {
> + --container->num_objects;
> + drm_gem_object_put(obj);
> + }
> + return ret;
> +}
> +
> +/* 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->locked, obj, 0);
> + if (unlikely(ret))
> + dma_resv_unlock(obj->resv);
> +
> +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->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 == -EALREADY)) {
> + struct drm_exec_objects *container = &exec->duplicates;
> +
> + /*
> + * If this is the first locked GEM object it was most likely
> + * just contended. So don't add it to the duplicates, just
> + * reserve the fence slots.
> + */
> + if (exec->locked.num_objects && exec->locked.objects[0] == obj)
> + container = NULL;
> +
> + ret = drm_exec_obj_locked(container, obj, num_fences);
> + if (ret)
> + return ret;
> +
> + } else if (unlikely(ret)) {
> + return ret;
> +
> + } else {
> + ret = drm_exec_obj_locked(&exec->locked, obj, num_fences);
> + if (ret)
> + goto error_unlock;
> + }
> +
> + drm_gem_object_get(obj);
> + return 0;
> +
> +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..f73981c6292e
> --- /dev/null
> +++ b/include/drm/drm_exec.h
> @@ -0,0 +1,144 @@
> +/* 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_objects - Container for GEM objects in a drm_exec
> + */
> +struct drm_exec_objects {
> + unsigned int num_objects;
> + unsigned int max_objects;
> + struct drm_gem_object **objects;
> +};
> +
> +/**
> + * drm_exec_objects_for_each - iterate over all the objects inside the container
> + */
> +#define drm_exec_objects_for_each(array, index, obj) \
> + for (index = 0, obj = (array)->objects[0]; \
> + index < (array)->num_objects; \
> + ++index, obj = (array)->objects[index])
> +
> +/**
> + * 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;
> +
> + /**
> + * @locked: container for the locked GEM objects
> + */
> + struct drm_exec_objects locked;
> +
> + /**
> + * @duplicates: container for the duplicated GEM objects
> + */
> + struct drm_exec_objects duplicates;
> +
> + /**
> + * @contended: contended GEM object we backet of for.
> + */
> + struct drm_gem_object *contended;
> +};
> +
> +/**
> + * drm_exec_for_each_locked_object - iterate over all the locked objects
> + * @exec: drm_exec object
> + * @index: unsigned long index for the iteration
> + * @obj: the current GEM object
> + *
> + * Iterate over all the locked GEM objects inside the drm_exec object.
> + */
> +#define drm_exec_for_each_locked_object(exec, index, obj) \
> + drm_exec_objects_for_each(&(exec)->locked, index, obj)
> +
> +/**
> + * drm_exec_for_each_duplicate_object - iterate over all the duplicate objects
> + * @exec: drm_exec object
> + * @index: unsigned long index for the iteration
> + * @obj: the current GEM object
> + *
> + * Iterate over all the duplicate GEM objects inside the drm_exec object.
> + */
> +#define drm_exec_for_each_duplicate_object(exec, index, obj) \
> + drm_exec_objects_for_each(&(exec)->duplicates, index, obj)
> +
> +/**
> + * drm_exec_while_not_all_locked - loop until all GEM objects are prepared
> + * @exec: drm_exec object
> + *
> + * Core functionality of the drm_exec object. Loops until all GEM objects are
> + * prepared and no more contention exists.
> + *
> + * At the beginning of the loop it is guaranteed that no GEM object is locked.
> + */
> +#define drm_exec_while_not_all_locked(exec) \
> + while (drm_exec_cleanup(exec))
> +
> +/**
> + * drm_exec_continue_on_contention - continue the loop when we need to cleanup
> + * @exec: drm_exec object
> + *
> + * Control flow helper to continue when a contention was detected and we need to
> + * clean up and re-start the loop to prepare all GEM objects.
> + */
> +#define drm_exec_continue_on_contention(exec) \
> + if (unlikely(drm_exec_is_contended(exec))) \
> + continue
> +
> +/**
> + * drm_exec_break_on_contention - break a subordinal loop on contention
> + * @exec: drm_exec object
> + *
> + * Control flow helper to break a subordinal loop when a contention was detected
> + * and we need to clean up and re-start the loop to prepare all GEM objects.
> + */
> +#define drm_exec_break_on_contention(exec) \
> + if (unlikely(drm_exec_is_contended(exec))) \
> + break
> +
> +/**
> + * drm_exec_is_contended - check for contention
> + * @exec: drm_exec object
> + *
> + * Returns true if the drm_exec object has run into some contention while
> + * locking a GEM object and needs to clean up.
> + */
> +static inline bool drm_exec_is_contended(struct drm_exec *exec)
> +{
> + return !!exec->contended;
> +}
> +
> +/**
> + * drm_exec_has_duplicates - check for duplicated GEM object
> + * @exec: drm_exec object
> + *
> + * Return true if the drm_exec object has encountered some already locked GEM
> + * objects while trying to lock them. This can happen if multiple GEM objects
> + * share the same underlying resv object.
> + */
> +static inline bool drm_exec_has_duplicates(struct drm_exec *exec)
> +{
> + return exec->duplicates.num_objects > 0;
Definitely an aside, but in our i915 efforts to get rid of temporary pins
we run into some fun where the eviction code couldn't differentiate from
memory we need reserved for the CS and memory we just keep locked because
we evicted it and fun stuff like that. So maybe we need a bit more
tracking here eventually, but that's only when we have this somehow glued
into ttm eviction code.
Also the even more massive step would be to glue this into dma-buf so you
can do dynamic dma-buf eviction and still keep track of all the buffers. I
think with some clever pointer tagging and a bit more indirection we could
nest drm_exec structures (so that a driver could insert it's entire
drm_exec structure with a drm_exec-level callback for handling refcounting
and stuff like that).
So anyway I think this all looks good, just one more thing before I think
we should land this:
gem helpers in drm_gem_lock_reservations() has something which is
practically compatible already, except that you bulk-add the entire set of
objects. I think if you add a bulk-prepare function then we could also
replace all those. Maybe even nicer if the bulk-prepare takes the array of
handles and does the handle lookup too, but at least something which can
subsititue drm_gem_lock_reservations with drm_exec would be nice to
validate the helpers a bit more and really make sure we only have one of
them left.
Thoughts?
-Daniel
> +}
> +
> +void drm_exec_init(struct drm_exec *exec, bool interruptible);
> +void drm_exec_fini(struct drm_exec *exec);
> +bool drm_exec_cleanup(struct drm_exec *exec);
> +int drm_exec_prepare_obj(struct drm_exec *exec, struct drm_gem_object *obj,
> + unsigned int num_fences);
> +
> +#endif
> --
> 2.25.1
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
