[Intel-gfx] [PATCH 17/39] drm/i915: Make the semaphore saturation mask global

[Chris Wilson]

The idea behind keeping the saturation mask local to a context backfired
spectacularly. The premise with the local mask was that we would be more
proactive in attempting to use semaphores after each time the 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. The
new contexts would not switch off using semaphores until their initial
batch of low priority 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 8e299c631575..3d660c8ce414 100644
--- a/drivers/gpu/drm/i915/gt/intel_context.c
+++ b/drivers/gpu/drm/i915/gt/intel_context.c
@@ -142,7 +142,6 @@ 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);
@@ -223,7 +222,6 @@ 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 b565c3ff4378..4c0e211c715d 100644
--- a/drivers/gpu/drm/i915/gt/intel_context_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_context_types.h
@@ -58,8 +58,6 @@ 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: Active tracker for the rq activity (inc. external) 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 3c448a061abd..25287140fb61 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
+++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
@@ -100,6 +100,8 @@ 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 11a25f060fed..1cbe10a0fec7 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
@@ -258,6 +258,8 @@ 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 0563fe8398c5..bbbdc63906c6 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -3198,7 +3198,6 @@ 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]);
 }
@@ -3397,6 +3396,7 @@ 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 a82f3ff82b39..865b49928ff9 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -418,7 +418,7 @@ 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);
@@ -799,7 +799,7 @@ 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