Re: [PATCH v2] drm/i915: Track the last-active inside the i915_vma

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2018-07-05 12:38:46)
> 
> On 04/07/2018 09:34, Chris Wilson wrote:
> > Using a VMA on more than one timeline concurrently is the exception
> > rather than the rule (using it concurrently on multiple engines). As we
> > expect to only use one active tracker, store the most recently used
> > tracker inside the i915_vma itself and only fallback to the rbtree if
> > we need a second or more concurrent active trackers.
> > 
> > v2: Comments on how we overwrite any existing last_active cache.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > ---
> >   drivers/gpu/drm/i915/i915_vma.c | 50 +++++++++++++++++++++++++++++++--
> >   drivers/gpu/drm/i915/i915_vma.h |  1 +
> >   2 files changed, 49 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
> > index cd94ffc7f079..33925e00f7e8 100644
> > --- a/drivers/gpu/drm/i915/i915_vma.c
> > +++ b/drivers/gpu/drm/i915/i915_vma.c
> > @@ -119,6 +119,12 @@ i915_vma_retire(struct i915_gem_active *base, struct i915_request *rq)
> >       __i915_vma_retire(active->vma, rq);
> >   }
> >   
> > +static void
> > +i915_vma_last_retire(struct i915_gem_active *base, struct i915_request *rq)
> > +{
> > +     __i915_vma_retire(container_of(base, struct i915_vma, last_active), rq);
> > +}
> > +
> >   static struct i915_vma *
> >   vma_create(struct drm_i915_gem_object *obj,
> >          struct i915_address_space *vm,
> > @@ -136,6 +142,7 @@ vma_create(struct drm_i915_gem_object *obj,
> >   
> >       vma->active = RB_ROOT;
> >   
> > +     init_request_active(&vma->last_active, i915_vma_last_retire);
> >       init_request_active(&vma->last_fence, NULL);
> >       vma->vm = vm;
> >       vma->ops = &vm->vma_ops;
> > @@ -895,6 +902,22 @@ static struct i915_gem_active *lookup_active(struct i915_vma *vma, u64 idx)
> >   {
> >       struct i915_vma_active *active;
> >       struct rb_node **p, *parent;
> > +     struct i915_request *old;
> > +
> > +     /*
> > +      * We track the most recently used timeline to skip a rbtree search
> > +      * for the common case, under typical loads we never need the rbtree
> > +      * at all. We can reuse the last_active slot if it is empty, that is
> > +      * after the previous activity has been retired, or if the active
> > +      * matches the current timeline.
> > +      */
> > +     old = i915_gem_active_raw(&vma->last_active,
> > +                               &vma->vm->i915->drm.struct_mutex);
> > +     if (!old || old->fence.context == idx)
> > +             goto out;
> 
> Is the situation that retire can be out of order relative to 
> move_to_active? In other words, last_active can retire before the rbtree 
> record, and so the following new move_to_active will find last_active 
> empty and so could create a double entry for the same timeline?

We don't mind a double entry, and do expect that last_active and the
rbtree entry will still be active, tracking different requests.
-Chris
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