[PATCH 02/16] drm/i915: Convert non-blocking waits for requests over to using RCU

Chris Wilson
2016-08-01T18:22:52Z

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.c | 112 +++++++++++++++++-----------------------
1 file changed, 47 insertions(+), 65 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index d8d3e728230d..f164ad482bdc 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -346,24 +346,20 @@ i915_gem_object_wait_rendering(struct drm_i915_gem_object *obj,
return 0;
}

-/* A nonblocking variant of the above wait. This is a highly dangerous routine
- * as the object state may change during this call.
+/* A nonblocking variant of the above wait. Must be called prior to
+ * acquiring the mutex for the object, as the object state may change
+ * during this call. A reference must be held by the caller for the object.
*/
static __must_check int
-i915_gem_object_wait_rendering__nonblocking(struct drm_i915_gem_object *obj,
- struct intel_rps_client *rps,
- bool readonly)
+__unsafe_wait_rendering(struct drm_i915_gem_object *obj,
+ struct intel_rps_client *rps,
+ bool readonly)
{
- struct drm_device *dev = obj->base.dev;
- struct drm_i915_gem_request *requests[I915_NUM_ENGINES];
struct i915_gem_active *active;
unsigned long active_mask;
- int ret, i, n = 0;
-
- lockdep_assert_held(&dev->struct_mutex);
- GEM_BUG_ON(!to_i915(dev)->mm.interruptible);
+ int idx;

- active_mask = i915_gem_object_get_active(obj);
+ active_mask = __I915_BO_ACTIVE(obj);
if (!active_mask)
return 0;

@@ -374,25 +370,16 @@ i915_gem_object_wait_rendering__nonblocking(struct drm_i915_gem_object *obj,
active = &obj->last_write;
}

- for_each_active(active_mask, i) {
- struct drm_i915_gem_request *req;
+ for_each_active(active_mask, idx) {
+ int ret;

- req = i915_gem_active_get(&active[i],
- &obj->base.dev->struct_mutex);
- if (req)
- requests[n++] = req;
+ ret = i915_gem_active_wait_unlocked(&active[idx],
+ true, NULL, rps);
+ if (ret)
+ return ret;
}

- mutex_unlock(&dev->struct_mutex);
- ret = 0;
- for (i = 0; ret == 0 && i < n; i++)
- ret = i915_wait_request(requests[i], true, NULL, rps);
- mutex_lock(&dev->struct_mutex);
-
- for (i = 0; i < n; i++)
- i915_gem_request_put(requests[i]);
-
- return ret;
+ return 0;
}

static struct intel_rps_client *to_rps_client(struct drm_file *file)
@@ -1461,10 +1448,7 @@ i915_gem_set_domain_ioctl(struct drm_device *dev, void *data,
int ret;

/* Only handle setting domains to types used by the CPU. */
- if (write_domain & I915_GEM_GPU_DOMAINS)
- return -EINVAL;
-
- if (read_domains & I915_GEM_GPU_DOMAINS)
+ if ((write_domain | read_domains) & I915_GEM_GPU_DOMAINS)
return -EINVAL;

/* Having something in the write domain implies it's in the read
@@ -1473,25 +1457,21 @@ i915_gem_set_domain_ioctl(struct drm_device *dev, void *data,
if (write_domain != 0 && read_domains != write_domain)
return -EINVAL;

- ret = i915_mutex_lock_interruptible(dev);
- if (ret)
- return ret;
-
obj = i915_gem_object_lookup(file, args->handle);
- if (!obj) {
- ret = -ENOENT;
- goto unlock;
- }
+ if (!obj)
+ return -ENOENT;

/* Try to flush the object off the GPU without holding the lock.
* We will repeat the flush holding the lock in the normal manner
* to catch cases where we are gazumped.
*/
- ret = i915_gem_object_wait_rendering__nonblocking(obj,
- to_rps_client(file),
- !write_domain);
+ ret = __unsafe_wait_rendering(obj, to_rps_client(file), !write_domain);
if (ret)
- goto unref;
+ goto out_unlocked;
+
+ ret = i915_mutex_lock_interruptible(dev);
+ if (ret)
+ goto out_unlocked;

if (read_domains & I915_GEM_DOMAIN_GTT)
ret = i915_gem_object_set_to_gtt_domain(obj, write_domain != 0);
@@ -1501,11 +1481,13 @@ i915_gem_set_domain_ioctl(struct drm_device *dev, void *data,
if (write_domain != 0)
intel_fb_obj_invalidate(obj, write_origin(obj, write_domain));

-unref:
i915_gem_object_put(obj);
-unlock:
mutex_unlock(&dev->struct_mutex);
return ret;
+
+out_unlocked:
+ i915_gem_object_put_unlocked(obj);
+ return ret;
}

/**
@@ -1647,6 +1629,15 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
int ret = 0;
bool write = !!(vmf->flags & FAULT_FLAG_WRITE);

+ /* Try to flush the object off the GPU first without holding the lock.
+ * Upon acquiring the lock, we will perform our sanity checks and then
+ * repeat the flush holding the lock in the normal manner to catch cases
+ * where we are gazumped.
+ */
+ ret = __unsafe_wait_rendering(obj, NULL, !write);
+ if (ret)
+ goto err;
+
intel_runtime_pm_get(dev_priv);

/* We don't use vmf->pgoff since that has the fake offset */
@@ -1655,23 +1646,14 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)

ret = i915_mutex_lock_interruptible(dev);
if (ret)
- goto out;
+ goto err_rpm;

trace_i915_gem_object_fault(obj, page_offset, true, write);

- /* Try to flush the object off the GPU first without holding the lock.
- * Upon reacquiring the lock, we will perform our sanity checks and then
- * repeat the flush holding the lock in the normal manner to catch cases
- * where we are gazumped.
- */
- ret = i915_gem_object_wait_rendering__nonblocking(obj, NULL, !write);
- if (ret)
- goto unlock;
-
/* Access to snoopable pages through the GTT is incoherent. */
if (obj->cache_level != I915_CACHE_NONE && !HAS_LLC(dev)) {
ret = -EFAULT;
- goto unlock;
+ goto err_unlock;
}

/* Use a partial view if the object is bigger than the aperture. */
@@ -1692,15 +1674,15 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
/* Now pin it into the GTT if needed */
ret = i915_gem_object_ggtt_pin(obj, &view, 0, 0, PIN_MAPPABLE);
if (ret)
- goto unlock;
+ goto err_unlock;

ret = i915_gem_object_set_to_gtt_domain(obj, write);
if (ret)
- goto unpin;
+ goto err_unpin;

ret = i915_gem_object_get_fence(obj);
if (ret)
- goto unpin;
+ goto err_unpin;

/* Finally, remap it using the new GTT offset */
pfn = ggtt->mappable_base +
@@ -1745,11 +1727,13 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
(unsigned long)vmf->virtual_address,
pfn + page_offset);
}
-unpin:
+err_unpin:
i915_gem_object_ggtt_unpin_view(obj, &view);
-unlock:
+err_unlock:
mutex_unlock(&dev->struct_mutex);
-out:
+err_rpm:
+ intel_runtime_pm_put(dev_priv);
+err:
switch (ret) {
case -EIO:
/*
@@ -1790,8 +1774,6 @@ out:
ret = VM_FAULT_SIGBUS;
break;
}
-
- intel_runtime_pm_put(dev_priv);
return ret;
}

--
2.8.1