Re: [PATCH v5 07/12] drm/ttm: Use the LRU walker for eviction

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On Tue, Jun 18, 2024 at 09:18:15AM +0200, Thomas Hellström wrote:
> Use the LRU walker for eviction. This helps
> removing a lot of code with weird locking
> semantics.
> 
> The functionality is slightly changed so that
> when trylocked buffer objects are exhausted, we
> continue to interleave walks with ticket-locks while
> there is still progress made. The list walks are
> not restarted in-between evictions.
> 
> Also provide a separate ttm_bo_evict_first()
> function for its single user. The context of that
> user allows sleeping dma_resv locks.
> 
> 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/ttm/ttm_bo.c       | 350 ++++++++++++-----------------
>  drivers/gpu/drm/ttm/ttm_resource.c |  20 +-
>  include/drm/ttm/ttm_bo.h           |   8 +-
>  3 files changed, 145 insertions(+), 233 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
> index 63a91b77f7da..316afe19a325 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -224,80 +224,6 @@ static void ttm_bo_flush_all_fences(struct ttm_buffer_object *bo)
>  	dma_resv_iter_end(&cursor);
>  }
>  
> -/**
> - * ttm_bo_cleanup_refs
> - * If bo idle, remove from lru lists, and unref.
> - * If not idle, block if possible.
> - *
> - * Must be called with lru_lock and reservation held, this function
> - * will drop the lru lock and optionally the reservation lock before returning.
> - *
> - * @bo:                    The buffer object to clean-up
> - * @interruptible:         Any sleeps should occur interruptibly.
> - * @no_wait_gpu:           Never wait for gpu. Return -EBUSY instead.
> - * @unlock_resv:           Unlock the reservation lock as well.
> - */
> -
> -static int ttm_bo_cleanup_refs(struct ttm_buffer_object *bo,
> -			       bool interruptible, bool no_wait_gpu,
> -			       bool unlock_resv)
> -{
> -	struct dma_resv *resv = &bo->base._resv;
> -	int ret;
> -
> -	if (dma_resv_test_signaled(resv, DMA_RESV_USAGE_BOOKKEEP))
> -		ret = 0;
> -	else
> -		ret = -EBUSY;
> -
> -	if (ret && !no_wait_gpu) {
> -		long lret;
> -
> -		if (unlock_resv)
> -			dma_resv_unlock(bo->base.resv);
> -		spin_unlock(&bo->bdev->lru_lock);
> -
> -		lret = dma_resv_wait_timeout(resv, DMA_RESV_USAGE_BOOKKEEP,
> -					     interruptible,
> -					     30 * HZ);
> -
> -		if (lret < 0)
> -			return lret;
> -		else if (lret == 0)
> -			return -EBUSY;
> -
> -		spin_lock(&bo->bdev->lru_lock);
> -		if (unlock_resv && !dma_resv_trylock(bo->base.resv)) {
> -			/*
> -			 * We raced, and lost, someone else holds the reservation now,
> -			 * and is probably busy in ttm_bo_cleanup_memtype_use.
> -			 *
> -			 * Even if it's not the case, because we finished waiting any
> -			 * delayed destruction would succeed, so just return success
> -			 * here.
> -			 */
> -			spin_unlock(&bo->bdev->lru_lock);
> -			return 0;
> -		}
> -		ret = 0;
> -	}
> -
> -	if (ret) {
> -		if (unlock_resv)
> -			dma_resv_unlock(bo->base.resv);
> -		spin_unlock(&bo->bdev->lru_lock);
> -		return ret;
> -	}
> -
> -	spin_unlock(&bo->bdev->lru_lock);
> -	ttm_bo_cleanup_memtype_use(bo);
> -
> -	if (unlock_resv)
> -		dma_resv_unlock(bo->base.resv);
> -
> -	return 0;
> -}
> -
>  /*
>   * Block for the dma_resv object to become idle, lock the buffer and clean up
>   * the resource and tt object.
