Quoting Thomas Hellström (Intel) (2020-06-23 21:31:38)
>
> On 6/23/20 8:41 PM, Chris Wilson wrote:
> > Quoting Thomas Hellström (Intel) (2020-06-23 19:21:28)
> >> On 6/23/20 6:36 PM, Chris Wilson wrote:
> >>> Quoting Thomas Hellström (Intel) (2020-06-23 12:22:11)
> >>>> Hi, Chris,
> >>>>
> >>>> On 6/22/20 11:59 AM, Chris Wilson wrote:
> >>>>> In order to actually handle eviction and what not, we need to process
> >>>>> all the objects together under a common lock, reservation_ww_class. As
> >>>>> such, do a memory reservation pass after looking up the object/vma,
> >>>>> which then feeds into the rest of execbuf [relocation, cmdparsing,
> >>>>> flushing and ofc execution].
> >>>>>
> >>>>> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> >>>>> ---
> >>>>> .../gpu/drm/i915/gem/i915_gem_execbuffer.c | 91 ++++++++++++++-----
> >>>>> 1 file changed, 70 insertions(+), 21 deletions(-)
> >>>>>
> >>>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> >>>>> index 46fcbdf8161c..8db2e013465f 100644
> >>>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> >>>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> >>>>> @@ -53,10 +53,9 @@ struct eb_vma_array {
> >>>>>
> >>>>> #define __EXEC_OBJECT_HAS_PIN BIT(31)
> >>>>> #define __EXEC_OBJECT_HAS_FENCE BIT(30)
> >>>>> -#define __EXEC_OBJECT_HAS_PAGES BIT(29)
> >>>>> -#define __EXEC_OBJECT_NEEDS_MAP BIT(28)
> >>>>> -#define __EXEC_OBJECT_NEEDS_BIAS BIT(27)
> >>>>> -#define __EXEC_OBJECT_INTERNAL_FLAGS (~0u << 27) /* all of the above */
> >>>>> +#define __EXEC_OBJECT_NEEDS_MAP BIT(29)
> >>>>> +#define __EXEC_OBJECT_NEEDS_BIAS BIT(28)
> >>>>> +#define __EXEC_OBJECT_INTERNAL_FLAGS (~0u << 28) /* all of the above */
> >>>>>
> >>>>> #define __EXEC_HAS_RELOC BIT(31)
> >>>>> #define __EXEC_INTERNAL_FLAGS (~0u << 31)
> >>>>> @@ -241,6 +240,8 @@ struct i915_execbuffer {
> >>>>> struct intel_context *context; /* logical state for the request */
> >>>>> struct i915_gem_context *gem_context; /** caller's context */
> >>>>>
> >>>>> + struct dma_fence *mm_fence;
> >>>>> +
> >>>>> struct i915_request *request; /** our request to build */
> >>>>> struct eb_vma *batch; /** identity of the batch obj/vma */
> >>>>> struct i915_vma *trampoline; /** trampoline used for chaining */
> >>>>> @@ -331,12 +332,7 @@ static inline void eb_unreserve_vma(struct eb_vma *ev)
> >>>>> if (ev->flags & __EXEC_OBJECT_HAS_PIN)
> >>>>> __i915_vma_unpin(vma);
> >>>>>
> >>>>> - if (ev->flags & __EXEC_OBJECT_HAS_PAGES)
> >>>>> - i915_gem_object_unpin_pages(vma->obj);
> >>>>> -
> >>>>> - ev->flags &= ~(__EXEC_OBJECT_HAS_PIN |
> >>>>> - __EXEC_OBJECT_HAS_FENCE |
> >>>>> - __EXEC_OBJECT_HAS_PAGES);
> >>>>> + ev->flags &= ~(__EXEC_OBJECT_HAS_PIN | __EXEC_OBJECT_HAS_FENCE);
> >>>>> }
> >>>>>
> >>>>> static void eb_vma_array_destroy(struct kref *kref)
> >>>>> @@ -667,6 +663,55 @@ eb_add_vma(struct i915_execbuffer *eb,
> >>>>> list_add_tail(&ev->lock_link, &eb->lock);
> >>>>> }
> >>>>>
> >>>>> +static int eb_vma_get_pages(struct i915_execbuffer *eb,
> >>>>> + struct eb_vma *ev,
> >>>>> + u64 idx)
> >>>>> +{
> >>>>> + struct i915_vma *vma = ev->vma;
> >>>>> + int err;
> >>>>> +
> >>>>> + /* XXX also preallocate PD for vma */
> >>>>> +
> >>>>> + err = ____i915_gem_object_get_pages_async(vma->obj);
> >>>>> + if (err)
> >>>>> + return err;
> >>>>> +
> >>>>> + return i915_active_ref(&vma->obj->mm.active, idx, eb->mm_fence);
> >>>>> +}
> >>>>> +
