Re: [Intel-gfx] [PATCH 10/20] drm/i915: Export a preallocate variant of i915_active_acquire()

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 06/07/2020 07:19, Chris Wilson wrote:
> Sometimes we have to be very careful not to allocate underneath a mutex
> (or spinlock) and yet still want to track activity. Enter
> i915_active_acquire_for_context(). This raises the activity counter on
> i915_active prior to use and ensures that the fence-tree contains a slot
> for the context.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   .../gpu/drm/i915/gem/i915_gem_execbuffer.c    |   2 +-
>   drivers/gpu/drm/i915/gt/intel_timeline.c      |   4 +-
>   drivers/gpu/drm/i915/i915_active.c            | 113 +++++++++++++++---
>   drivers/gpu/drm/i915/i915_active.h            |  14 ++-
>   4 files changed, 113 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> index 1015c4fd9f3e..6d20be29ff3c 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> @@ -1789,7 +1789,7 @@ __parser_mark_active(struct i915_vma *vma,
>   {
>   	struct intel_gt_buffer_pool_node *node = vma->private;
>   
> -	return i915_active_ref(&node->active, tl, fence);
> +	return i915_active_ref(&node->active, tl->fence_context, fence);
>   }
>   
>   static int
> diff --git a/drivers/gpu/drm/i915/gt/intel_timeline.c b/drivers/gpu/drm/i915/gt/intel_timeline.c
> index 4546284fede1..e4a5326633b8 100644
> --- a/drivers/gpu/drm/i915/gt/intel_timeline.c
> +++ b/drivers/gpu/drm/i915/gt/intel_timeline.c
> @@ -479,7 +479,9 @@ __intel_timeline_get_seqno(struct intel_timeline *tl,
>   	 * free it after the current request is retired, which ensures that
>   	 * all writes into the cacheline from previous requests are complete.
>   	 */
> -	err = i915_active_ref(&tl->hwsp_cacheline->active, tl, &rq->fence);
> +	err = i915_active_ref(&tl->hwsp_cacheline->active,
> +			      tl->fence_context,
> +			      &rq->fence);
>   	if (err)
>   		goto err_cacheline;
>   
> diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
> index d960d0be5bd2..3f595446fd44 100644
> --- a/drivers/gpu/drm/i915/i915_active.c
> +++ b/drivers/gpu/drm/i915/i915_active.c
> @@ -217,11 +217,10 @@ excl_retire(struct dma_fence *fence, struct dma_fence_cb *cb)
>   }
>   
>   static struct i915_active_fence *
> -active_instance(struct i915_active *ref, struct intel_timeline *tl)
> +active_instance(struct i915_active *ref, u64 idx)
>   {
>   	struct active_node *node, *prealloc;
>   	struct rb_node **p, *parent;
> -	u64 idx = tl->fence_context;
>   
>   	/*
>   	 * We track the most recently used timeline to skip a rbtree search
> @@ -353,21 +352,17 @@ __active_del_barrier(struct i915_active *ref, struct active_node *node)
>   	return ____active_del_barrier(ref, node, barrier_to_engine(node));
>   }
>   
> -int i915_active_ref(struct i915_active *ref,
> -		    struct intel_timeline *tl,
> -		    struct dma_fence *fence)
> +int i915_active_ref(struct i915_active *ref, u64 idx, struct dma_fence *fence)
>   {
>   	struct i915_active_fence *active;
>   	int err;
>   
> -	lockdep_assert_held(&tl->mutex);
> -
>   	/* Prevent reaping in case we malloc/wait while building the tree */
>   	err = i915_active_acquire(ref);
>   	if (err)
>   		return err;
>   
> -	active = active_instance(ref, tl);
> +	active = active_instance(ref, idx);
>   	if (!active) {
>   		err = -ENOMEM;
>   		goto out;
> @@ -384,32 +379,104 @@ int i915_active_ref(struct i915_active *ref,
>   		atomic_dec(&ref->count);
>   	}
>   	if (!__i915_active_fence_set(active, fence))
> -		atomic_inc(&ref->count);
> +		__i915_active_acquire(ref);
>   
>   out:
>   	i915_active_release(ref);
>   	return err;
>   }
>   
> -struct dma_fence *
> -i915_active_set_exclusive(struct i915_active *ref, struct dma_fence *f)
> +static struct dma_fence *
> +__i915_active_set_fence(struct i915_active *ref,
> +			struct i915_active_fence *active,
> +			struct dma_fence *fence)
>   {
>   	struct dma_fence *prev;
>   
>   	/* We expect the caller to manage the exclusive timeline ordering */
>   	GEM_BUG_ON(i915_active_is_idle(ref));
>   
> +	if (is_barrier(active)) { /* proto-node used by our idle barrier */
> +		/*
> +		 * This request is on the kernel_context timeline, and so
> +		 * we can use it to substitute for the pending idle-barrer
> +		 * request that we want to emit on the kernel_context.
> +		 */
> +		__active_del_barrier(ref, node_from_active(active));
> +		RCU_INIT_POINTER(active->fence, NULL);

The condition, comments and operation up to here is duplicated from 
i915_active_ref not shown in the diff.

More important question is on the relationship between i915_active_ref 
and __i915_active_ref. Latter actually calls this new 
__i915_active_set_fence which is kind of surprising considering the 
usual patterns.

Is there any duplication of the APIs ie. opportunity to consolidate? For 
instance should i915_active_ref be broken up into 
preallocate/lookup/install helpers too?

Another confusing end result is that we'd end up with both 
__i915_active_fence_set(active-fence, dma-fence) and
__i915_active_set_fence(active, active-fence, dma-fence).

