[PATCH v2 09/13] drm/gem: Add LRU/shrinker helper

Rob Clark
2022-07-19T17:25:17Z

Add a simple LRU helper to assist with driver's shrinker implementation.
It handles tracking the number of backing pages associated with a given
LRU, and provides a helper to implement shrinker_scan.

A driver can use multiple LRU instances to track objects in various
states, for example a dontneed LRU for purgeable objects, a willneed LRU
for evictable objects, and an unpinned LRU for objects without backing
pages.

All LRUs that the object can be moved between must share a single lock.

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>
---
drivers/gpu/drm/drm_gem.c | 183 ++++++++++++++++++++++++++++++++++++++
include/drm/drm_gem.h | 56 ++++++++++++
2 files changed, 239 insertions(+)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index eb0c2d041f13..684db28cc71c 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -165,6 +165,7 @@ void drm_gem_private_object_init(struct drm_device *dev,
obj->resv = &obj->_resv;

drm_vma_node_reset(&obj->vma_node);
+ INIT_LIST_HEAD(&obj->lru_node);
}
EXPORT_SYMBOL(drm_gem_private_object_init);

@@ -951,6 +952,7 @@ drm_gem_object_release(struct drm_gem_object *obj)

dma_resv_fini(&obj->_resv);
drm_gem_free_mmap_offset(obj);
+ drm_gem_lru_remove(obj);
}
EXPORT_SYMBOL(drm_gem_object_release);

@@ -1274,3 +1276,184 @@ drm_gem_unlock_reservations(struct drm_gem_object **objs, int count,
ww_acquire_fini(acquire_ctx);
}
EXPORT_SYMBOL(drm_gem_unlock_reservations);
+
+/**
+ * drm_gem_lru_init - initialize a LRU
+ *
+ * @lru: The LRU to initialize
+ * @lock: The lock protecting the LRU
+ */
+void
+drm_gem_lru_init(struct drm_gem_lru *lru, struct mutex *lock)
+{
+ lru->lock = lock;
+ lru->count = 0;
+ INIT_LIST_HEAD(&lru->list);
+}
+EXPORT_SYMBOL(drm_gem_lru_init);
+
+static void
+lru_remove(struct drm_gem_object *obj)
+{
+ obj->lru->count -= obj->size >> PAGE_SHIFT;
+ WARN_ON(obj->lru->count < 0);
+ list_del(&obj->lru_node);
+ obj->lru = NULL;
+}
+
+/**
+ * drm_gem_lru_remove - remove object from whatever LRU it is in
+ *
+ * If the object is currently in any LRU, remove it.
+ *
+ * @obj: The GEM object to remove from current LRU
+ */
+void
+drm_gem_lru_remove(struct drm_gem_object *obj)
+{
+ struct drm_gem_lru *lru = obj->lru;
+
+ if (!lru)
+ return;
+
+ mutex_lock(lru->lock);
+ lru_remove(obj);
+ mutex_unlock(lru->lock);
+}
+EXPORT_SYMBOL(drm_gem_lru_remove);
+
+/**
+ * drm_gem_lru_move_tail - move the object to the tail of the LRU
+ *
+ * If the object is already in this LRU it will be moved to the
+ * tail. Otherwise it will be removed from whichever other LRU
+ * it is in (if any) and moved into this LRU.
+ *
+ * @lru: The LRU to move the object into.
+ * @obj: The GEM object to move into this LRU
+ */
+void
+drm_gem_lru_move_tail(struct drm_gem_lru *lru, struct drm_gem_object *obj)
+{
+ mutex_lock(lru->lock);
+ drm_gem_lru_move_tail_locked(lru, obj);
+ mutex_unlock(lru->lock);
+}
+EXPORT_SYMBOL(drm_gem_lru_move_tail);
+
+/**
+ * drm_gem_lru_move_tail_locked - move the object to the tail of the LRU
+ *
+ * If the object is already in this LRU it will be moved to the
+ * tail. Otherwise it will be removed from whichever other LRU
+ * it is in (if any) and moved into this LRU.
+ *
+ * Call with LRU lock held.
+ *
+ * @lru: The LRU to move the object into.
+ * @obj: The GEM object to move into this LRU
+ */
+void
+drm_gem_lru_move_tail_locked(struct drm_gem_lru *lru, struct drm_gem_object *obj)
+{
+ WARN_ON(!mutex_is_locked(lru->lock));
+
+ if (obj->lru)
+ lru_remove(obj);
+
+ lru->count += obj->size >> PAGE_SHIFT;
+ list_add_tail(&obj->lru_node, &lru->list);
+ obj->lru = lru;
+}
+EXPORT_SYMBOL(drm_gem_lru_move_tail_locked);
+
+/**
+ * drm_gem_lru_scan - helper to implement shrinker.scan_objects
+ *
+ * If the shrink callback succeeds, it is expected that the driver
+ * move the object out of this LRU.
+ *
+ * If the LRU possibly contain active buffers, it is the responsibility
+ * of the shrink callback to check for this (ie. dma_resv_test_signaled())
+ * or if necessary block until the buffer becomes idle.
+ *
+ * @lru: The LRU to scan
+ * @nr_to_scan: The number of pages to try to reclaim
+ * @shrink: Callback to try to shrink/reclaim the object.
+ */
+unsigned long
+drm_gem_lru_scan(struct drm_gem_lru *lru, unsigned nr_to_scan,
+ bool (*shrink)(struct drm_gem_object *obj))
+{
+ struct drm_gem_lru still_in_lru;
+ struct drm_gem_object *obj;
+ unsigned freed = 0;
+
+ drm_gem_lru_init(&still_in_lru, lru->lock);
+
+ mutex_lock(lru->lock);
+
+ while (freed < nr_to_scan) {
+ obj = list_first_entry_or_null(&lru->list, typeof(*obj), lru_node);
+
+ if (!obj)
+ break;
+
+ drm_gem_lru_move_tail_locked(&still_in_lru, obj);
+
+ /*
+ * If it's in the process of being freed, gem_object->free()
+ * may be blocked on lock waiting to remove it. So just
+ * skip it.
+ */
+ if (!kref_get_unless_zero(&obj->refcount))
+ continue;
+
+ /*
+ * Now that we own a reference, we can drop the lock for the
+ * rest of the loop body, to reduce contention with other
+ * code paths that need the LRU lock
+ */
+ mutex_unlock(lru->lock);
+
+ /*
+ * Note that this still needs to be trylock, since we can
+ * hit shrinker in response to trying to get backing pages
+ * for this obj (ie. while it's lock is already held)
+ */
+ if (!dma_resv_trylock(obj->resv))
+ goto tail;
+
+ if (shrink(obj)) {
+ freed += obj->size >> PAGE_SHIFT;
+
+ /*
+ * If we succeeded in releasing the object's backing
+ * pages, we expect the driver to have moved the object
+ * out of this LRU
+ */
+ WARN_ON(obj->lru == &still_in_lru);
+ WARN_ON(obj->lru == lru);
+ }
+
+ dma_resv_unlock(obj->resv);
+
+tail:
+ drm_gem_object_put(obj);
+ mutex_lock(lru->lock);
+ }
+
+ /*
+ * Move objects we've skipped over out of the temporary still_in_lru
+ * back into this LRU
+ */
+ list_for_each_entry (obj, &still_in_lru.list, lru_node)
+ obj->lru = lru;
+ list_splice_tail(&still_in_lru.list, &lru->list);
+ lru->count += still_in_lru.count;
+
+ mutex_unlock(lru->lock);
+
+ return freed;
+}
+EXPORT_SYMBOL(drm_gem_lru_scan);
diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
index 87cffc9efa85..f13a9080af37 100644
--- a/include/drm/drm_gem.h
+++ b/include/drm/drm_gem.h
@@ -174,6 +174,41 @@ struct drm_gem_object_funcs {
const struct vm_operations_struct *vm_ops;
};

