Re: [PATCH 22/43] drm/i915: Introduce intel_context.pin_mutex for pin management

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On 06/03/2019 14:24, Chris Wilson wrote:
Introduce a mutex to start locking the HW contexts independently of
struct_mutex, with a view to reducing the coarse struct_mutex. The
intel_context.pin_mutex is used to guard the transition to and from being
pinned on the gpu, and so is required before starting to build any
request. The intel_context will then remain pinned until the request
completes, but the mutex can be released immediately unpin completion of
pinning the context.

A slight variant of the above is used by per-context sseu that wants to
inspect the pinned status of the context, and requires that it remains
stable (either !pinned or pinned) across its operation. By using the
pin_mutex to serialise operations while pin_count==0, we can take that
pin_mutex for stabilise the boolean pin status.

Change log!

Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
---
  drivers/gpu/drm/i915/i915_gem.c               |  2 +-
  drivers/gpu/drm/i915/i915_gem_context.c       | 76 +++++++------------
  drivers/gpu/drm/i915/intel_context.c          | 56 ++++++++++++--
  drivers/gpu/drm/i915/intel_context.h          | 38 ++++++----
  drivers/gpu/drm/i915/intel_context_types.h    |  8 +-
  drivers/gpu/drm/i915/intel_lrc.c              |  4 +-
  drivers/gpu/drm/i915/intel_ringbuffer.c       |  4 +-
  .../gpu/drm/i915/selftests/i915_gem_context.c |  2 +-
  drivers/gpu/drm/i915/selftests/mock_engine.c  |  2 +-
  9 files changed, 110 insertions(+), 82 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 71d8ce38f42c..8863dd414cde 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4662,7 +4662,7 @@ static int __intel_engines_record_defaults(struct drm_i915_private *i915)
  		if (!state)
  			continue;
- GEM_BUG_ON(ce->pin_count);
+		GEM_BUG_ON(intel_context_is_pinned(ce));
/*
  		 * As we will hold a reference to the logical state, it will
diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index f7a075efccad..35541f99b37d 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -1098,23 +1098,27 @@ static int gen8_emit_rpcs_config(struct i915_request *rq,
  }
static int
-gen8_modify_rpcs_gpu(struct intel_context *ce,
-		     struct intel_engine_cs *engine,
-		     struct intel_sseu sseu)
+gen8_modify_rpcs(struct intel_context *ce, struct intel_sseu sseu)
  {
-	struct drm_i915_private *i915 = engine->i915;
+	struct drm_i915_private *i915 = ce->engine->i915;
  	struct i915_request *rq, *prev;
  	intel_wakeref_t wakeref;
  	int ret;
- GEM_BUG_ON(!ce->pin_count);
+	lockdep_assert_held(&ce->pin_mutex);
- lockdep_assert_held(&i915->drm.struct_mutex);
+	/*
+	 * If context is not idle we have to submit an ordered request to modify
+	 * its context image via the kernel context. Pristine and idle contexts
+	 * will be configured on pinning.
+	 */
+	if (!intel_context_is_pinned(ce))
+		return 0;
/* Submitting requests etc needs the hw awake. */
  	wakeref = intel_runtime_pm_get(i915);
- rq = i915_request_alloc(engine, i915->kernel_context);
+	rq = i915_request_alloc(ce->engine, i915->kernel_context);
  	if (IS_ERR(rq)) {
  		ret = PTR_ERR(rq);
  		goto out_put;
@@ -1158,53 +1162,30 @@ gen8_modify_rpcs_gpu(struct intel_context *ce,
  }
static int
-__i915_gem_context_reconfigure_sseu(struct i915_gem_context *ctx,
-				    struct intel_engine_cs *engine,
-				    struct intel_sseu sseu)
+i915_gem_context_reconfigure_sseu(struct i915_gem_context *ctx,
+				  struct intel_engine_cs *engine,
+				  struct intel_sseu sseu)
  {
  	struct intel_context *ce;
  	int ret = 0;
- ce = intel_context_instance(ctx, engine);
-	if (IS_ERR(ce))
-		return PTR_ERR(ce);
-
  	GEM_BUG_ON(INTEL_GEN(ctx->i915) < 8);
  	GEM_BUG_ON(engine->id != RCS0);
+ ce = intel_context_pin_lock(ctx, engine);
+	if (IS_ERR(ce))
+		return PTR_ERR(ce);
+
  	/* Nothing to do if unmodified. */
  	if (!memcmp(&ce->sseu, &sseu, sizeof(sseu)))
-		return 0;
-
-	/*
-	 * If context is not idle we have to submit an ordered request to modify
-	 * its context image via the kernel context. Pristine and idle contexts
-	 * will be configured on pinning.
