Re: [PATCH 2/8] drm/ttm: Use LRU hitches

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

Am 29.03.24 um 15:57 schrieb Thomas Hellström:
> Have iterators insert themselves into the list they are iterating
> over using hitch list nodes. Since only the iterator owner
> can remove these list nodes from the list, it's safe to unlock
> the list and when continuing, use them as a starting point. Due to
> the way LRU bumping works in TTM, newly added items will not be
> missed, and bumped items will be iterated over a second time before
> reaching the end of the list.
>
> The exception is list with bulk move sublists. When bumping a
> sublist, a hitch that is part of that sublist will also be moved
> and we might miss items if restarting from it. This will be
> addressed in a later patch.
>
> v2:
> - Updated ttm_resource_cursor_fini() documentation.
>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Somalapuram Amaranath <Amaranath.Somalapuram@amd.com>
> Cc: <dri-devel@lists.freedesktop.org>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>   drivers/gpu/drm/ttm/ttm_bo.c       |  1 +
>   drivers/gpu/drm/ttm/ttm_device.c   |  9 ++-
>   drivers/gpu/drm/ttm/ttm_resource.c | 94 ++++++++++++++++++++----------
>   include/drm/ttm/ttm_resource.h     | 16 +++--
>   4 files changed, 82 insertions(+), 38 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
> index e059b1e1b13b..b6f75a0ff2e5 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -622,6 +622,7 @@ int ttm_mem_evict_first(struct ttm_device *bdev,
>   		if (locked)
>   			dma_resv_unlock(res->bo->base.resv);
>   	}
> +	ttm_resource_cursor_fini_locked(&cursor);
>   
>   	if (!bo) {
>   		if (busy_bo && !ttm_bo_get_unless_zero(busy_bo))
> diff --git a/drivers/gpu/drm/ttm/ttm_device.c b/drivers/gpu/drm/ttm/ttm_device.c
> index f27406e851e5..e8a6a1dab669 100644
> --- a/drivers/gpu/drm/ttm/ttm_device.c
> +++ b/drivers/gpu/drm/ttm/ttm_device.c
> @@ -169,12 +169,17 @@ int ttm_device_swapout(struct ttm_device *bdev, struct ttm_operation_ctx *ctx,
>   			num_pages = PFN_UP(bo->base.size);
>   			ret = ttm_bo_swapout(bo, ctx, gfp_flags);
>   			/* ttm_bo_swapout has dropped the lru_lock */
> -			if (!ret)
> +			if (!ret) {
> +				ttm_resource_cursor_fini(&cursor);
>   				return num_pages;
> -			if (ret != -EBUSY)
> +			}
> +			if (ret != -EBUSY) {
> +				ttm_resource_cursor_fini(&cursor);
>   				return ret;
> +			}
>   		}
>   	}
> +	ttm_resource_cursor_fini_locked(&cursor);
>   	spin_unlock(&bdev->lru_lock);
>   	return 0;
>   }
> diff --git a/drivers/gpu/drm/ttm/ttm_resource.c b/drivers/gpu/drm/ttm/ttm_resource.c
> index 7aa5ca5c0e33..ccc31ad85e3b 100644
> --- a/drivers/gpu/drm/ttm/ttm_resource.c
> +++ b/drivers/gpu/drm/ttm/ttm_resource.c
> @@ -32,6 +32,37 @@
>   
>   #include <drm/drm_util.h>
>   
> +/**
> + * ttm_resource_cursor_fini_locked() - Finalize the LRU list cursor usage
> + * @cursor: The struct ttm_resource_cursor to finalize.
> + *
> + * The function pulls the LRU list cursor off any lists it was previusly
> + * attached to. Needs to be called with the LRU lock held. The function
> + * can be called multiple times after eachother.
> + */
> +void ttm_resource_cursor_fini_locked(struct ttm_resource_cursor *cursor)
> +{
> +	lockdep_assert_held(&cursor->man->bdev->lru_lock);
> +	list_del_init(&cursor->hitch.link);
> +}
> +
> +/**
> + * ttm_resource_cursor_fini() - Finalize the LRU list cursor usage
> + * @cursor: The struct ttm_resource_cursor to finalize.
> + *
> + * The function pulls the LRU list cursor off any lists it was previusly
> + * attached to. Needs to be called without the LRU list lock held. The
> + * function can be called multiple times after eachother.
> + */
> +void ttm_resource_cursor_fini(struct ttm_resource_cursor *cursor)
> +{
> +	spinlock_t *lru_lock = &cursor->man->bdev->lru_lock;
> +
> +	spin_lock(lru_lock);
> +	ttm_resource_cursor_fini_locked(cursor);
> +	spin_unlock(lru_lock);
> +}
> +
>   /**
>    * ttm_lru_bulk_move_init - initialize a bulk move structure
>    * @bulk: the structure to init
> @@ -484,62 +515,63 @@ void ttm_resource_manager_debug(struct ttm_resource_manager *man,
>   EXPORT_SYMBOL(ttm_resource_manager_debug);
>   
>   /**
> - * ttm_resource_manager_first
> - *
> - * @man: resource manager to iterate over
> + * ttm_resource_manager_next() - Continue iterating over the resource manager
> + * resources
>    * @cursor: cursor to record the position
>    *
> - * Returns the first resource from the resource manager.
> + * Return: The next resource from the resource manager.
>    */
>   struct ttm_resource *
> -ttm_resource_manager_first(struct ttm_resource_manager *man,
> -			   struct ttm_resource_cursor *cursor)
> +ttm_resource_manager_next(struct ttm_resource_cursor *cursor)

Why are you switching first/next here? It took me a moment to realize 
that the two functions switched places.