> @@ -505,151 +431,154 @@ bool ttm_bo_eviction_valuable(struct ttm_buffer_object *bo,
>  }
>  EXPORT_SYMBOL(ttm_bo_eviction_valuable);
>  
> -/*
> - * Check the target bo is allowable to be evicted or swapout, including cases:
> - *
> - * a. if share same reservation object with ctx->resv, have assumption
> - * reservation objects should already be locked, so not lock again and
> - * return true directly when either the opreation allow_reserved_eviction
> - * or the target bo already is in delayed free list;
> +/**
> + * ttm_bo_evict_first() - Evict the first bo on the manager's LRU list.
> + * @bdev: The ttm device.
> + * @man: The manager whose bo to evict.
> + * @ctx: The TTM operation ctx governing the eviction.
>   *
> - * b. Otherwise, trylock it.
> + * Return: 0 if successful or the resource disappeared. Negative error code on error.
>   */
> -static bool ttm_bo_evict_swapout_allowable(struct ttm_buffer_object *bo,
> -					   struct ttm_operation_ctx *ctx,
> -					   const struct ttm_place *place,
> -					   bool *locked, bool *busy)
> +int ttm_bo_evict_first(struct ttm_device *bdev, struct ttm_resource_manager *man,
> +		       struct ttm_operation_ctx *ctx)
>  {
> -	bool ret = false;
> +	struct ttm_resource_cursor cursor;
> +	struct ttm_buffer_object *bo;
> +	struct ttm_resource *res;
> +	unsigned int mem_type;
> +	int ret = 0;
>  
> -	if (bo->pin_count) {
> -		*locked = false;
> -		if (busy)
> -			*busy = false;
> -		return false;
> +	spin_lock(&bdev->lru_lock);
> +	res = ttm_resource_manager_first(man, &cursor);
> +	if (!res) {
> +		ret = -ENOENT;
> +		goto out_no_ref;
>  	}
> +	bo = res->bo;
> +	if (!ttm_bo_get_unless_zero(bo))
> +		goto out_no_ref;
> +	mem_type = res->mem_type;
> +	spin_unlock(&bdev->lru_lock);
> +	ret = ttm_bo_reserve(bo, ctx->interruptible, ctx->no_wait_gpu, NULL);
> +	if (ret)
> +		goto out_no_lock;
> +	if (bo->resource != res || res->mem_type != mem_type)
> +		goto out_bad_res;
>  
> -	if (bo->base.resv == ctx->resv) {
> -		dma_resv_assert_held(bo->base.resv);
> -		if (ctx->allow_res_evict)
> -			ret = true;
> -		*locked = false;
> -		if (busy)
> -			*busy = false;
> +	if (bo->deleted) {
> +		ret = ttm_bo_wait_ctx(bo, ctx);
> +		if (ret)
> +			ttm_bo_cleanup_memtype_use(bo);
>  	} else {
> -		ret = dma_resv_trylock(bo->base.resv);
> -		*locked = ret;
> -		if (busy)
> -			*busy = !ret;
> -	}
> -
> -	if (ret && place && (bo->resource->mem_type != place->mem_type ||
> -		!bo->bdev->funcs->eviction_valuable(bo, place))) {
> -		ret = false;
> -		if (*locked) {
> -			dma_resv_unlock(bo->base.resv);
> -			*locked = false;
> -		}
> +		ret = ttm_bo_evict(bo, ctx);
>  	}
> -
> +out_bad_res:
> +	dma_resv_unlock(bo->base.resv);
> +out_no_lock:
> +	ttm_bo_put(bo);
> +	ttm_resource_cursor_fini(&cursor);
>  	return ret;
> +
> +out_no_ref:
> +	ttm_resource_cursor_fini_locked(&cursor);
> +	spin_unlock(&bdev->lru_lock);
> +	return -ENOENT;
>  }
>  
>  /**
> - * ttm_mem_evict_wait_busy - wait for a busy BO to become available
> - *
> - * @busy_bo: BO which couldn't be locked with trylock
> - * @ctx: operation context
> - * @ticket: acquire ticket
> - *
> - * Try to lock a busy buffer object to avoid failing eviction.
