[PATCH 03/16] drm/i915: Convert non-blocking userptr waits for requests over to using RCU

Chris Wilson
2016-08-01T18:22:53Z

We can completely avoid taking the struct_mutex around the non-blocking
waits by switching over to the RCU request management (trading the mutex
for a RCU read lock and some complex atomic operations). The improvement
is that we gain further contention reduction, and overall the code
become simpler due to the reduced mutex dancing.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---
drivers/gpu/drm/i915/i915_gem_userptr.c | 34 +++++++--------------------------
1 file changed, 7 insertions(+), 27 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 53f64fcc89ef..96ab6161903a 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -63,32 +63,12 @@ struct i915_mmu_object {

static void wait_rendering(struct drm_i915_gem_object *obj)
{
- struct drm_device *dev = obj->base.dev;
- struct drm_i915_gem_request *requests[I915_NUM_ENGINES];
- int i, n;
-
- if (!i915_gem_object_is_active(obj))
- return;
-
- n = 0;
- for (i = 0; i < I915_NUM_ENGINES; i++) {
- struct drm_i915_gem_request *req;
+ unsigned long active = __I915_BO_ACTIVE(obj);
+ int idx;

- req = i915_gem_active_get(&obj->last_read[i],
- &obj->base.dev->struct_mutex);
- if (req)
- requests[n++] = req;
- }
-
- mutex_unlock(&dev->struct_mutex);
-
- for (i = 0; i < n; i++)
- i915_wait_request(requests[i], false, NULL, NULL);
-
- mutex_lock(&dev->struct_mutex);
-
- for (i = 0; i < n; i++)
- i915_gem_request_put(requests[i]);
+ for_each_active(active, idx)
+ i915_gem_active_wait_unlocked(&obj->last_read[idx],
+ false, NULL, NULL);
}

static void cancel_userptr(struct work_struct *work)
@@ -97,6 +77,8 @@ static void cancel_userptr(struct work_struct *work)
struct drm_i915_gem_object *obj = mo->obj;
struct drm_device *dev = obj->base.dev;

+ wait_rendering(obj);
+
mutex_lock(&dev->struct_mutex);
/* Cancel any active worker and force us to re-evaluate gup */
obj->userptr.work = NULL;
@@ -105,8 +87,6 @@ static void cancel_userptr(struct work_struct *work)
struct drm_i915_private *dev_priv = to_i915(dev);
bool was_interruptible;

- wait_rendering(obj);
-
was_interruptible = dev_priv->mm.interruptible;
dev_priv->mm.interruptible = false;

--
2.8.1