Tvrtko noticed that the comments describing the interaction of RCU and the deferred worker for freeing drm_i915_gem_object were a little confusing, so attempt to bring some sense to them. Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx> --- drivers/gpu/drm/i915/i915_gem.c | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c index 87937c4f9dff..98bacaee2720 100644 --- a/drivers/gpu/drm/i915/i915_gem.c +++ b/drivers/gpu/drm/i915/i915_gem.c @@ -4699,7 +4699,8 @@ static void __i915_gem_free_work(struct work_struct *work) container_of(work, struct drm_i915_private, mm.free_work); struct llist_node *freed; - /* All file-owned VMA should have been released by this point through + /* + * All file-owned VMA should have been released by this point through * i915_gem_close_object(), or earlier by i915_gem_context_close(). * However, the object may also be bound into the global GTT (e.g. * older GPUs without per-process support, or for direct access through @@ -4726,10 +4727,15 @@ static void __i915_gem_free_object_rcu(struct rcu_head *head) container_of(head, typeof(*obj), rcu); struct drm_i915_private *i915 = to_i915(obj->base.dev); - /* We can't simply use call_rcu() from i915_gem_free_object() - * as we need to block whilst unbinding, and the call_rcu - * task may be called from softirq context. So we take a - * detour through a worker. + /* + * Since we require blocking on struct_mutex to unbind the freed + * object from the GPU before releasing resources back to the + * system, we can not do that directly from the RCU callback (which may + * be a softirq context), but must intend then defer that work onto a + * kthread. We use the RCU callback rather than move the freed object + * directly onto the work queue so that we can mix between using the + * worker and performing frees directly from subsequent allocations for + * crude but effective memory throttling. */ if (llist_add(&obj->freed, &i915->mm.free_list)) queue_work(i915->wq, &i915->mm.free_work); @@ -4745,7 +4751,8 @@ void i915_gem_free_object(struct drm_gem_object *gem_obj) if (discard_backing_storage(obj)) obj->mm.madv = I915_MADV_DONTNEED; - /* Before we free the object, make sure any pure RCU-only + /* + * Before we free the object, make sure any pure RCU-only * read-side critical sections are complete, e.g. * i915_gem_busy_ioctl(). For the corresponding synchronized * lookup see i915_gem_object_lookup_rcu(). -- 2.15.1 _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