>   {
> +	struct ttm_resource_manager *man = cursor->man;
>   	struct ttm_lru_item *lru;
>   
>   	lockdep_assert_held(&man->bdev->lru_lock);
>   
> -	for (cursor->priority = 0; cursor->priority < TTM_MAX_BO_PRIORITY;
> -	     ++cursor->priority)
> -		list_for_each_entry(lru, &man->lru[cursor->priority], link) {
> -			if (ttm_lru_item_is_res(lru))
> +	do {
> +		lru = &cursor->hitch;
> +		list_for_each_entry_continue(lru, &man->lru[cursor->priority], link) {
> +			if (ttm_lru_item_is_res(lru)) {
> +				list_move(&cursor->hitch.link, &lru->link);
>   				return ttm_lru_item_to_res(lru);
> +			}
>   		}
>   
> +		if (++cursor->priority >= TTM_MAX_BO_PRIORITY)
> +			break;
> +
> +		list_move(&cursor->hitch.link, &man->lru[cursor->priority]);
> +	} while (true);
> +
> +	list_del_init(&cursor->hitch.link);
> +
>   	return NULL;
>   }
>   
>   /**
> - * ttm_resource_manager_next
> - *
> + * ttm_resource_manager_first() - Start iterating over the resources
> + * of a resource manager
>    * @man: resource manager to iterate over
>    * @cursor: cursor to record the position
> - * @res: the current resource pointer
>    *
> - * Returns the next resource from the resource manager.
> + * Initializes the cursor and starts iterating. When done iterating,
> + * the caller must explicitly call ttm_resource_cursor_fini().
> + *
> + * Return: The first resource from the resource manager.
>    */
>   struct ttm_resource *
> -ttm_resource_manager_next(struct ttm_resource_manager *man,
> -			  struct ttm_resource_cursor *cursor,
> -			  struct ttm_resource *res)
> +ttm_resource_manager_first(struct ttm_resource_manager *man,
> +			   struct ttm_resource_cursor *cursor)
>   {
> -	struct ttm_lru_item *lru = &res->lru;
> -
>   	lockdep_assert_held(&man->bdev->lru_lock);
>   
> -	list_for_each_entry_continue(lru, &man->lru[cursor->priority], link) {
> -		if (ttm_lru_item_is_res(lru))
> -			return ttm_lru_item_to_res(lru);
> -	}
> +	cursor->priority = 0;
> +	cursor->man = man;
> +	ttm_lru_item_init(&cursor->hitch, TTM_LRU_HITCH);
> +	list_move(&cursor->hitch.link, &man->lru[cursor->priority]);

That should probably be a list_add() instead.

Need to take a closer look what actually changed when the functions keep 
their original order.

Christian.

>   
> -	for (++cursor->priority; cursor->priority < TTM_MAX_BO_PRIORITY;
> -	     ++cursor->priority)
> -		list_for_each_entry(lru, &man->lru[cursor->priority], link) {
> -			if (ttm_lru_item_is_res(lru))
> -				ttm_lru_item_to_res(lru);
> -		}
> -
> -	return NULL;
> +	return ttm_resource_manager_next(cursor);
>   }
>   
>   static void ttm_kmap_iter_iomap_map_local(struct ttm_kmap_iter *iter,
> diff --git a/include/drm/ttm/ttm_resource.h b/include/drm/ttm/ttm_resource.h
> index 4babc4ff10b0..dfc01258c981 100644
> --- a/include/drm/ttm/ttm_resource.h
> +++ b/include/drm/ttm/ttm_resource.h
> @@ -271,15 +271,23 @@ ttm_lru_item_to_res(struct ttm_lru_item *item)
>   
>   /**
>    * struct ttm_resource_cursor
> - *
> + * @man: The resource manager currently being iterated over
> + * @hitch: A hitch list node inserted before the next resource
> + * to iterate over.
>    * @priority: the current priority
>    *
>    * Cursor to iterate over the resources in a manager.
>    */
>   struct ttm_resource_cursor {
> +	struct ttm_resource_manager *man;
> +	struct ttm_lru_item hitch;
>   	unsigned int priority;
>   };
>   
> +void ttm_resource_cursor_fini_locked(struct ttm_resource_cursor *cursor);
> +
> +void ttm_resource_cursor_fini(struct ttm_resource_cursor *cursor);
> +
>   /**
>    * struct ttm_lru_bulk_move_pos
>    *
> @@ -438,9 +446,7 @@ struct ttm_resource *
>   ttm_resource_manager_first(struct ttm_resource_manager *man,
>   			   struct ttm_resource_cursor *cursor);
>   struct ttm_resource *
> -ttm_resource_manager_next(struct ttm_resource_manager *man,
> -			  struct ttm_resource_cursor *cursor,
> -			  struct ttm_resource *res);
> +ttm_resource_manager_next(struct ttm_resource_cursor *cursor);
>   
>   /**
>    * ttm_resource_manager_for_each_res - iterate over all resources
> @@ -452,7 +458,7 @@ ttm_resource_manager_next(struct ttm_resource_manager *man,
>    */
>   #define ttm_resource_manager_for_each_res(man, cursor, res)		\
>   	for (res = ttm_resource_manager_first(man, cursor); res;	\
> -	     res = ttm_resource_manager_next(man, cursor, res))
> +	     res = ttm_resource_manager_next(cursor))
>   
>   struct ttm_kmap_iter *
>   ttm_kmap_iter_iomap_init(struct ttm_kmap_iter_iomap *iter_io,