> >>>>> +static int eb_reserve_mm(struct i915_execbuffer *eb)
> >>>>> +{
> >>>>> + const u64 idx = eb->context->timeline->fence_context;
> >>>>> + struct ww_acquire_ctx acquire;
> >>>>> + struct eb_vma *ev;
> >>>>> + int err;
> >>>>> +
> >>>>> + eb->mm_fence = __dma_fence_create_proxy(0, 0);
> >>>>> + if (!eb->mm_fence)
> >>>>> + return -ENOMEM;
> >>>> Question: eb is local to this thread, right, so eb->mm_fence is not
> >>>> considered "published" yet?
> >>>>
> >>>>> +
> >>>>> + ww_acquire_init(&acquire, &reservation_ww_class);
> >>>>> +
> >>>>> + err = eb_lock_vma(eb, &acquire);
> >>>>> + if (err)
> >>>>> + goto out;
> >>>>> +
> >>>>> + ww_acquire_done(&acquire);
> >>>>> +
> >>>>> + list_for_each_entry(ev, &eb->lock, lock_link) {
> >>>>> + struct i915_vma *vma = ev->vma;
> >>>>> +
> >>>>> + if (err == 0)
> >>>>> + err = eb_vma_get_pages(eb, ev, idx);
> >>>> I figure this is where you publish the proxy fence? If so, the fence
> >>>> signaling critical path starts with this loop,
> >>> Hmm, actually at this moment, the fence is still very much internal
> >>> being only used as a reference token,
> >> I think as long as another thread, running in this driver or another gpu
> >> driver can theoretically reference the fence pointer from the
> >> reservation object and wait for the fence it's considered published.
> > It's not in the reservation object.
> >
> >> Also the ww_mutexes in this context are really all about grabbing a
> >> random set of resources and associate them with a point in a timeline,
> >> as the ww_mutexes are released, the fence pointer(s) need to point to
> >> published fence(s).
> > That's not the purpose of these fences, though. They exist to provide
> > reference counting on the backing store, along side the migration fence.
> > It's extra detail tacked on the equivalent of bo->moving.
> >
> > That is not to say that one could build up an async migration chain which
> > form a graph back to these, that chain could only be formed once the
> > operation itself has been published in the dma_resv though.
>
> Hmm. So let's say another thread grabs one of the just released
> ww_mutexes and wants to schedule a blit from one of the buffers in the
> current operation with high priority. How would that thread know how to
> order that blit operation w r t the current operation?
Why would it order?
At this moment in time all that has been reserved is the backing store.
Both threads will issue an await on the same fence. As the high priority
thread, it will be scheduled first (providing no other ordering is
imposed). Neither thread will block the other inside execbuf, and their
active reference fence is never published directly (indirect coupling
would be after eviction, and if both threads are holding a reference to
the current backing store they are before that eviction) so I do not see
how even someone else can do a locked wait on their unpublished fences.
After that, it is a race as to which thread hits the implicit fencing
ww_mutex lock. (That is if they are using implicit write fencing on a
shared buffer, a pair of reads to a common buffer are not ordered.) In
which case GEM rules apply, first to install their write fence is ahead
in the queue. So if the high priority arrives after the low priority, the
low priority thread receives the priority boost to high.
-Chris
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