> + * struct ttm_bo_evict_walk - Parameters for the evict walk.
>   */
> -static int ttm_mem_evict_wait_busy(struct ttm_buffer_object *busy_bo,
> -				   struct ttm_operation_ctx *ctx,
> -				   struct ww_acquire_ctx *ticket)
> -{
> -	int r;
> -
> -	if (!busy_bo || !ticket)
> -		return -EBUSY;
> -
> -	if (ctx->interruptible)
> -		r = dma_resv_lock_interruptible(busy_bo->base.resv,
> -							  ticket);
> -	else
> -		r = dma_resv_lock(busy_bo->base.resv, ticket);
> -
> -	/*
> -	 * TODO: It would be better to keep the BO locked until allocation is at
> -	 * least tried one more time, but that would mean a much larger rework
> -	 * of TTM.
> -	 */
> -	if (!r)
> -		dma_resv_unlock(busy_bo->base.resv);
> -
> -	return r == -EDEADLK ? -EBUSY : r;
> -}
> +struct ttm_bo_evict_walk {
> +	/** @walk: The walk base parameters. */
> +	struct ttm_lru_walk walk;
> +	/** @place: The place passed to the resource allocation. */
> +	const struct ttm_place *place;
> +	/** @evictor: The buffer object we're trying to make room for. */
> +	struct ttm_buffer_object *evictor;
> +	/** @res: The allocated resource if any. */
> +	struct ttm_resource **res;
> +	/** @evicted: The number of evicted pages. */
> +	unsigned long evicted;
> +};
>  
> -int ttm_mem_evict_first(struct ttm_device *bdev,
> -			struct ttm_resource_manager *man,
> -			const struct ttm_place *place,
> -			struct ttm_operation_ctx *ctx,
> -			struct ww_acquire_ctx *ticket)
> +static long ttm_bo_evict_cb(struct ttm_lru_walk *walk, struct ttm_buffer_object *bo)
>  {
> -	struct ttm_buffer_object *bo = NULL, *busy_bo = NULL;
> -	struct ttm_resource_cursor cursor;
> -	struct ttm_resource *res;
> -	bool locked = false;
> -	int ret;
> +	struct ttm_bo_evict_walk *evict_walk =
> +		container_of(walk, typeof(*evict_walk), walk);
> +	long lret;
>  
> -	spin_lock(&bdev->lru_lock);
> -	ttm_resource_manager_for_each_res(man, &cursor, res) {
> -		bool busy;
> -
> -		if (!ttm_bo_evict_swapout_allowable(res->bo, ctx, place,
> -						    &locked, &busy)) {
> -			if (busy && !busy_bo && ticket !=
> -			    dma_resv_locking_ctx(res->bo->base.resv))
> -				busy_bo = res->bo;
> -			continue;
> -		}
> +	if (!bo->bdev->funcs->eviction_valuable(bo, evict_walk->place))
> +		return 0;
>  
> -		if (ttm_bo_get_unless_zero(res->bo)) {
> -			bo = res->bo;
> -			break;
> -		}
> -		if (locked)
> -			dma_resv_unlock(res->bo->base.resv);
> +	if (bo->deleted) {
> +		lret = ttm_bo_wait_ctx(bo, walk->ctx);
> +		if (!lret)
> +			ttm_bo_cleanup_memtype_use(bo);
> +	} else {
> +		lret = ttm_bo_evict(bo, walk->ctx);
>  	}
> -	ttm_resource_cursor_fini_locked(&cursor);
>  
> -	if (!bo) {
> -		if (busy_bo && !ttm_bo_get_unless_zero(busy_bo))
> -			busy_bo = NULL;
> -		spin_unlock(&bdev->lru_lock);
> -		ret = ttm_mem_evict_wait_busy(busy_bo, ctx, ticket);
> -		if (busy_bo)
> -			ttm_bo_put(busy_bo);
> -		return ret;
> -	}
> +	if (lret)
> +		goto out;
>  
> -	if (bo->deleted) {
