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

Christian König <christian.koenig@xxxxxxx> · Wed, 22 May 2024 15:28:23 +0200

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@xxxxxxx>
Cc: Somalapuram Amaranath <Amaranath.Somalapuram@xxxxxxx>
Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
Cc: <dri-devel@xxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
---
  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);







