Re: [RFC PATCH v3 16/21] drm/exec: Introduce an evict mode

["Christian König" <christian.koenig@amd.com>]

Am 21.05.24 um 09:16 schrieb Thomas Hellström:
> Locking for eviction is in some way different from locking for
> submission:
>
> 1) We can't lock objects that are already locked for submission,
> hence DRM_EXEC_IGNORE_DUPLICATES must be unset.
> 2) We must be able to re-lock objects locked for eviction,
> either for submission or for yet another eviction, in
> particular objects sharing a single resv must be considered.

Yeah, I was already thinking about that as well.

My idea so far was to have a separate function for locking eviction BOs. 
This function would then use trylock or blocking depending on some setting.

> 3) There is no point to keep a contending object after the
> transaction restart. We don't know whether we actually want to use
> it again.

Well that isn't true as far as I know.

If we don't use trylock we still need to lock the object after rollback 
to make sure that we waited for it to become available.

Regards,
Christian.

> So introduce a drm_exec evict mode, and for now instead of
> explicitly setting it using a function call or implement separate
> locking functions that use evict mode, assume evict mode if
> there is a snapshot registered. This can easily be changed later.
>
> To keep track of resvs locked for eviction, use a pointer set
> implemented by an xarray. This is probably not the most efficient
> data structure but used as an easy-to-implement first approach.
> If the set is empty (evict mode never used), the performance-
> and memory usage impact will be very small.
>
> TODO: Probably want to implement the set using an open addressing
> hash table.
>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Somalapuram Amaranath <Amaranath.Somalapuram@amd.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: <dri-devel@lists.freedesktop.org>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>   drivers/gpu/drm/drm_exec.c | 77 ++++++++++++++++++++++++++++++++++----
>   include/drm/drm_exec.h     | 15 ++++++++
>   2 files changed, 85 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_exec.c b/drivers/gpu/drm/drm_exec.c
> index 9eea5d0d3a98..ea79d96f5439 100644
> --- a/drivers/gpu/drm/drm_exec.c
> +++ b/drivers/gpu/drm/drm_exec.c
> @@ -65,6 +65,10 @@ static void drm_exec_unlock_all(struct drm_exec *exec)
>   
>   	drm_gem_object_put(exec->prelocked);
>   	exec->prelocked = NULL;
> +
> +	/* garbage collect */
> +	xa_destroy(&exec->resv_set);
> +	xa_init(&exec->resv_set);
>   }
>   
>   /**
> @@ -92,6 +96,8 @@ void drm_exec_init(struct drm_exec *exec, u32 flags, unsigned nr)
>   	exec->contended = DRM_EXEC_DUMMY;
>   	exec->prelocked = NULL;
>   	exec->snap = NULL;
> +	exec->drop_contended = false;
> +	xa_init(&exec->resv_set);
>   }
>   EXPORT_SYMBOL(drm_exec_init);
>   
> @@ -110,6 +116,7 @@ void drm_exec_fini(struct drm_exec *exec)
>   		drm_gem_object_put(exec->contended);
>   		ww_acquire_fini(&exec->ticket);
>   	}
> +	xa_destroy(&exec->resv_set);
>   }
>   EXPORT_SYMBOL(drm_exec_fini);
>   
> @@ -139,6 +146,30 @@ bool drm_exec_cleanup(struct drm_exec *exec)
>   }
>   EXPORT_SYMBOL(drm_exec_cleanup);
>   
> +static unsigned long drm_exec_resv_to_key(const struct dma_resv *resv)
> +{
> +	return (unsigned long)resv / __alignof__(typeof(*resv));
> +}
> +
> +static void
> +drm_exec_resv_set_erase(struct drm_exec *exec, unsigned long key)
> +{
> +	if (xa_load(&exec->resv_set, key))
> +		xa_erase(&exec->resv_set, key);
> +}
> +
> +static bool drm_exec_in_evict_mode(struct drm_exec *exec)
> +{
> +	return !!exec->snap;
> +}
> +
> +static void drm_exec_set_evict_mode(struct drm_exec *exec,
> +				    struct drm_exec_snapshot *snap)
> +{
> +	exec->snap = snap;
> +	exec->flags &= ~DRM_EXEC_IGNORE_DUPLICATES;
> +}
> +
>   /* Track the locked object in the array */
>   static int drm_exec_obj_locked(struct drm_exec *exec,
>   			       struct drm_gem_object *obj,
> @@ -161,6 +192,14 @@ static int drm_exec_obj_locked(struct drm_exec *exec,
>   	drm_gem_object_get(obj);
>   	exec->objects[exec->num_objects++] = obj;
>   
> +	/*
> +	 * Errors here are not fatal, It means the object we locked
> +	 * for eviction can't be locked again. If that is problematic
> +	 * we may need to reconsider this.
> +	 */
> +	if (drm_exec_in_evict_mode(exec))
