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@xxxxxxxxxxxxxxxxxx>
> > ---
> > 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
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