-	 */
-	if (ce->pin_count)
-		ret = gen8_modify_rpcs_gpu(ce, engine, sseu);
+		goto unlock;
+ ret = gen8_modify_rpcs(ce, sseu);
  	if (!ret)
  		ce->sseu = sseu;
- return ret;
-}
-
-static int
-i915_gem_context_reconfigure_sseu(struct i915_gem_context *ctx,
-				  struct intel_engine_cs *engine,
-				  struct intel_sseu sseu)
-{
-	int ret;
-
-	ret = mutex_lock_interruptible(&ctx->i915->drm.struct_mutex);
-	if (ret)
-		return ret;
-
-	ret = __i915_gem_context_reconfigure_sseu(ctx, engine, sseu);
-
-	mutex_unlock(&ctx->i915->drm.struct_mutex);
-
+unlock:
+	intel_context_pin_unlock(ce);
  	return ret;
  }
@@ -1622,11 +1603,12 @@ static int clone_sseu(struct i915_gem_context *dst,
  		if (!memcmp(&sseu, &default_sseu, sizeof(sseu)))
  			continue;
- ce = intel_context_instance(dst, engine);
+		ce = intel_context_pin_lock(dst, engine);
  		if (IS_ERR(ce))
  			return PTR_ERR(ce);
ce->sseu = sseu;
+		intel_context_pin_unlock(ce);
  	}
return 0;
@@ -1806,7 +1788,6 @@ static int get_sseu(struct i915_gem_context *ctx,
  	struct intel_engine_cs *engine;
  	struct intel_context *ce;
  	unsigned long lookup;
-	int ret;
if (args->size == 0)
  		goto out;
@@ -1833,21 +1814,16 @@ static int get_sseu(struct i915_gem_context *ctx,
  	if (!engine)
  		return -EINVAL;
- ce = intel_context_instance(ctx, engine);
+	ce = intel_context_pin_lock(ctx, engine); /* serialise with set_sseu */
  	if (IS_ERR(ce))
  		return PTR_ERR(ce);
- /* Only use for mutex here is to serialize get_param and set_param. */
-	ret = mutex_lock_interruptible(&ctx->i915->drm.struct_mutex);
-	if (ret)
-		return ret;
-
  	user_sseu.slice_mask = ce->sseu.slice_mask;
  	user_sseu.subslice_mask = ce->sseu.subslice_mask;
  	user_sseu.min_eus_per_subslice = ce->sseu.min_eus_per_subslice;
  	user_sseu.max_eus_per_subslice = ce->sseu.max_eus_per_subslice;
- mutex_unlock(&ctx->i915->drm.struct_mutex);
+	intel_context_pin_unlock(ce);
if (copy_to_user(u64_to_user_ptr(args->value), &user_sseu,
  			 sizeof(user_sseu)))
diff --git a/drivers/gpu/drm/i915/intel_context.c b/drivers/gpu/drm/i915/intel_context.c
index 4c1dacb69f51..8bf0d38d3a08 100644
--- a/drivers/gpu/drm/i915/intel_context.c
+++ b/drivers/gpu/drm/i915/intel_context.c
@@ -102,7 +102,7 @@ void __intel_context_remove(struct intel_context *ce)
  	spin_unlock(&ctx->hw_contexts_lock);
  }
-struct intel_context *
+static struct intel_context *
  intel_context_instance(struct i915_gem_context *ctx,
  		       struct intel_engine_cs *engine)
  {
@@ -126,6 +126,21 @@ intel_context_instance(struct i915_gem_context *ctx,
  	return pos;
  }
+struct intel_context *
+intel_context_pin_lock(struct i915_gem_context *ctx,
+		       struct intel_engine_cs *engine)
+	__acquires(ce->pin_mutex)
+{
+	struct intel_context *ce;
+
+	ce = intel_context_instance(ctx, engine);
+	if (IS_ERR(ce))
+		return ce;
+
+	mutex_lock(&ce->pin_mutex);
+	return ce;
+}
+
  struct intel_context *
  intel_context_pin(struct i915_gem_context *ctx,
  		  struct intel_engine_cs *engine)