I am not offering any solutions just raising oddities at this point. :)

Regards,

Tvrtko

> +		atomic_dec(&ref->count);
> +	}
> +
>   	rcu_read_lock();
> -	prev = __i915_active_fence_set(&ref->excl, f);
> +	prev = __i915_active_fence_set(active, fence);
>   	if (prev)
>   		prev = dma_fence_get_rcu(prev);
>   	else
> -		atomic_inc(&ref->count);
> +		__i915_active_acquire(ref);
>   	rcu_read_unlock();
>   
>   	return prev;
>   }
>   
> +static struct i915_active_fence *
> +__active_lookup(struct i915_active *ref, u64 idx)
> +{
> +	struct active_node *node;
> +	struct rb_node *p;
> +
> +	/* Like active_instance() but with no malloc */
> +
> +	node = READ_ONCE(ref->cache);
> +	if (node && node->timeline == idx)
> +		return &node->base;
> +
> +	spin_lock_irq(&ref->tree_lock);
> +	GEM_BUG_ON(i915_active_is_idle(ref));
> +
> +	p = ref->tree.rb_node;
> +	while (p) {
> +		node = rb_entry(p, struct active_node, node);
> +		if (node->timeline == idx) {
> +			ref->cache = node;
> +			spin_unlock_irq(&ref->tree_lock);
> +			return &node->base;
> +		}
> +
> +		if (node->timeline < idx)
> +			p = p->rb_right;
> +		else
> +			p = p->rb_left;
> +	}
> +
> +	spin_unlock_irq(&ref->tree_lock);
> +
> +	return NULL;
> +}
> +
> +struct dma_fence *
> +__i915_active_ref(struct i915_active *ref, u64 idx, struct dma_fence *fence)
> +{
> +	struct dma_fence *prev = ERR_PTR(-ENOENT);
> +	struct i915_active_fence *active;
> +
> +	if (!i915_active_acquire_if_busy(ref))
> +		return ERR_PTR(-EINVAL);
> +
> +	active = __active_lookup(ref, idx);
> +	if (active)
> +		prev = __i915_active_set_fence(ref, active, fence);
> +
> +	i915_active_release(ref);
> +	return prev;
> +}
> +
> +struct dma_fence *
> +i915_active_set_exclusive(struct i915_active *ref, struct dma_fence *f)
> +{
> +	/* We expect the caller to manage the exclusive timeline ordering */
> +	return __i915_active_set_fence(ref, &ref->excl, f);
> +}
> +
>   bool i915_active_acquire_if_busy(struct i915_active *ref)
>   {
>   	debug_active_assert(ref);
> @@ -443,6 +510,24 @@ int i915_active_acquire(struct i915_active *ref)
>   	return err;
>   }
>   
> +int i915_active_acquire_for_context(struct i915_active *ref, u64 idx)
> +{
> +	struct i915_active_fence *active;
> +	int err;
> +
> +	err = i915_active_acquire(ref);
> +	if (err)
> +		return err;
> +
> +	active = active_instance(ref, idx);
> +	if (!active) {
> +		i915_active_release(ref);
> +		return -ENOMEM;
> +	}
> +
> +	return 0; /* return with active ref */
> +}
> +
>   void i915_active_release(struct i915_active *ref)
>   {
>   	debug_active_assert(ref);
> @@ -804,7 +889,7 @@ int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
>   			 */
>   			RCU_INIT_POINTER(node->base.fence, ERR_PTR(-EAGAIN));
>   			node->base.cb.node.prev = (void *)engine;
> -			atomic_inc(&ref->count);
> +			__i915_active_acquire(ref);
>   		}
>   		GEM_BUG_ON(rcu_access_pointer(node->base.fence) != ERR_PTR(-EAGAIN));
>   
> diff --git a/drivers/gpu/drm/i915/i915_active.h b/drivers/gpu/drm/i915/i915_active.h
> index cf4058150966..2e0bcb3289ec 100644
> --- a/drivers/gpu/drm/i915/i915_active.h
> +++ b/drivers/gpu/drm/i915/i915_active.h
> @@ -163,14 +163,18 @@ void __i915_active_init(struct i915_active *ref,
>   	__i915_active_init(ref, active, retire, &__mkey, &__wkey);	\
>   } while (0)
>   
> -int i915_active_ref(struct i915_active *ref,
> -		    struct intel_timeline *tl,
> -		    struct dma_fence *fence);
> +struct dma_fence *
> +__i915_active_ref(struct i915_active *ref, u64 idx, struct dma_fence *fence);
> +int i915_active_ref(struct i915_active *ref, u64 idx, struct dma_fence *fence);
>   
>   static inline int
>   i915_active_add_request(struct i915_active *ref, struct i915_request *rq)
>   {
> -	return i915_active_ref(ref, i915_request_timeline(rq), &rq->fence);
> +	struct intel_timeline *tl = i915_request_timeline(rq);
> +
> +	lockdep_assert_held(&tl->mutex);
> +
> +	return i915_active_ref(ref, tl->fence_context, &rq->fence);
>   }
>   
>   struct dma_fence *
> @@ -198,7 +202,9 @@ int i915_request_await_active(struct i915_request *rq,
>   #define I915_ACTIVE_AWAIT_BARRIER BIT(2)
>   
>   int i915_active_acquire(struct i915_active *ref);
> +int i915_active_acquire_for_context(struct i915_active *ref, u64 idx);
>   bool i915_active_acquire_if_busy(struct i915_active *ref);
> +
>   void i915_active_release(struct i915_active *ref);
>   
>   static inline void __i915_active_acquire(struct i915_active *ref)
> 
_______________________________________________
Intel-gfx mailing list
Intel-gfx@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/intel-gfx