[Intel-gfx] [PATCH 08/41] drm/i915: Improve DFS for priority inheritance

[Chris Wilson]

Quoting Tvrtko Ursulin (2021-01-26 16:22:58)
> 
> 
> On 25/01/2021 14:01, Chris Wilson wrote:
> > The core of the scheduling algorithm is that we compute the topological
> > order of the fence DAG. Knowing that we have a DAG, we should be able to
> > use a DFS to compute the topological sort in linear time. However,
> > during the conversion of the recursive algorithm into an iterative one,
> > the memoization of how far we had progressed down a branch was
> > forgotten. The result was that instead of running in linear time, it was
> > running in geometric time and could easily run for a few hundred
> > milliseconds given a wide enough graph, not the microseconds as required.
> > 
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > ---
> >   drivers/gpu/drm/i915/i915_scheduler.c | 58 ++++++++++++++++-----------
> >   1 file changed, 34 insertions(+), 24 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
> > index 4802c9b1081d..9139a91f0aa3 100644
> > --- a/drivers/gpu/drm/i915/i915_scheduler.c
> > +++ b/drivers/gpu/drm/i915/i915_scheduler.c
> > @@ -234,6 +234,26 @@ void __i915_priolist_free(struct i915_priolist *p)
> >       kmem_cache_free(global.slab_priorities, p);
> >   }
> >   
> > +static struct i915_request *
> > +stack_push(struct i915_request *rq,
> > +        struct i915_request *stack,
> > +        struct list_head *pos)
> > +{
> > +     stack->sched.dfs.prev = pos;
> > +     rq->sched.dfs.next = (struct list_head *)stack;
> > +     return rq;
> > +}
> > +
> > +static struct i915_request *
> > +stack_pop(struct i915_request *rq,
> > +       struct list_head **pos)
> > +{
> > +     rq = (struct i915_request *)rq->sched.dfs.next;
> > +     if (rq)
> > +             *pos = rq->sched.dfs.prev;
> > +     return rq;
> > +}
> > +
> >   static inline bool need_preempt(int prio, int active)
> >   {
> >       /*
> > @@ -298,11 +318,10 @@ static void ipi_priority(struct i915_request *rq, int prio)
> >   static void __i915_request_set_priority(struct i915_request *rq, int prio)
> >   {
> >       struct intel_engine_cs *engine = rq->engine;
> > -     struct i915_request *rn;
> > +     struct list_head *pos = &rq->sched.signalers_list;
> >       struct list_head *plist;
> > -     LIST_HEAD(dfs);
> >   
> > -     list_add(&rq->sched.dfs, &dfs);
> > +     plist = i915_sched_lookup_priolist(engine, prio);
> >   
> >       /*
> >        * Recursively bump all dependent priorities to match the new request.
> > @@ -322,40 +341,31 @@ static void __i915_request_set_priority(struct i915_request *rq, int prio)
> >        * end result is a topological list of requests in reverse order, the
> >        * last element in the list is the request we must execute first.
> >        */
> > -     list_for_each_entry(rq, &dfs, sched.dfs) {
> > -             struct i915_dependency *p;
> > -
> > -             /* Also release any children on this engine that are ready */
> > -             GEM_BUG_ON(rq->engine != engine);
> > -
> > -             for_each_signaler(p, rq) {
> > +     rq->sched.dfs.next = NULL;
> > +     do {
> > +             list_for_each_continue(pos, &rq->sched.signalers_list) {
> > +                     struct i915_dependency *p =
> > +                             list_entry(pos, typeof(*p), signal_link);
> >                       struct i915_request *s =
> >                               container_of(p->signaler, typeof(*s), sched);
> >   
> > -                     GEM_BUG_ON(s == rq);
> > -
> >                       if (rq_prio(s) >= prio)
> >                               continue;
> >   
> >                       if (__i915_request_is_complete(s))
> >                               continue;
> >   
> > -                     if (s->engine != rq->engine) {
> > +                     if (s->engine != engine) {
> >                               ipi_priority(s, prio);
> >                               continue;
> >                       }
> >   
> > -                     list_move_tail(&s->sched.dfs, &dfs);
> > +                     /* Remember our position along this branch */
> > +                     rq = stack_push(s, rq, pos);
> > +                     pos = &rq->sched.signalers_list;
> >               }
> > -     }
> >   
> > -     plist = i915_sched_lookup_priolist(engine, prio);
> > -
> > -     /* Fifo and depth-first replacement ensure our deps execute first */
> > -     list_for_each_entry_safe_reverse(rq, rn, &dfs, sched.dfs) {
> > -             GEM_BUG_ON(rq->engine != engine);
> > -
> > -             INIT_LIST_HEAD(&rq->sched.dfs);
> > +             RQ_TRACE(rq, "set-priority:%d\n", prio);
> >               WRITE_ONCE(rq->sched.attr.priority, prio);
> >   
> >               /*
> > @@ -369,12 +379,13 @@ static void __i915_request_set_priority(struct i915_request *rq, int prio)
> >               if (!i915_request_is_ready(rq))
> >                       continue;
> >   
> > +             GEM_BUG_ON(rq->engine != engine);
> >               if (i915_request_in_priority_queue(rq))
> >                       list_move_tail(&rq->sched.link, plist);
> >   
> >               /* Defer (tasklet) submission until after all updates. */
> >               kick_submission(engine, rq, prio);
> > -     }
> > +     } while ((rq = stack_pop(rq, &pos)));
> >   }
> >   
> >   void i915_request_set_priority(struct i915_request *rq, int prio)
> > @@ -444,7 +455,6 @@ void i915_sched_node_init(struct i915_sched_node *node)
> >       INIT_LIST_HEAD(&node->signalers_list);
> >       INIT_LIST_HEAD(&node->waiters_list);
> >       INIT_LIST_HEAD(&node->link);
> > -     INIT_LIST_HEAD(&node->dfs);
> >   
> >       node->ipi_link = NULL;
> >   
> > 
> 
> Pen and paper was needed here but it looks good.

If you highlight the areas that need more commentary, I guess
a theory-of-operation for stack_push/stack_pop?
-Chris