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

Imre Deak
2015-11-05T22:57:44Z

On Thu, 2015-11-05 at 11:56 +0000, Chris Wilson wrote:
> 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.

I think I understand the documentation part, not sure how that could be
solved. Perhaps RPM-held asserts right before the point where the HW
needs to be on?

What do you mean by hard to track down bugs? Couldn't that also be
tackled by asserts?

> 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:

Ok, looks interesting. But as you said we would have to then make
callers of i915_gem_release_mmap() wake up the device, which isn't
strictly necessary. (and imo as such goes somewhat against the
documentation argument)

--Imre

> 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