> -		ret = ttm_bo_cleanup_refs(bo, ctx->interruptible,
> -					  ctx->no_wait_gpu, locked);
> -		ttm_bo_put(bo);
> -		return ret;
> -	}
> +	evict_walk->evicted++;
> +	if (evict_walk->res)
> +		lret = ttm_resource_alloc(evict_walk->evictor, evict_walk->place,
> +					  evict_walk->res);
> +	if (lret == 0)
> +		return 1;
> +out:
> +	/* Errors that should terminate the walk. */
> +	if (lret == -ENOMEM || lret == -EINTR || lret == -ERESTARTSYS ||
> +	    lret == -EAGAIN)
> +		return lret;
>  
> -	spin_unlock(&bdev->lru_lock);
> +	return 0;
> +}
>  
> -	ret = ttm_bo_evict(bo, ctx);
> -	if (locked)
> -		ttm_bo_unreserve(bo);
> -	else
> -		ttm_bo_move_to_lru_tail_unlocked(bo);
> +static const struct ttm_lru_walk_ops ttm_evict_walk_ops = {
> +	.process_bo = ttm_bo_evict_cb,
> +};
>  
> -	ttm_bo_put(bo);
> -	return ret;
> +static int ttm_bo_evict_alloc(struct ttm_device *bdev,
> +			      struct ttm_resource_manager *man,
> +			      const struct ttm_place *place,
> +			      struct ttm_buffer_object *evictor,
> +			      struct ttm_operation_ctx *ctx,
> +			      struct ww_acquire_ctx *ticket,
> +			      struct ttm_resource **res)
> +{
> +	struct ttm_bo_evict_walk evict_walk = {
> +		.walk = {
> +			.ops = &ttm_evict_walk_ops,
> +			.ctx = ctx,
> +			.ticket = ticket,
> +		},
> +		.place = place,
> +		.evictor = evictor,
> +		.res = res,
> +	};
> +	long lret;
> +
> +	evict_walk.walk.trylock_only = true;
> +	lret = ttm_lru_walk_for_evict(&evict_walk.walk, bdev, man, 1);
> +	if (lret || !ticket)
> +		goto out;
> +
> +	/* If ticket-locking, repeat while making progress. */
> +	evict_walk.walk.trylock_only = false;
> +	do {
> +		/* The walk may clear the evict_walk.walk.ticket field */
> +		evict_walk.walk.ticket = ticket;
> +		evict_walk.evicted = 0;
> +		lret = ttm_lru_walk_for_evict(&evict_walk.walk, bdev, man, 1);
> +	} while (!lret && evict_walk.evicted);
> +out:
> +	if (lret < 0)
> +		return lret;
> +	if (lret == 0)
> +		return -EBUSY;
> +	return 0;
>  }
>  
>  /**
> @@ -760,6 +689,7 @@ static int ttm_bo_alloc_resource(struct ttm_buffer_object *bo,
>  	for (i = 0; i < placement->num_placement; ++i) {
>  		const struct ttm_place *place = &placement->placement[i];
>  		struct ttm_resource_manager *man;
> +		bool may_evict;
>  
>  		man = ttm_manager_type(bdev, place->mem_type);
>  		if (!man || !ttm_resource_manager_used(man))
> @@ -769,22 +699,21 @@ static int ttm_bo_alloc_resource(struct ttm_buffer_object *bo,
>  				    TTM_PL_FLAG_FALLBACK))
>  			continue;
>  
> -		do {
> -			ret = ttm_resource_alloc(bo, place, res);
> -			if (unlikely(ret && ret != -ENOSPC))
> +		may_evict = (force_space && place->mem_type != TTM_PL_SYSTEM);
> +		ret = ttm_resource_alloc(bo, place, res);
> +		if (ret) {
> +			if (ret != -ENOSPC)
>  				return ret;
> -			if (likely(!ret) || !force_space)
> -				break;
> -
> -			ret = ttm_mem_evict_first(bdev, man, place, ctx,
> -						  ticket);
