[PATCH 17/25] drm/i915/execlists: Suppress redundant preemption

Chris Wilson
2019-02-19T12:22:44Z

On unwinding the active request we give it a small (limited to internal
priority levels) boost to prevent it from being gazumped a second time.
However, this means that it can be promoted to above the request that
triggered the preemption request, causing a preempt-to-idle cycle for no
change. We can avoid this if we take the boost into account when
checking if the preemption request is valid.

v2: After preemption the active request will be after the preemptee if
they end up with equal priority.

v3: Tvrtko pointed out that this, the existing logic, makes
I915_PRIORITY_WAIT non-preemptible. Document this interesting quirk!

v4: Prove Tvrtko was right about WAIT being non-preemptible and test it.
v5: Except not all priorities were made equal, and the WAIT not preempting
is only if we start off as !NEWCLIENT.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
---
drivers/gpu/drm/i915/intel_lrc.c | 38 ++++++++++++++++++++++++++++----
1 file changed, 34 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index ed53de5335c3..5a3bbf4ded35 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -164,6 +164,8 @@
#define WA_TAIL_DWORDS 2
#define WA_TAIL_BYTES (sizeof(u32) * WA_TAIL_DWORDS)

+#define ACTIVE_PRIORITY (I915_PRIORITY_NEWCLIENT)
+
static int execlists_context_deferred_alloc(struct i915_gem_context *ctx,
struct intel_engine_cs *engine,
struct intel_context *ce);
@@ -190,8 +192,30 @@ static inline int rq_prio(const struct i915_request *rq)

static int effective_prio(const struct i915_request *rq)
{
+ int prio = rq_prio(rq);
+
+ /*
+ * On unwinding the active request, we give it a priority bump
+ * equivalent to a freshly submitted request. This protects it from
+ * being gazumped again, but it would be preferable if we didn't
+ * let it be gazumped in the first place!
+ *
+ * See __unwind_incomplete_requests()
+ */
+ if (~prio & ACTIVE_PRIORITY && __i915_request_has_started(rq)) {
+ /*
+ * After preemption, we insert the active request at the
+ * end of the new priority level. This means that we will be
+ * _lower_ priority than the preemptee all things equal (and
+ * so the preemption is valid), so adjust our comparison
+ * accordingly.
+ */
+ prio |= ACTIVE_PRIORITY;
+ prio--;
+ }
+
/* Restrict mere WAIT boosts from triggering preemption */
- return rq_prio(rq) | __NO_PREEMPTION;
+ return prio | __NO_PREEMPTION;
}

static int queue_prio(const struct intel_engine_execlists *execlists)
@@ -359,7 +383,7 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
{
struct i915_request *rq, *rn, *active = NULL;
struct list_head *uninitialized_var(pl);
- int prio = I915_PRIORITY_INVALID | I915_PRIORITY_NEWCLIENT;
+ int prio = I915_PRIORITY_INVALID | ACTIVE_PRIORITY;

lockdep_assert_held(&engine->timeline.lock);

@@ -390,9 +414,15 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
* The active request is now effectively the start of a new client
* stream, so give it the equivalent small priority bump to prevent
* it being gazumped a second time by another peer.
+ *
+ * One consequence of this preemption boost is that we may jump
+ * over lesser priorities (such as I915_PRIORITY_WAIT), effectively
+ * making those priorities non-preemptible. They will be moved forward
+ * in the priority queue, but they will not gain immediate access to
+ * the GPU.
*/
- if (!(prio & I915_PRIORITY_NEWCLIENT)) {
- prio |= I915_PRIORITY_NEWCLIENT;
+ if (~prio & ACTIVE_PRIORITY && __i915_request_has_started(active)) {
+ prio |= ACTIVE_PRIORITY;
active->sched.attr.priority = prio;
list_move_tail(&active->sched.link,
i915_sched_lookup_priolist(engine, prio));