+/**
+ * struct drm_gem_lru - A simple LRU helper
+ *
+ * A helper for tracking GEM objects in a given state, to aid in
+ * driver's shrinker implementation. Tracks the count of pages
+ * for lockless &shrinker.count_objects, and provides
+ * &drm_gem_lru_scan for driver's &shrinker.scan_objects
+ * implementation.
+ */
+struct drm_gem_lru {
+ /**
+ * @lock:
+ *
+ * Lock protecting movement of GEM objects between LRUs. All
+ * LRUs that the object can move between should be protected
+ * by the same lock.
+ */
+ struct mutex *lock;
+
+ /**
+ * @count:
+ *
+ * The total number of backing pages of the GEM objects in
+ * this LRU.
+ */
+ long count;
+
+ /**
+ * @list:
+ *
+ * The LRU list.
+ */
+ struct list_head list;
+};
+
/**
* struct drm_gem_object - GEM buffer object
*
@@ -312,6 +347,20 @@ struct drm_gem_object {
*
*/
const struct drm_gem_object_funcs *funcs;
+
+ /**
+ * @lru_node:
+ *
+ * List node in a &drm_gem_lru.
+ */
+ struct list_head lru_node;
+
+ /**
+ * @lru:
+ *
+ * The current LRU list that the GEM object is on.
+ */
+ struct drm_gem_lru *lru;
};

/**
@@ -420,4 +469,11 @@ void drm_gem_unlock_reservations(struct drm_gem_object **objs, int count,
int drm_gem_dumb_map_offset(struct drm_file *file, struct drm_device *dev,
u32 handle, u64 *offset);

+void drm_gem_lru_init(struct drm_gem_lru *lru, struct mutex *lock);
+void drm_gem_lru_remove(struct drm_gem_object *obj);
+void drm_gem_lru_move_tail(struct drm_gem_lru *lru, struct drm_gem_object *obj);
+void drm_gem_lru_move_tail_locked(struct drm_gem_lru *lru, struct drm_gem_object *obj);
+unsigned long drm_gem_lru_scan(struct drm_gem_lru *lru, unsigned nr_to_scan,
+ bool (*shrink)(struct drm_gem_object *obj));
+
#endif /* __DRM_GEM_H__ */
--
2.36.1