On 16/07/2020 12:33, Chris Wilson wrote:
Since the breadcrumb enabling/cancelling itself is serialised by the
breadcrumbs.irq_lock, with a bit of care we can remove the outer
serialisation with i915_request.lock for concurrent
dma_fence_enable_signaling(). This has the important side-effect of
eliminating the nested i915_request.lock within request submission.
The challenge in serialisation is around the unsubmission where we take
an active request that wants a breadcrumb on the signaling engine and
put it to sleep. We do not want a concurrent
dma_fence_enable_signaling() to attach a breadcrumb as we unsubmit, so
we must mark the request as no longer active before serialising with the
concurrent enable-signaling.
On retire, we serialise with the concurrent enable-signaling, but
instead of clearing ACTIVE, we mark it as SIGNALED.
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 130 +++++++++++++-------
drivers/gpu/drm/i915/gt/intel_lrc.c | 14 ---
drivers/gpu/drm/i915/i915_request.c | 39 +++---
3 files changed, 100 insertions(+), 83 deletions(-)
diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
index 91786310c114..a0f52417238c 100644
--- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
+++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
@@ -220,17 +220,17 @@ static void signal_irq_work(struct irq_work *work)
}
}
-static bool __intel_breadcrumbs_arm_irq(struct intel_breadcrumbs *b)
+static void __intel_breadcrumbs_arm_irq(struct intel_breadcrumbs *b)
{
struct intel_engine_cs *engine =
container_of(b, struct intel_engine_cs, breadcrumbs);
lockdep_assert_held(&b->irq_lock);
if (b->irq_armed)
- return true;
+ return;
if (!intel_gt_pm_get_if_awake(engine->gt))
- return false;
+ return;
/*
* The breadcrumb irq will be disarmed on the interrupt after the
@@ -250,8 +250,6 @@ static bool __intel_breadcrumbs_arm_irq(struct intel_breadcrumbs *b)
if (!b->irq_enabled++)
irq_enable(engine);
-
- return true;
}
void intel_engine_init_breadcrumbs(struct intel_engine_cs *engine)
@@ -310,57 +308,99 @@ void intel_engine_fini_breadcrumbs(struct intel_engine_cs *engine)
{
}
-bool i915_request_enable_breadcrumb(struct i915_request *rq)
+static void insert_breadcrumb(struct i915_request *rq,
+ struct intel_breadcrumbs *b)
{
- lockdep_assert_held(&rq->lock);
+ struct intel_context *ce = rq->context;
+ struct list_head *pos;
- if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &rq->fence.flags))
- return true;
+ if (test_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags))
+ return;
- if (test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags)) {
- struct intel_breadcrumbs *b = &rq->engine->breadcrumbs;
- struct intel_context *ce = rq->context;
- struct list_head *pos;
+ __intel_breadcrumbs_arm_irq(b);
- spin_lock(&b->irq_lock);
+ /*
+ * We keep the seqno in retirement order, so we can break
+ * inside intel_engine_signal_breadcrumbs as soon as we've
+ * passed the last completed request (or seen a request that
+ * hasn't event started). We could walk the timeline->requests,
+ * but keeping a separate signalers_list has the advantage of
+ * hopefully being much smaller than the full list and so
+ * provides faster iteration and detection when there are no
+ * more interrupts required for this context.
+ *
+ * We typically expect to add new signalers in order, so we
+ * start looking for our insertion point from the tail of
+ * the list.
+ */
+ list_for_each_prev(pos, &ce->signals) {
+ struct i915_request *it =
+ list_entry(pos, typeof(*it), signal_link);
- if (test_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags))
- goto unlock;
+ if (i915_seqno_passed(rq->fence.seqno, it->fence.seqno))
+ break;
+ }
+ list_add(&rq->signal_link, pos);
+ if (pos == &ce->signals) /* catch transitions from empty list */
+ list_move_tail(&ce->signal_link, &b->signalers);
+ GEM_BUG_ON(!check_signal_order(ce, rq));
- if (!__intel_breadcrumbs_arm_irq(b))
- goto unlock;
+ set_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
+}
- /*
- * We keep the seqno in retirement order, so we can break
- * inside intel_engine_signal_breadcrumbs as soon as we've
- * passed the last completed request (or seen a request that
- * hasn't event started). We could walk the timeline->requests,
- * but keeping a separate signalers_list has the advantage of
- * hopefully being much smaller than the full list and so
- * provides faster iteration and detection when there are no
- * more interrupts required for this context.
- *
- * We typically expect to add new signalers in order, so we
- * start looking for our insertion point from the tail of
- * the list.
- */
- list_for_each_prev(pos, &ce->signals) {
- struct i915_request *it =
- list_entry(pos, typeof(*it), signal_link);
+bool i915_request_enable_breadcrumb(struct i915_request *rq)
+{
+ struct intel_breadcrumbs *b;
- if (i915_seqno_passed(rq->fence.seqno, it->fence.seqno))
- break;
- }
- list_add(&rq->signal_link, pos);
- if (pos == &ce->signals) /* catch transitions from empty list */
- list_move_tail(&ce->signal_link, &b->signalers);
- GEM_BUG_ON(!check_signal_order(ce, rq));
+ /* Seralises with i915_request_retire() using rq->lock */
Serialises
+ if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &rq->fence.flags))
+ return true;
- set_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
-unlock:
+ /*
+ * Peek at i915_request_submit()/i915_request_unsubmit() status.
+ *
+ * If the request is not yet active (and not signaled), we will
+ * attach the breadcrumb later.
