From: Chris Wilson chris@chris-wilson.co.uk
commit 44d89409a12eb8333735958509d7d591b461d13d upstream.
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.
v2: Explain why we need to assume the worst wrt virtual engines.
Fixes: ca6e56f654e7 ("drm/i915: Disable semaphore busywaits on saturated systems") Signed-off-by: Chris Wilson chris@chris-wilson.co.uk Cc: Tvrtko Ursulin tvrtko.ursulin@intel.com Cc: Dmitry Rogozhkin dmitry.v.rogozhkin@intel.com Cc: Dmitry Ermilov dmitry.ermilov@intel.com Reviewed-by: Tvrtko Ursulin tvrtko.ursulin@intel.com Link: https://patchwork.freedesktop.org/patch/msgid/20190618074153.16055-8-chris@c... Signed-off-by: Joonas Lahtinen joonas.lahtinen@linux.intel.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/gpu/drm/i915/i915_request.c | 4 ++-- drivers/gpu/drm/i915/intel_context.c | 1 - drivers/gpu/drm/i915/intel_context_types.h | 2 -- drivers/gpu/drm/i915/intel_engine_cs.c | 1 + drivers/gpu/drm/i915/intel_engine_types.h | 2 ++ 5 files changed, 5 insertions(+), 5 deletions(-)
--- a/drivers/gpu/drm/i915/i915_request.c +++ b/drivers/gpu/drm/i915/i915_request.c @@ -443,7 +443,7 @@ void __i915_request_submit(struct i915_r */ 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); @@ -829,7 +829,7 @@ already_busywaiting(struct i915_request * * 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 --- a/drivers/gpu/drm/i915/intel_context.c +++ b/drivers/gpu/drm/i915/intel_context.c @@ -230,7 +230,6 @@ intel_context_init(struct intel_context ce->gem_context = ctx; ce->engine = engine; ce->ops = engine->cops; - ce->saturated = 0;
INIT_LIST_HEAD(&ce->signal_link); INIT_LIST_HEAD(&ce->signals); --- a/drivers/gpu/drm/i915/intel_context_types.h +++ b/drivers/gpu/drm/i915/intel_context_types.h @@ -59,8 +59,6 @@ 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. --- a/drivers/gpu/drm/i915/intel_engine_cs.c +++ b/drivers/gpu/drm/i915/intel_engine_cs.c @@ -1200,6 +1200,7 @@ void intel_engines_park(struct drm_i915_
i915_gem_batch_pool_fini(&engine->batch_pool); engine->execlists.no_priolist = false; + engine->saturated = 0; }
i915->gt.active_engines = 0; --- a/drivers/gpu/drm/i915/intel_engine_types.h +++ b/drivers/gpu/drm/i915/intel_engine_types.h @@ -285,6 +285,8 @@ 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? */ + struct drm_i915_gem_object *default_state; void *pinned_default_state;