In order to avoid preempting ourselves, we currently refuse to schedule
the tasklet if we reschedule an inflight context. However, this glosses
over a few issues such as what happens after a CS completion event and
we then preempt the newly executing context with itself, or if something
else causes a tasklet_schedule triggering the same evaluation to
preempt the active context with itself.
To avoid the extra complications, after deciding that we have
potentially queued a request with higher priority than the currently
executing request, inspect the head of the queue to see if it is indeed
higher priority from another context.
v2: We can simplify a bunch of tests based on the knowledge that PI will
ensure that earlier requests along the same context will have the highest
priority.
References: a2bf92e8cc16 ("drm/i915/execlists: Avoid kicking priority on the current context")
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/i915_scheduler.c | 20 ++++--
drivers/gpu/drm/i915/intel_lrc.c | 91 ++++++++++++++++++++++++---
2 files changed, 100 insertions(+), 11 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
index 340faea6c08a..fb5d953430e5 100644
--- a/drivers/gpu/drm/i915/i915_scheduler.c
+++ b/drivers/gpu/drm/i915/i915_scheduler.c
@@ -239,6 +239,18 @@ sched_lock_engine(struct i915_sched_node *node, struct intel_engine_cs *locked)
return engine;
}
+static bool inflight(const struct i915_request *rq,
+ const struct intel_engine_cs *engine)
+{
+ const struct i915_request *active;
+
+ if (!rq->global_seqno)
+ return false;
+
+ active = port_request(engine->execlists.port);
+ return active->hw_context == rq->hw_context;
+}
+
static void __i915_schedule(struct i915_request *rq,
const struct i915_sched_attr *attr)
{
@@ -328,6 +340,7 @@ static void __i915_schedule(struct i915_request *rq,
INIT_LIST_HEAD(&dep->dfs_link);
engine = sched_lock_engine(node, engine);
+ lockdep_assert_held(&engine->timeline.lock);
/* Recheck after acquiring the engine->timeline.lock */
if (prio <= node->attr.priority || node_signaled(node))
@@ -356,17 +369,16 @@ static void __i915_schedule(struct i915_request *rq,
if (prio <= engine->execlists.queue_priority)
continue;
+ engine->execlists.queue_priority = prio;
+
/*
* If we are already the currently executing context, don't
* bother evaluating if we should preempt ourselves.
*/
- if (node_to_request(node)->global_seqno &&
- i915_seqno_passed(port_request(engine->execlists.port)->global_seqno,
- node_to_request(node)->global_seqno))
+ if (inflight(node_to_request(node), engine))
continue;
/* Defer (tasklet) submission until after all of our updates. */
- engine->execlists.queue_priority = prio;
tasklet_hi_schedule(&engine->execlists.tasklet);
}
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 8aa8a4862543..b61235304734 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -181,13 +181,89 @@ static inline int rq_prio(const struct i915_request *rq)
return rq->sched.attr.priority;
}
+static int queue_prio(const struct intel_engine_execlists *execlists)
+{
+ struct i915_priolist *p;
+ struct rb_node *rb;
+
+ rb = rb_first_cached(&execlists->queue);
+ if (!rb)
+ return INT_MIN;
+
+ /*
+ * As the priolist[] are inverted, with the highest priority in [0],
+ * we have to flip the index value to become priority.
+ */
+ p = to_priolist(rb);
+ return ((p->priority + 1) << I915_USER_PRIORITY_SHIFT) - ffs(p->used);
+}
+
static inline bool need_preempt(const struct intel_engine_cs *engine,
- const struct i915_request *last,
- int prio)
+ const struct i915_request *rq,
+ int q_prio)
{
- return (intel_engine_has_preemption(engine) &&
- __execlists_need_preempt(prio, rq_prio(last)) &&
- !i915_request_completed(last));
+ const struct intel_context *ctx = rq->hw_context;
+ const int last_prio = rq_prio(rq);
+
+ if (!intel_engine_has_preemption(engine))
+ return false;
+
+ if (i915_request_completed(rq))
+ return false;
+
+ /*
+ * Check if the current queue_priority merits a preemption attempt.
+ *
+ * However, the queue_priority is a mere hint that we may need to
+ * preempt. If that hint is stale or we may be trying to preempt
+ * ourselves, ignore the request.
+ */
+ if (!__execlists_need_preempt(q_prio, last_prio))
+ return false;
+
+ /*
+ * Check against the first request in ELSP[1], it will, thanks to the
+ * power of PI, be the highest priority of that context.
+ */
+ if (!list_is_last(&rq->link, &engine->timeline.requests)) {
+ rq = list_next_entry(rq, link);
+ GEM_BUG_ON(rq->hw_context == ctx);
+ if (rq_prio(rq) > last_prio)
+ return true;
+ }
+
+ /*
+ * If the inflight context did not trigger the preemption, then maybe
+ * it was the set of queued requests? Pick the highest priority in
+ * the queue (the first active priolist) and see if it deserves to be
+ * running instead of ELSP[0].
+ *
+ * The highest priority request in the queue can not be either
+ * ELSP[0] or ELSP[1] as, thanks again to PI, if it was the same
+ * context, it's priority would not exceed ELSP[0] aka last_prio.
+ */
+ return queue_prio(&engine->execlists) > last_prio;
+}
+
+__maybe_unused static inline bool
+assert_priority_queue(const struct intel_engine_execlists *execlists,
+ const struct i915_request *prev,
+ const struct i915_request *next)
+{
+ if (!prev)
+ return true;
+
+ /*
+ * Without preemption, the prev may refer to the still active element
+ * which we refuse to let go.
+ *
+ * Even with premption, there are times when we think it is better not
+ * to preempt and leave an ostensibly lower priority request in flight.
+ */
+ if (port_request(execlists->port) == prev)
+ return true;
+
+ return rq_prio(prev) >= rq_prio(next);
}
/*
@@ -626,8 +702,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
int i;
priolist_for_each_request_consume(rq, rn, p, i) {
- GEM_BUG_ON(last &&
- need_preempt(engine, last, rq_prio(rq)));
+ GEM_BUG_ON(!assert_priority_queue(execlists, last, rq));
/*
* Can we combine this request with the current port?
@@ -872,6 +947,8 @@ static void process_csb(struct intel_engine_cs *engine)
const u32 * const buf = execlists->csb_status;
u8 head, tail;
+ lockdep_assert_held(&engine->timeline.lock);
+
/*
* Note that csb_write, csb_status may be either in HWSP or mmio.
* When reading from the csb_write mmio register, we have to be
--
2.20.1
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