+ */
+ if (!test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags))
+ return true;
+
+ /*
+ * rq->engine is locked by rq->engine->active.lock. That however
+ * is not known until after rq->engine has been dereferenced and
+ * the lock acquired. Hence we acquire the lock and then validate
+ * that rq->engine still matches the lock we hold for it.
+ *
+ * Here, we are using the breadcrumb lock as a proxy for the
+ * rq->engine->active.lock, and we know that since the breadcrumb
+ * will be serialised within i915_request_submit/i915_request_unsubmit,
+ * the engine cannot change while active as long as we hold the
+ * breadcrumb lock on that engine.
+ *
+ * From the dma_fence_enable_signaling() path, we are outside of the
+ * request submit/unsubmit path, and so we must be more careful to
+ * acquire the right lock.
+ */
+ b = &READ_ONCE(rq->engine)->breadcrumbs;
+ spin_lock(&b->irq_lock);
+ while (unlikely(b != &READ_ONCE(rq->engine)->breadcrumbs)) {
spin_unlock(&b->irq_lock);
+ b = &READ_ONCE(rq->engine)->breadcrumbs;
+ spin_lock(&b->irq_lock);
}
+ /*
+ * Now that we are finally serialised with request submit/unsubmit,
+ * [with b->irq_lock] and with i915_request_reitre() [via checking
retire
+ * SIGNALED with rq->lock] confirm the request is indeed active. If
+ * it is no longer active, the breadcrumb will be attached upon
+ * i915_request_submit().
+ */
+ if (test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags))
+ insert_breadcrumb(rq, b);
+
+ spin_unlock(&b->irq_lock);
+
return !__request_completed(rq);
}
@@ -368,8 +408,6 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq)
{
struct intel_breadcrumbs *b = &rq->engine->breadcrumbs;
- lockdep_assert_held(&rq->lock);
-
/*
* We must wait for b->irq_lock so that we know the interrupt handler
* has released its reference to the intel_context and has completed
diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 29c0fde8b4df..21c16e31c4fe 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -1148,20 +1148,6 @@ __unwind_incomplete_requests(struct intel_engine_cs *engine)
} else {
struct intel_engine_cs *owner = rq->context->engine;
- /*
- * Decouple the virtual breadcrumb before moving it
- * back to the virtual engine -- we don't want the
- * request to complete in the background and try
- * and cancel the breadcrumb on the virtual engine
- * (instead of the old engine where it is linked)!
- */
- if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
- &rq->fence.flags)) {
- spin_lock_nested(&rq->lock,
- SINGLE_DEPTH_NESTING);
- i915_request_cancel_breadcrumb(rq);
- spin_unlock(&rq->lock);
- }
WRITE_ONCE(rq->engine, owner);
owner->submit_request(rq);
active = NULL;
diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index 2ef17b11ca4b..8c345ead04a6 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -320,11 +320,12 @@ bool i915_request_retire(struct i915_request *rq)
dma_fence_signal_locked(&rq->fence);
if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &rq->fence.flags))
i915_request_cancel_breadcrumb(rq);
+ spin_unlock_irq(&rq->lock);
+
if (i915_request_has_waitboost(rq)) {
GEM_BUG_ON(!atomic_read(&rq->engine->gt->rps.num_waiters));
atomic_dec(&rq->engine->gt->rps.num_waiters);
}
- spin_unlock_irq(&rq->lock);
/*
* We only loosely track inflight requests across preemption,
@@ -608,17 +609,9 @@ bool __i915_request_submit(struct i915_request *request)
*/
__notify_execute_cb_irq(request);
- /* We may be recursing from the signal callback of another i915 fence */
- if (!i915_request_signaled(request)) {
- spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
-
- if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
- &request->fence.flags) &&
- !i915_request_enable_breadcrumb(request))
- intel_engine_signal_breadcrumbs(engine);
-
- spin_unlock(&request->lock);
- }
+ if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &request->fence.flags) &&
+ !i915_request_enable_breadcrumb(request))
+ intel_engine_signal_breadcrumbs(engine);
return result;
}
@@ -640,27 +633,27 @@ void __i915_request_unsubmit(struct i915_request *request)
{
struct intel_engine_cs *engine = request->engine;
+ /*
+ * Only unwind in reverse order, required so that the per-context list
+ * is kept in seqno/ring order.
+ */
RQ_TRACE(request, "\n");
GEM_BUG_ON(!irqs_disabled());
lockdep_assert_held(&engine->active.lock);
/*
- * Only unwind in reverse order, required so that the per-context list
- * is kept in seqno/ring order.
+ * Before we remove this breadcrumb from the signal list, we have
+ * to ensure that a concurrent dma_fence_enable_signaling() does not
+ * attach itself. We first mark the request as no longer active and
+ * make sure that is visible to other cores, and then remove the
+ * breadcrumb if attached.
*/
-
- /* We may be recursing from the signal callback of another i915 fence */
- spin_lock_nested(&request->lock, SINGLE_DEPTH_NESTING);
-
+ GEM_BUG_ON(!test_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags));
+ clear_bit_unlock(I915_FENCE_FLAG_ACTIVE, &request->fence.flags);
if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &request->fence.flags))
i915_request_cancel_breadcrumb(request);
- GEM_BUG_ON(!test_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags));
- clear_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags);
-
- spin_unlock(&request->lock);
-
/* We've already spun, don't charge on resubmitting. */
if (request->sched.semaphores && i915_request_started(request))
request->sched.semaphores = 0;
I did not find a hole (race) after quite a bit of straining the grey
matter so.. lets see..
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Regards,
Tvrtko
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