@@ -133,16 +148,20 @@ intel_context_pin(struct i915_gem_context *ctx,
  	struct intel_context *ce;
  	int err;
- lockdep_assert_held(&ctx->i915->drm.struct_mutex);
-
  	ce = intel_context_instance(ctx, engine);
  	if (IS_ERR(ce))
  		return ce;
- if (unlikely(!ce->pin_count++)) {
+	if (likely(atomic_inc_not_zero(&ce->pin_count)))
+		return ce;
+
+	if (mutex_lock_interruptible(&ce->pin_mutex))
+		return ERR_PTR(-EINTR);
+
+	if (likely(!atomic_read(&ce->pin_count))) {
  		err = ce->ops->pin(ce);
  		if (err)
-			goto err_unpin;
+			goto err;
mutex_lock(&ctx->mutex);
  		list_add(&ce->active_link, &ctx->active_engines);
@@ -150,16 +169,35 @@ intel_context_pin(struct i915_gem_context *ctx,
i915_gem_context_get(ctx);
  		GEM_BUG_ON(ce->gem_context != ctx);
+
+		smp_mb__before_atomic();
  	}
-	GEM_BUG_ON(!ce->pin_count); /* no overflow! */
+ atomic_inc(&ce->pin_count);
+	GEM_BUG_ON(!intel_context_is_pinned(ce)); /* no overflow! */
+
+	mutex_unlock(&ce->pin_mutex);
  	return ce;
-err_unpin:
-	ce->pin_count = 0;
+err:
+	mutex_unlock(&ce->pin_mutex);
  	return ERR_PTR(err);
  }
+void intel_context_unpin(struct intel_context *ce)
+{
+	if (likely(atomic_add_unless(&ce->pin_count, -1, 1)))
+		return;
+
+	/* We may be called from inside intel_context_pin() to evict another */
+	mutex_lock_nested(&ce->pin_mutex, SINGLE_DEPTH_NESTING);
+
+	if (likely(atomic_dec_and_test(&ce->pin_count)))
+		ce->ops->unpin(ce);
+
+	mutex_unlock(&ce->pin_mutex);
+}
+
  static void intel_context_retire(struct i915_active_request *active,
  				 struct i915_request *rq)
  {
@@ -181,6 +219,8 @@ intel_context_init(struct intel_context *ce,
  	INIT_LIST_HEAD(&ce->signal_link);
  	INIT_LIST_HEAD(&ce->signals);
+ mutex_init(&ce->pin_mutex);
+
  	/* Use the whole device by default */
  	ce->sseu = intel_device_default_sseu(ctx->i915);
diff --git a/drivers/gpu/drm/i915/intel_context.h b/drivers/gpu/drm/i915/intel_context.h
index ea21473891f7..9546d932406a 100644
--- a/drivers/gpu/drm/i915/intel_context.h
+++ b/drivers/gpu/drm/i915/intel_context.h
@@ -7,6 +7,8 @@
  #ifndef __INTEL_CONTEXT_H__
  #define __INTEL_CONTEXT_H__
+#include <linux/lockdep.h>
+
  #include "intel_context_types.h"
  #include "intel_engine_types.h"
@@ -29,18 +31,30 @@ intel_context_lookup(struct i915_gem_context *ctx,
  		     struct intel_engine_cs *engine);
/**
- * intel_context_instance - Lookup or allocate the HW context for (ctx, engine)
+ * intel_context_pin_lock - Stablises the 'pinned' status of the HW context
   * @ctx - the parent GEM context
   * @engine - the target HW engine
   *
- * Returns the existing HW context for this pair of (GEM context, engine), or
- * allocates and initialises a fresh context. Once allocated, the HW context
- * remains resident until the GEM context is destroyed.
+ * Acquire a lock on the pinned status of the HW context, such that the context
+ * can neither be bound to the GPU or unbound whilst the lock is held, i.e.
+ * intel_context_is_pinned() remains stable.
