Re: [PATCH] drm/i915: get runtime PM reference around GEM set_caching IOCTL

Chris Wilson
2015-11-05T11:56:32Z

On Thu, Nov 05, 2015 at 01:28:32PM +0200, Imre Deak wrote:
> On ke, 2015-11-04 at 20:57 +0000, Chris Wilson wrote:
> > On Wed, Nov 04, 2015 at 09:25:32PM +0200, Imre Deak wrote:
> > > After Damien's D3 fix I started to get runtime suspend residency for the
> > > first time and that revealed a breakage on the set_caching IOCTL path
> > > that accesses the HW but doesn't take an RPM ref. Fix this up.
> >
> > Why here (and in so many random places) and not around the PTE write
> > itself?
>
> Imo we should take the RPM ref outside of any of our locks. Otherwise we
> need hacks like we have currently in the runtime suspend handler to work
> around lock inversions. It works now, but we couldn't do the same trick
> if we needed to take struct_mutex for example in the resume handler too
> for some reason.

On the other hand, it leads to hard to track down bugs and improper
documentation. Neither solution is perfect.

Note since intel_runtime_suspend has ample barriers of its own, you can
avoid the struct_mutex if you have a dedicated dev_priv->mm.fault_list.

Something along the lines of:

diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 86734be..fe91ce5 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -180,11 +180,11 @@ describe_obj(struct seq_file *m, struct drm_i915_gem_object *obj)
}
if (obj->stolen)
seq_printf(m, " (stolen: %08llx)", obj->stolen->start);
- if (obj->pin_display || obj->fault_mappable) {
+ if (obj->pin_display || !list_empty(&obj->fault_link)) {
char s[3], *t = s;
if (obj->pin_display)
*t++ = 'p';
- if (obj->fault_mappable)
+ if (!list_empty(&obj->fault_link))
*t++ = 'f';
*t = '\0';
seq_printf(m, " (%s mappable)", s);
@@ -474,7 +474,7 @@ static int i915_gem_object_info(struct seq_file *m, void* data)

size = count = mappable_size = mappable_count = 0;
list_for_each_entry(obj, &dev_priv->mm.bound_list, global_list) {
- if (obj->fault_mappable) {
+ if (!list_empty(&obj->fault_link)) {
size += i915_gem_obj_ggtt_size(obj);
++count;
}
diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index 1d88745..179594e 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -1447,28 +1447,10 @@ static int intel_runtime_suspend(struct device *device)
DRM_DEBUG_KMS("Suspending device\n");

/*
- * We could deadlock here in case another thread holding struct_mutex
- * calls RPM suspend concurrently, since the RPM suspend will wait
- * first for this RPM suspend to finish. In this case the concurrent
- * RPM resume will be followed by its RPM suspend counterpart. Still
- * for consistency return -EAGAIN, which will reschedule this suspend.
- */
- if (!mutex_trylock(&dev->struct_mutex)) {
- DRM_DEBUG_KMS("device lock contention, deffering suspend\n");
- /*
- * Bump the expiration timestamp, otherwise the suspend won't
- * be rescheduled.
- */
- pm_runtime_mark_last_busy(device);
-
- return -EAGAIN;
- }
- /*
* We are safe here against re-faults, since the fault handler takes
* an RPM reference.
*/
i915_gem_release_all_mmaps(dev_priv);
- mutex_unlock(&dev->struct_mutex);

intel_suspend_gt_powersave(dev);
intel_runtime_pm_disable_interrupts(dev_priv);
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 55611d8..5e4904a 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1268,6 +1268,8 @@ struct i915_gem_mm {
*/
struct list_head unbound_list;

+ struct list_head fault_list;
+
/** Usable portion of the GTT for GEM */
unsigned long stolen_base; /* limited to low memory (32-bit) */

@@ -2025,6 +2027,8 @@ struct drm_i915_gem_object {

struct list_head batch_pool_link;

+ struct list_head fault_link;
+
/**
* This is set if the object is on the active lists (has pending
* rendering and so a non-zero seqno), and is not set if it i s on
@@ -2069,13 +2073,6 @@ struct drm_i915_gem_object {
*/
unsigned int map_and_fenceable:1;

- /**
- * Whether the current gtt mapping needs to be mappable (and isn't just
- * mappable by accident). Track pin and fault separate for a more
- * accurate mappable working set.
- */
- unsigned int fault_mappable:1;
-
/*
* Is the object to be mapped as read-only to the GPU
* Only honoured if hardware has relevant pte bit
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 407b6b3..a90d1d8 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1814,9 +1814,10 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
break;
}

- obj->fault_mappable = true;
+ if (list_empty(&obj->fault_link))
+ list_add(&obj->fault_link, &dev_priv->mm.fault_list);
} else {
- if (!obj->fault_mappable) {
+ if (list_empty(&obj->fault_link)) {
unsigned long size = min_t(unsigned long,
vma->vm_end - vma->vm_start,
obj->base.size);
@@ -1830,7 +1831,7 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
break;
}

- obj->fault_mappable = true;
+ list_add(&obj->fault_link, &dev_priv->mm.fault_list);
} else
ret = vm_insert_pfn(vma,
(unsigned long)vmf->virtual_address,
@@ -1903,20 +1904,20 @@ out:
void
i915_gem_release_mmap(struct drm_i915_gem_object *obj)
{
- if (!obj->fault_mappable)
+ if (list_empty(&obj->fault_link))
return;

drm_vma_node_unmap(&obj->base.vma_node,
obj->base.dev->anon_inode->i_mapping);
- obj->fault_mappable = false;
+ list_del_init(&obj->fault_link);
}

void
i915_gem_release_all_mmaps(struct drm_i915_private *dev_priv)
{
- struct drm_i915_gem_object *obj;
+ struct drm_i915_gem_object *obj, *n;

- list_for_each_entry(obj, &dev_priv->mm.bound_list, global_list)
+ list_for_each_entry_safe(obj, n, &dev_priv->mm.fault_list, fault_link)
i915_gem_release_mmap(obj);
}

@@ -4224,6 +4229,7 @@ void i915_gem_object_init(struct drm_i915_gem_object *obj,
INIT_LIST_HEAD(&obj->obj_exec_link);
INIT_LIST_HEAD(&obj->vma_list);
INIT_LIST_HEAD(&obj->batch_pool_link);
+ INIT_LIST_HEAD(&obj->fault_link);

obj->ops = ops;

@@ -4858,6 +4864,7 @@ i915_gem_load(struct drm_device *dev)
INIT_LIST_HEAD(&dev_priv->context_list);
INIT_LIST_HEAD(&dev_priv->mm.unbound_list);
INIT_LIST_HEAD(&dev_priv->mm.bound_list);
+ INIT_LIST_HEAD(&dev_priv->mm.fault_list);
INIT_LIST_HEAD(&dev_priv->mm.fence_list);
for (i = 0; i < I915_NUM_RINGS; i++)
init_ring_lists(&dev_priv->ring[i]);

The tricky part is reviewing the i915_gem_release_mmap() callers to
ensure that they have the right barrier. If you make
i915_gem_release_mmap() assert it holds an rpm ref, and then make the
i915_gem_release_all_mmaps() unlink the node directly that should do the
trick.
-Chris

--
Chris Wilson, InteleOpen Source Technology Centre