> -			if (unlikely(ret == -EBUSY))
> -				break;
> -			if (unlikely(ret))
> +			if (!may_evict)
> +				continue;
> +
> +			ret = ttm_bo_evict_alloc(bdev, man, place, bo, ctx,
> +						 ticket, res);
> +			if (ret == -EBUSY)
> +				continue;
> +			if (ret)
>  				return ret;
> -		} while (1);
> -		if (ret)
> -			continue;
> +		}
>  
>  		ret = ttm_bo_add_move_fence(bo, man, ctx->no_wait_gpu);
>  		if (unlikely(ret)) {
> @@ -796,7 +725,6 @@ static int ttm_bo_alloc_resource(struct ttm_buffer_object *bo,
>  		}
>  		return 0;
>  	}
> -
>  	return -ENOSPC;
>  }
>  
> diff --git a/drivers/gpu/drm/ttm/ttm_resource.c b/drivers/gpu/drm/ttm/ttm_resource.c
> index a03090683e79..6d0c66fc36e3 100644
> --- a/drivers/gpu/drm/ttm/ttm_resource.c
> +++ b/drivers/gpu/drm/ttm/ttm_resource.c
> @@ -508,24 +508,10 @@ int ttm_resource_manager_evict_all(struct ttm_device *bdev,
>  	};
>  	struct dma_fence *fence;
>  	int ret;
> -	unsigned i;
> -
> -	/*
> -	 * Can't use standard list traversal since we're unlocking.
> -	 */
>  
> -	spin_lock(&bdev->lru_lock);
> -	for (i = 0; i < TTM_MAX_BO_PRIORITY; ++i) {
> -		while (!list_empty(&man->lru[i])) {
> -			spin_unlock(&bdev->lru_lock);
> -			ret = ttm_mem_evict_first(bdev, man, NULL, &ctx,
> -						  NULL);
> -			if (ret)
> -				return ret;
> -			spin_lock(&bdev->lru_lock);
> -		}
> -	}
> -	spin_unlock(&bdev->lru_lock);
> +	do {
> +		ret = ttm_bo_evict_first(bdev, man, &ctx);

Ooo, just noticed this after my initial review.

This function, ttm_bo_evict_first, will return -ENOENT if
ttm_bo_get_unless_zero returns false breaking this loop. I don't think
that is the correct behavior. If ttm_bo_get_unless_zero returns false on
the head of LRU, that means this is getting destroyed. I don't think in
that what we want do to here. Shouldn't continue the LRU walk evicting
all other BOs on the LRU list?

Matt 

> +	} while (!ret);
>  
>  	spin_lock(&man->move_lock);
>  	fence = dma_fence_get(man->move);
> diff --git a/include/drm/ttm/ttm_bo.h b/include/drm/ttm/ttm_bo.h
> index 472a55b69afb..148f49f625e4 100644
> --- a/include/drm/ttm/ttm_bo.h
> +++ b/include/drm/ttm/ttm_bo.h
> @@ -415,11 +415,9 @@ long ttm_bo_swapout(struct ttm_device *bdev, struct ttm_operation_ctx *ctx,
>  		    pgoff_t target);
>  void ttm_bo_pin(struct ttm_buffer_object *bo);
>  void ttm_bo_unpin(struct ttm_buffer_object *bo);
> -int ttm_mem_evict_first(struct ttm_device *bdev,
> -			struct ttm_resource_manager *man,
> -			const struct ttm_place *place,
> -			struct ttm_operation_ctx *ctx,
> -			struct ww_acquire_ctx *ticket);
> +int ttm_bo_evict_first(struct ttm_device *bdev,
> +		       struct ttm_resource_manager *man,
> +		       struct ttm_operation_ctx *ctx);
>  vm_fault_t ttm_bo_vm_reserve(struct ttm_buffer_object *bo,
>  			     struct vm_fault *vmf);
>  vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
> -- 
> 2.44.0
> 



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