   */
  struct intel_context *
-intel_context_instance(struct i915_gem_context *ctx,
+intel_context_pin_lock(struct i915_gem_context *ctx,
  		       struct intel_engine_cs *engine);
+static inline bool
+intel_context_is_pinned(struct intel_context *ce)
+{
+	return atomic_read(&ce->pin_count);
+}
+
+static inline void intel_context_pin_unlock(struct intel_context *ce)
+__releases(ce->pin_mutex)
+{
+	mutex_unlock(&ce->pin_mutex);
+}
+
  struct intel_context *
  __intel_context_insert(struct i915_gem_context *ctx,
  		       struct intel_engine_cs *engine,
@@ -53,18 +67,10 @@ intel_context_pin(struct i915_gem_context *ctx, struct intel_engine_cs *engine);
static inline void __intel_context_pin(struct intel_context *ce)
  {
-	GEM_BUG_ON(!ce->pin_count);
-	ce->pin_count++;
+	GEM_BUG_ON(!intel_context_is_pinned(ce));
+	atomic_inc(&ce->pin_count);
  }
-static inline void intel_context_unpin(struct intel_context *ce)
-{
-	GEM_BUG_ON(!ce->pin_count);
-	if (--ce->pin_count)
-		return;
-
-	GEM_BUG_ON(!ce->ops);
-	ce->ops->unpin(ce);
-}
+void intel_context_unpin(struct intel_context *ce);
#endif /* __INTEL_CONTEXT_H__ */
diff --git a/drivers/gpu/drm/i915/intel_context_types.h b/drivers/gpu/drm/i915/intel_context_types.h
index 804fa93de853..6dc9b4b9067b 100644
--- a/drivers/gpu/drm/i915/intel_context_types.h
+++ b/drivers/gpu/drm/i915/intel_context_types.h
@@ -8,6 +8,7 @@
  #define __INTEL_CONTEXT_TYPES__
#include <linux/list.h>
+#include <linux/mutex.h>
  #include <linux/rbtree.h>
  #include <linux/types.h>
@@ -38,14 +39,19 @@ struct intel_context {
  	struct i915_gem_context *gem_context;
  	struct intel_engine_cs *engine;
  	struct intel_engine_cs *active;
+
  	struct list_head active_link;
  	struct list_head signal_link;
  	struct list_head signals;
+
  	struct i915_vma *state;
  	struct intel_ring *ring;
+
  	u32 *lrc_reg_state;
  	u64 lrc_desc;
-	int pin_count;
+
+	atomic_t pin_count;
+	struct mutex pin_mutex; /* guards pinning and associated on-gpuing */
/**
  	 * active_tracker: Active tracker for the external rq activity
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index f7134d650187..7b938eaff9c5 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1244,7 +1244,7 @@ static void __execlists_context_fini(struct intel_context *ce)
static void execlists_context_destroy(struct intel_context *ce)
  {
-	GEM_BUG_ON(ce->pin_count);
+	GEM_BUG_ON(intel_context_is_pinned(ce));
if (ce->state)
  		__execlists_context_fini(ce);
@@ -1481,7 +1481,7 @@ static int execlists_request_alloc(struct i915_request *request)
  {
  	int ret;
- GEM_BUG_ON(!request->hw_context->pin_count);
+	GEM_BUG_ON(!intel_context_is_pinned(request->hw_context));
/*
  	 * Flush enough space to reduce the likelihood of waiting after
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 03656021d04c..cc4fcd89b845 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1357,7 +1357,7 @@ static void __ring_context_fini(struct intel_context *ce)
static void ring_context_destroy(struct intel_context *ce)
  {
-	GEM_BUG_ON(ce->pin_count);
+	GEM_BUG_ON(intel_context_is_pinned(ce));
if (ce->state)
  		__ring_context_fini(ce);
@@ -1918,7 +1918,7 @@ static int ring_request_alloc(struct i915_request *request)
  {
  	int ret;
- GEM_BUG_ON(!request->hw_context->pin_count);
+	GEM_BUG_ON(!intel_context_is_pinned(request->hw_context));
  	GEM_BUG_ON(request->timeline->has_initial_breadcrumb);
/*
diff --git a/drivers/gpu/drm/i915/selftests/i915_gem_context.c b/drivers/gpu/drm/i915/selftests/i915_gem_context.c
index df1380498612..b6ed055abf5c 100644
--- a/drivers/gpu/drm/i915/selftests/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/selftests/i915_gem_context.c
@@ -1024,7 +1024,7 @@ __sseu_test(struct drm_i915_private *i915,
  	if (ret)
  		goto out_context;
- ret = __i915_gem_context_reconfigure_sseu(ctx, engine, sseu);
+	ret = i915_gem_context_reconfigure_sseu(ctx, engine, sseu);
  	if (ret)
  		goto out_spin;
diff --git a/drivers/gpu/drm/i915/selftests/mock_engine.c b/drivers/gpu/drm/i915/selftests/mock_engine.c
index 99d7463218d0..f6d120e05ee4 100644
--- a/drivers/gpu/drm/i915/selftests/mock_engine.c
+++ b/drivers/gpu/drm/i915/selftests/mock_engine.c
@@ -131,7 +131,7 @@ static void mock_context_unpin(struct intel_context *ce)
static void mock_context_destroy(struct intel_context *ce)
  {
-	GEM_BUG_ON(ce->pin_count);
+	GEM_BUG_ON(intel_context_is_pinned(ce));
if (ce->ring)
  		mock_ring_free(ce->ring);


Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Regards,

Tvrtko
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