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

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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
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