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authorChris Wilson <chris@chris-wilson.co.uk>2019-03-01 17:08:58 +0000
committerChris Wilson <chris@chris-wilson.co.uk>2019-03-01 17:40:32 +0000
commit1e3f697e47f61293351c72c35e3045b1774851c2 (patch)
tree941cc80dfc187a58352b5855f189a817e3548f0d
parent34ae8455f4d30ddc7c26d914d0f246de37488a99 (diff)
downloadlinux-1e3f697e47f61293351c72c35e3045b1774851c2.tar.bz2
drm/i915/execlists: Suppress redundant preemption
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. v6: More commentary after coming to an understanding about what I had forgotten to say. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20190301170901.8340-1-chris@chris-wilson.co.uk
-rw-r--r--drivers/gpu/drm/i915/intel_lrc.c44
1 files changed, 40 insertions, 4 deletions
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 4f2187aa44e4..3fd0c45a2920 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 +414,21 @@ __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.
+ *
+ * Note we have to be careful not to apply a priority boost to a request
+ * still spinning on its semaphores. If the request hasn't started, that
+ * means it is still waiting for its dependencies to be signaled, and
+ * if we apply a priority boost to this request, we will boost it past
+ * its signalers and so break PI.
+ *
+ * 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));