[Intel-gfx] [PATCH] drm/i915: Make the semaphore saturation mask global

[Chris Wilson]

The idea behind keeping the saturation mask local to a context backfired
spectaculary. The premise with the local mask was that we would be more
proactive in attempting to use semaphores everytime after each time th
context idled, and that all new contexts would attempt to use semaphores
ignoring the current state of the system. This turns out to be horribly
optimistic if the system state is still oversaturated and the existing
workloads have all stopped using semaphores. The new workloads would
attempt to use semaphores and be deprioritised behind real work, and
would not switch off using semaphores until their initial batch of
work had completed. Given sufficient backload load of equal user priority,
this would completely starve the new work of any GPU time.

To compensate, remove the local tracking in favour of keeping it as
global state on the engine -- once the system is saturated and
semaphores are disabled, everyone stops attempting to use semaphores
until the system is idle again. One of the reason for preferring local
context tracking was that it worked with virtual engines, so for
switching to global state we could either do a complete check of all the
virtual siblings or simply disable semaphores for those requests. This
takes the simpler approach of disabling semaphores on virtual engines.

The downside is that the decision that the engine is saturated is a
local measure -- we are only checking whether or not this context was
scheduled in a timely fashion, it may be legitimately delayed due to user
priorities. We still have the same dilemma though, that we do not want
to employ the semaphore poll unless it will be used.

Fixes: ca6e56f654e7 ("drm/i915: Disable semaphore busywaits on saturated systems")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin at intel.com>
Cc: Dmitry Ermilov <dmitry.ermilov at intel.com>
---
 drivers/gpu/drm/i915/gt/intel_context.c       | 2 --
 drivers/gpu/drm/i915/gt/intel_context_types.h | 2 --
 drivers/gpu/drm/i915/gt/intel_engine_pm.c     | 2 ++
 drivers/gpu/drm/i915/gt/intel_engine_types.h  | 2 ++
 drivers/gpu/drm/i915/gt/intel_lrc.c           | 2 +-
 drivers/gpu/drm/i915/i915_request.c           | 4 ++--
 6 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
index c78ec0b58e77..7e2b18ddda19 100644
--- a/drivers/gpu/drm/i915/gt/intel_context.c
+++ b/drivers/gpu/drm/i915/gt/intel_context.c
@@ -118,7 +118,6 @@ intel_context_init(struct intel_context *ce,
 	ce->engine = engine;
 	ce->ops = engine->cops;
 	ce->sseu = engine->sseu;
-	ce->saturated = 0;
 
 	INIT_LIST_HEAD(&ce->signal_link);
 	INIT_LIST_HEAD(&ce->signals);
@@ -161,7 +160,6 @@ void intel_context_enter_engine(struct intel_context *ce)
 
 void intel_context_exit_engine(struct intel_context *ce)
 {
-	ce->saturated = 0;
 	intel_engine_pm_put(ce->engine);
 }
 
diff --git a/drivers/gpu/drm/i915/gt/intel_context_types.h b/drivers/gpu/drm/i915/gt/intel_context_types.h
index 825fcf0ac9c4..e47d5b7af5d4 100644
--- a/drivers/gpu/drm/i915/gt/intel_context_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_context_types.h
@@ -53,8 +53,6 @@ struct intel_context {
 	atomic_t pin_count;
 	struct mutex pin_mutex; /* guards pinning and associated on-gpuing */
 
-	intel_engine_mask_t saturated; /* submitting semaphores too late? */
-
 	/**
 	 * active_tracker: Active tracker for the external rq activity
 	 * on this intel_context object.
diff --git a/drivers/gpu/drm/i915/gt/intel_engine_pm.c b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
index ccf034764741..7bffa0c6f5a2 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
+++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
@@ -98,6 +98,8 @@ static int __engine_park(struct intel_wakeref *wf)
 	struct intel_engine_cs *engine =
 		container_of(wf, typeof(*engine), wakeref);
 
+	engine->saturated = 0;
+
 	/*
 	 * If one and only one request is completed between pm events,
 	 * we know that we are inside the kernel context and it is
diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
index 01223864237a..d2dff9056ea0 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
@@ -292,6 +292,8 @@ struct intel_engine_cs {
 	struct intel_context *kernel_context; /* pinned */
 	struct intel_context *preempt_context; /* pinned; optional */
 
+	intel_engine_mask_t saturated; /* submitting semaphores too late? */
+
 	unsigned long serial;
 
 	unsigned long wakeref_serial;
diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index fed704802c57..9ba14fa7b7fb 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -3144,7 +3144,6 @@ static void virtual_context_exit(struct intel_context *ce)
 	struct virtual_engine *ve = container_of(ce, typeof(*ve), context);
 	unsigned int n;
 
-	ce->saturated = 0;
 	for (n = 0; n < ve->num_siblings; n++)
 		intel_engine_pm_put(ve->siblings[n]);
 }
@@ -3339,6 +3338,7 @@ intel_execlists_create_virtual(struct i915_gem_context *ctx,
 	ve->base.uabi_class = I915_ENGINE_CLASS_INVALID;
 	ve->base.instance = I915_ENGINE_CLASS_INVALID_VIRTUAL;
 	ve->base.flags = I915_ENGINE_IS_VIRTUAL;
+	ve->base.saturated = ALL_ENGINES;
 
 	snprintf(ve->base.name, sizeof(ve->base.name), "virtual");
 
diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index da1e6984a8cc..8100ccf04c0b 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -459,7 +459,7 @@ void __i915_request_submit(struct i915_request *request)
 	 */
 	if (request->sched.semaphores &&
 	    i915_sw_fence_signaled(&request->semaphore))
-		request->hw_context->saturated |= request->sched.semaphores;
+		engine->saturated |= request->sched.semaphores;
 
 	/* We may be recursing from the signal callback of another i915 fence */
 	spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
@@ -839,7 +839,7 @@ already_busywaiting(struct i915_request *rq)
 	 *
 	 * See the are-we-too-late? check in __i915_request_submit().
 	 */
-	return rq->sched.semaphores | rq->hw_context->saturated;
+	return rq->sched.semaphores | rq->engine->saturated;
 }
 
 static int
-- 
2.20.1