drm/i915: Bump signaler priority on adding a waiter

The handling of the no-preemption priority level imposes the restriction
that we need to maintain the implied ordering even though preemption is
disabled. Otherwise we may end up with an AB-BA deadlock across multiple
engine due to a real preemption event reordering the no-preemption
WAITs. To resolve this issue we currently promote all requests to WAIT
on unsubmission, however this interferes with the timeslicing
requirement that we do not apply any implicit promotion that will defeat
the round-robin timeslice list. (If we automatically promote the active
request it will go back to the head of the queue and not the tail!)

So we need implicit promotion to prevent reordering around semaphores
where we are not allowed to preempt, and we must avoid implicit
promotion on unsubmission. So instead of at unsubmit, if we apply that
implicit promotion on adding the dependency, we avoid the semaphore
deadlock and we also reduce the gains made by the promotion for user
space waiting. Furthermore, by keeping the earlier dependencies at a
higher level, we reduce the search space for timeslicing without
altering runtime scheduling too badly (no dependencies at all will be
assigned a higher priority for rrul).

v2: Limit the bump to external edges (as originally intended) i.e.
between contexts and out to the user.

Testcase: igt/gem_concurrent_blit
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190515130052.4475-3-chris@chris-wilson.co.uk
(cherry picked from commit 6e7eb7a80769e7250e31652b96918cf7f3e0d285)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index f6c78c0fa74bf9bc93c6ee60d6c5b6e3272d2670..f258281bf3f3bfcefb77c132df55b0b4db482faa 100644 (file)
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -502,15 +502,6 @@ void __i915_request_unsubmit(struct i915_request *request)
        /* We may be recursing from the signal callback of another i915 fence */
        spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
 
-       /*
-        * As we do not allow WAIT to preempt inflight requests,
-        * once we have executed a request, along with triggering
-        * any execution callbacks, we must preserve its ordering
-        * within the non-preemptible FIFO.
-        */
-       BUILD_BUG_ON(__NO_PREEMPTION & ~I915_PRIORITY_MASK); /* only internal */
-       request->sched.attr.priority |= __NO_PREEMPTION;
-
        if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &request->fence.flags))
                i915_request_cancel_breadcrumb(request);
 

diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
index 3cfadb9db9880fe4d5cfadcad54c12466661ce61..108f52e1bf359dd72248b616dae39c475a5aba52 100644 (file)
--- a/drivers/gpu/drm/i915/i915_scheduler.c
+++ b/drivers/gpu/drm/i915/i915_scheduler.c
@@ -383,6 +383,16 @@ bool __i915_sched_node_add_dependency(struct i915_sched_node *node,
                    !node_started(signal))
                        node->flags |= I915_SCHED_HAS_SEMAPHORE_CHAIN;
 
+               /*
+                * As we do not allow WAIT to preempt inflight requests,
+                * once we have executed a request, along with triggering
+                * any execution callbacks, we must preserve its ordering
+                * within the non-preemptible FIFO.
+                */
+               BUILD_BUG_ON(__NO_PREEMPTION & ~I915_PRIORITY_MASK);
+               if (flags & I915_DEPENDENCY_EXTERNAL)
+                       __bump_priority(signal, __NO_PREEMPTION);
+
                ret = true;
        }
 
@@ -401,6 +411,7 @@ int i915_sched_node_add_dependency(struct i915_sched_node *node,
                return -ENOMEM;
 
        if (!__i915_sched_node_add_dependency(node, signal, dep,
+                                             I915_DEPENDENCY_EXTERNAL |
                                              I915_DEPENDENCY_ALLOC))
                i915_dependency_free(dep);
 

diff --git a/drivers/gpu/drm/i915/i915_scheduler_types.h b/drivers/gpu/drm/i915/i915_scheduler_types.h
index f1af3916a808e6578b4390aa64277bb36a64ec83..4f2b2eb7c3e513d9320845bdad6af31da65355b6 100644 (file)
--- a/drivers/gpu/drm/i915/i915_scheduler_types.h
+++ b/drivers/gpu/drm/i915/i915_scheduler_types.h
@@ -66,7 +66,8 @@ struct i915_dependency {
        struct list_head wait_link;
        struct list_head dfs_link;
        unsigned long flags;
-#define I915_DEPENDENCY_ALLOC BIT(0)
+#define I915_DEPENDENCY_ALLOC          BIT(0)
+#define I915_DEPENDENCY_EXTERNAL       BIT(1)
 };
 
 #endif /* _I915_SCHEDULER_TYPES_H_ */

diff --git a/drivers/gpu/drm/i915/selftests/intel_lrc.c b/drivers/gpu/drm/i915/selftests/intel_lrc.c
index fbee030db940157c5d5a9218905f0557f44fa9d8..e8b0b5dbcb2cb44be448b9781d6d2cbd12ea9750 100644 (file)
--- a/drivers/gpu/drm/i915/selftests/intel_lrc.c
+++ b/drivers/gpu/drm/i915/selftests/intel_lrc.c
@@ -99,12 +99,14 @@ static int live_busywait_preempt(void *arg)
        ctx_hi = kernel_context(i915);
        if (!ctx_hi)
                goto err_unlock;
-       ctx_hi->sched.priority = INT_MAX;
+       ctx_hi->sched.priority =
+               I915_USER_PRIORITY(I915_CONTEXT_MAX_USER_PRIORITY);
 
        ctx_lo = kernel_context(i915);
        if (!ctx_lo)
                goto err_ctx_hi;
-       ctx_lo->sched.priority = INT_MIN;
+       ctx_lo->sched.priority =
+               I915_USER_PRIORITY(I915_CONTEXT_MIN_USER_PRIORITY);
 
        obj = i915_gem_object_create_internal(i915, PAGE_SIZE);
        if (IS_ERR(obj)) {
@@ -954,12 +956,14 @@ static int live_preempt_hang(void *arg)
        ctx_hi = kernel_context(i915);
        if (!ctx_hi)
                goto err_spin_lo;
-       ctx_hi->sched.priority = I915_CONTEXT_MAX_USER_PRIORITY;
+       ctx_hi->sched.priority =
+               I915_USER_PRIORITY(I915_CONTEXT_MAX_USER_PRIORITY);
 
        ctx_lo = kernel_context(i915);
        if (!ctx_lo)
                goto err_ctx_hi;
-       ctx_lo->sched.priority = I915_CONTEXT_MIN_USER_PRIORITY;
+       ctx_lo->sched.priority =
+               I915_USER_PRIORITY(I915_CONTEXT_MIN_USER_PRIORITY);
 
        for_each_engine(engine, i915, id) {
                struct i915_request *rq;