Quoting Tvrtko Ursulin (2021-01-26 16:42:37)
>
> On 26/01/2021 16:26, Chris Wilson wrote:
> > 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?
>
> At some point I wanted to suggest you change dfs.list_head abuse to
> explicit rq and list head pointer to better represent how there are two
> pieces of information tracked in there.
Ok. While writing it I thought some places continued to use it as a
struct list_head, but it appears that this is the only user.
I'll give it a whirl.
-Chris
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