> +		(void)xa_store(&exec->resv_set, drm_exec_resv_to_key(obj->resv),
> +			       obj->resv, gfp | __GFP_NOWARN);
>   	return 0;
>   }
>   
> @@ -184,6 +223,9 @@ static int drm_exec_lock_contended(struct drm_exec *exec)
>   		dma_resv_lock_slow(obj->resv, &exec->ticket);
>   	}
>   
> +	if (exec->drop_contended)
> +		goto error_unlock;
> +
>   	ret = drm_exec_obj_locked(exec, obj, GFP_KERNEL);
>   	if (unlikely(ret))
>   		goto error_unlock;
> @@ -245,10 +287,19 @@ int drm_exec_trylock_obj(struct drm_exec *exec, struct drm_gem_object *obj)
>   	}
>   
>   	if (!dma_resv_trylock_ctx(obj->resv, &exec->ticket)) {
> -		if (dma_resv_locking_ctx(obj->resv) == &exec->ticket)
> -			return (exec->flags & DRM_EXEC_IGNORE_DUPLICATES) ? 0 : -EALREADY;
> -		else
> +		if (dma_resv_locking_ctx(obj->resv) == &exec->ticket) {
> +			unsigned long key = drm_exec_resv_to_key(obj->resv);
> +
> +			if (exec->flags & DRM_EXEC_IGNORE_DUPLICATES ||
> +			    xa_load(&exec->resv_set, key)) {
> +				if (!drm_exec_in_evict_mode(exec))
> +					drm_exec_resv_set_erase(exec, key);
> +				return 0;
> +			}
> +			return -EALREADY;
> +		} else {
>   			return -EBUSY;
> +		}
>   	}
>   
>   	ret = drm_exec_obj_locked(exec, obj, GFP_ATOMIC | __GFP_NOWARN);
> @@ -288,12 +339,20 @@ int drm_exec_lock_obj(struct drm_exec *exec, struct drm_gem_object *obj)
>   	if (unlikely(ret == -EDEADLK)) {
>   		drm_gem_object_get(obj);
>   		exec->contended = obj;
> +		exec->drop_contended = drm_exec_in_evict_mode(exec);
>   		return -EDEADLK;
>   	}
>   
> -	if (unlikely(ret == -EALREADY) &&
> -	    exec->flags & DRM_EXEC_IGNORE_DUPLICATES)
> -		return 0;
> +	if (unlikely(ret == -EALREADY)) {
> +		unsigned long key = drm_exec_resv_to_key(obj->resv);
> +
> +		if (exec->flags & DRM_EXEC_IGNORE_DUPLICATES ||
> +		    xa_load(&exec->resv_set, key)) {
> +			if (!drm_exec_in_evict_mode(exec))
> +				drm_exec_resv_set_erase(exec, key);
> +			return 0;
> +		}
> +	}
>   
>   	if (unlikely(ret))
>   		return ret;
> @@ -324,6 +383,7 @@ void drm_exec_unlock_obj(struct drm_exec *exec, struct drm_gem_object *obj)
>   
>   	for (i = exec->num_objects; i--;) {
>   		if (exec->objects[i] == obj) {
> +			drm_exec_resv_set_erase(exec, drm_exec_resv_to_key(obj->resv));
>   			dma_resv_unlock(obj->resv);
>   			for (++i; i < exec->num_objects; ++i)
>   				exec->objects[i - 1] = exec->objects[i];
> @@ -415,12 +475,14 @@ void drm_exec_restore(struct drm_exec *exec, struct drm_exec_snapshot *snap)
>   		if (index + 1 == snap->num_locked)
>   			break;
>   
> +		xa_erase(&exec->resv_set, drm_exec_resv_to_key(obj->resv));
>   		dma_resv_unlock(obj->resv);
>   		drm_gem_object_put(obj);
>   		exec->objects[index] = NULL;
>   	}
>   
>   	exec->num_objects = snap->num_locked;
> +	exec->flags = snap->flags;
>   
>   	if (!exec->prelocked)
>   		exec->prelocked = snap->prelocked;
> @@ -443,8 +505,9 @@ void drm_exec_snapshot(struct drm_exec *exec, struct drm_exec_snapshot *snap)
>   	snap->prelocked = exec->prelocked;
>   	if (snap->prelocked)
>   		drm_gem_object_get(snap->prelocked);
> +	snap->flags = exec->flags;
>   	snap->saved_snap = exec->snap;
> -	exec->snap = snap;
> +	drm_exec_set_evict_mode(exec, snap);
>   }
>   EXPORT_SYMBOL(drm_exec_snapshot);
>   
> diff --git a/include/drm/drm_exec.h b/include/drm/drm_exec.h
> index 0ce4d749511b..0b6d5ac0c092 100644
> --- a/include/drm/drm_exec.h
> +++ b/include/drm/drm_exec.h
> @@ -5,6 +5,7 @@
>   
>   #include <linux/compiler.h>
>   #include <linux/ww_mutex.h>
> +#include <linux/xarray.h>
>   
>   #define DRM_EXEC_INTERRUPTIBLE_WAIT	BIT(0)
>   #define DRM_EXEC_IGNORE_DUPLICATES	BIT(1)
> @@ -53,6 +54,17 @@ struct drm_exec {
>   	 * @snap: Pointer to the last snapshot taken or NULL if none.
>   	 */
>   	struct drm_exec_snapshot *snap;
> +
> +	/**
> +	 * @resv_set: Set of pointers to locked objects in evict mode.
> +	 */
> +	struct xarray resv_set;
> +
> +	/**
> +	 * @drop_contended: Drop the contended object after WW transaction
> +	 * relaxation.
> +	 */
> +	bool drop_contended;
>   };
>   
>   /**
> @@ -67,6 +79,9 @@ struct drm_exec_snapshot {
>   
>   	/** @num_locked: Number of locked objects at snapshot time. */
>   	unsigned long num_locked;
> +
> +	/** @flags: The drm_exec flags at snapshot time. */
> +	u32 flags;
>   };
>   
>   int drm_exec_handle_contended(struct drm_exec *exec);