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

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Fri, 2024-04-05 at 14:41 +0200, Christian König wrote:
> 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.

Hmm. That was probably when _first() started using _next(), before
realizing they already had declarations. I'll change that back for
clarity.

> 
> >   {
> > +	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.

Yes I can change that. The list head is initialized in
ttm_lru_item_init() so it's not really a bug, just slightly
inefficient.

/Thomas


> 
> 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,
>