Re: [Intel-gfx] [PATCH v2 07/16] drm/i915: Take trylock during eviction, v2.

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

On 07-12-2021 12:01, Matthew Auld wrote:
> On Mon, 29 Nov 2021 at 13:57, Maarten Lankhorst
> <maarten.lankhorst@linux.intel.com> wrote:
>> Now that freeing objects takes the object lock when destroying the
>> backing pages, we can confidently take the object lock even for dead
>> objects.
> That looks to be a future patch, at least with non-TTM backend? Does
> something need to be re-ordered in the series?
>
>> Use this fact to take the object lock in the shrinker, without requiring
>> a reference to the object, so all calls to unbind take the object lock.
> Hmm, the previous patch was talking about "we don't evict when
> refcount = 0", but this looks to be doing something else?

Yeah, a careful inspection of eviction code does allow evicting when refcount = 0 using trylocks, iff we take the object lock during destruction., in order to free all remaining backing.

However, you can't do a blocking lock for dead objects, because VM lock is the inner lock in the hierarchy. Lockdep will complain (rightfully).

To free dead objects to reclaim memory, you would need to call drain_freed_objects(), which will deadlock if you hold either vm lock or resv locks.

>> This is the last step to requiring the object lock for vma_unbind.
>>
>> Changes since v1:
>> - No longer require the refcount, as every freed object now holds the lock
>>   when unbinding VMA's.
>>
>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>> ---
>>  drivers/gpu/drm/i915/gem/i915_gem_shrinker.c |  6 ++++
>>  drivers/gpu/drm/i915/i915_gem_evict.c        | 34 +++++++++++++++++---
>>  2 files changed, 35 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c b/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c
>> index eebff4735781..ad2123369e0d 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shrinker.c
>> @@ -405,12 +405,18 @@ i915_gem_shrinker_vmap(struct notifier_block *nb, unsigned long event, void *ptr
>>         list_for_each_entry_safe(vma, next,
>>                                  &i915->ggtt.vm.bound_list, vm_link) {
>>                 unsigned long count = vma->node.size >> PAGE_SHIFT;
>> +               struct drm_i915_gem_object *obj = vma->obj;
>>
>>                 if (!vma->iomap || i915_vma_is_active(vma))
>>                         continue;
>>
>> +               if (!i915_gem_object_trylock(obj))
>> +                       continue;
>> +
>>                 if (__i915_vma_unbind(vma) == 0)
>>                         freed_pages += count;
>> +
>> +               i915_gem_object_unlock(obj);
>>         }
>>         mutex_unlock(&i915->ggtt.vm.mutex);
>>
>> diff --git a/drivers/gpu/drm/i915/i915_gem_evict.c b/drivers/gpu/drm/i915/i915_gem_evict.c
>> index 2b73ddb11c66..286efa462eca 100644
>> --- a/drivers/gpu/drm/i915/i915_gem_evict.c
>> +++ b/drivers/gpu/drm/i915/i915_gem_evict.c
>> @@ -58,6 +58,9 @@ mark_free(struct drm_mm_scan *scan,
>>         if (i915_vma_is_pinned(vma))
>>                 return false;
>>
>> +       if (!i915_gem_object_trylock(vma->obj))
>> +               return false;
>> +
>>         list_add(&vma->evict_link, unwind);
>>         return drm_mm_scan_add_block(scan, &vma->node);
>>  }
>> @@ -178,6 +181,7 @@ i915_gem_evict_something(struct i915_address_space *vm,
>>         list_for_each_entry_safe(vma, next, &eviction_list, evict_link) {
>>                 ret = drm_mm_scan_remove_block(&scan, &vma->node);
>>                 BUG_ON(ret);
>> +               i915_gem_object_unlock(vma->obj);
>>         }
>>
>>         /*
>> @@ -222,10 +226,12 @@ i915_gem_evict_something(struct i915_address_space *vm,
>>          * of any of our objects, thus corrupting the list).
>>          */
>>         list_for_each_entry_safe(vma, next, &eviction_list, evict_link) {
>> -               if (drm_mm_scan_remove_block(&scan, &vma->node))
>> +               if (drm_mm_scan_remove_block(&scan, &vma->node)) {
>>                         __i915_vma_pin(vma);
>> -               else
>> +               } else {
>>                         list_del(&vma->evict_link);
>> +                       i915_gem_object_unlock(vma->obj);
>> +               }
>>         }
>>
>>         /* Unbinding will emit any required flushes */
>> @@ -234,16 +240,22 @@ i915_gem_evict_something(struct i915_address_space *vm,
>>                 __i915_vma_unpin(vma);
>>                 if (ret == 0)
>>                         ret = __i915_vma_unbind(vma);
>> +
>> +               i915_gem_object_unlock(vma->obj);
>>         }
>>
>>         while (ret == 0 && (node = drm_mm_scan_color_evict(&scan))) {
>>                 vma = container_of(node, struct i915_vma, node);
>>
>> +
>>                 /* If we find any non-objects (!vma), we cannot evict them */
>> -               if (vma->node.color != I915_COLOR_UNEVICTABLE)
>> +               if (vma->node.color != I915_COLOR_UNEVICTABLE &&
>> +                   i915_gem_object_trylock(vma->obj)) {
>>                         ret = __i915_vma_unbind(vma);
>> -               else
>> -                       ret = -ENOSPC; /* XXX search failed, try again? */
>> +                       i915_gem_object_unlock(vma->obj);
>> +               } else {
>> +                       ret = -ENOSPC;
>> +               }
>>         }
>>
>>         return ret;
>> @@ -333,6 +345,11 @@ int i915_gem_evict_for_node(struct i915_address_space *vm,
>>                         break;
>>                 }
>>
>> +               if (!i915_gem_object_trylock(vma->obj)) {
>> +                       ret = -ENOSPC;
>> +                       break;
>> +               }
>> +
>>                 /*
>>                  * Never show fear in the face of dragons!
>>                  *
>> @@ -350,6 +367,8 @@ int i915_gem_evict_for_node(struct i915_address_space *vm,
>>                 __i915_vma_unpin(vma);
>>                 if (ret == 0)
>>                         ret = __i915_vma_unbind(vma);
>> +
>> +               i915_gem_object_unlock(vma->obj);
>>         }
>>
>>         return ret;
>> @@ -393,6 +412,9 @@ int i915_gem_evict_vm(struct i915_address_space *vm)
>>                         if (i915_vma_is_pinned(vma))
>>                                 continue;
>>
>> +                       if (!i915_gem_object_trylock(vma->obj))
>> +                               continue;
>> +
>>                         __i915_vma_pin(vma);
>>                         list_add(&vma->evict_link, &eviction_list);
>>                 }
>> @@ -406,6 +428,8 @@ int i915_gem_evict_vm(struct i915_address_space *vm)
>>                                 ret = __i915_vma_unbind(vma);
>>                         if (ret != -EINTR) /* "Get me out of here!" */
>>                                 ret = 0;
>> +
>> +                       i915_gem_object_unlock(vma->obj);
>>                 }
>>         } while (ret == 0);
>>
>> --
>> 2.34.0
>>