drm/i915: Reduce context HW ID lifetime

Future gen reduce the number of bits we will have available to
differentiate between contexts, so reduce the lifetime of the ID
assignment from that of the context to its current active cycle (i.e.
only while it is pinned for use by the HW, will it have a constant ID).
This means that instead of a max of 2k allocated contexts (worst case
before fun with bit twiddling), we instead have a limit of 2k in flight
contexts (minus a few that have been pinned by the kernel or by perf).

To reduce the number of contexts id we require, we allocate a context id
on first and mark it as pinned for as long as the GEM context itself is,
that is we keep it pinned it while active on each engine. If we exhaust
our context id space, then we try to reclaim an id from an idle context.
In the extreme case where all context ids are pinned by active contexts,
we force the system to idle in order to recover ids.

We cannot reduce the scope of an HW-ID to an engine (allowing the same
gem_context to have different ids on each engine) as in the future we
will need to preassign an id before we know which engine the
context is being executed on.

v2: Improved commentary (Tvrtko) [I tried at least]

References: https://bugs.freedesktop.org/show_bug.cgi?id=107788
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904153117.3907-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -1953,7 +1953,10 @@ static int i915_context_status(struct seq_file *m, void *unused)
 		return ret;
 
 	list_for_each_entry(ctx, &dev_priv->contexts.list, link) {
-		seq_printf(m, "HW context %u ", ctx->hw_id);
+		seq_puts(m, "HW context ");
+		if (!list_empty(&ctx->hw_id_link))
+			seq_printf(m, "%x [pin %u]", ctx->hw_id,
+				   atomic_read(&ctx->hw_id_pin_count));
 		if (ctx->pid) {
 			struct task_struct *task;
 

drivers/gpu/drm/i915/i915_drv.h

@@ -1861,6 +1861,7 @@ struct drm_i915_private {
 	struct mutex av_mutex;
 
 	struct {
+		struct mutex mutex;
 		struct list_head list;
 		struct llist_head free_list;
 		struct work_struct free_work;
@@ -1873,6 +1874,7 @@ struct drm_i915_private {
 #define MAX_CONTEXT_HW_ID (1<<21) /* exclusive */
 #define MAX_GUC_CONTEXT_HW_ID (1 << 20) /* exclusive */
 #define GEN11_MAX_CONTEXT_HW_ID (1<<11) /* exclusive */
+		struct list_head hw_id_list;
 	} contexts;
 
 	u32 fdi_rx_config;

drivers/gpu/drm/i915/i915_gem_context.c

@@ -115,13 +115,103 @@ static void lut_close(struct i915_gem_context *ctx)
 	rcu_read_unlock();
 }
 
+static inline int new_hw_id(struct drm_i915_private *i915, gfp_t gfp)
+{
+	unsigned int max;
+
+	lockdep_assert_held(&i915->contexts.mutex);
+
+	if (INTEL_GEN(i915) >= 11)
+		max = GEN11_MAX_CONTEXT_HW_ID;
+	else if (USES_GUC_SUBMISSION(i915))
+		/*
+		 * When using GuC in proxy submission, GuC consumes the
+		 * highest bit in the context id to indicate proxy submission.
+		 */
+		max = MAX_GUC_CONTEXT_HW_ID;
+	else
+		max = MAX_CONTEXT_HW_ID;
+
+	return ida_simple_get(&i915->contexts.hw_ida, 0, max, gfp);
+}
+
+static int steal_hw_id(struct drm_i915_private *i915)
+{
+	struct i915_gem_context *ctx, *cn;
+	LIST_HEAD(pinned);
+	int id = -ENOSPC;
+
+	lockdep_assert_held(&i915->contexts.mutex);
+
+	list_for_each_entry_safe(ctx, cn,
+				 &i915->contexts.hw_id_list, hw_id_link) {
+		if (atomic_read(&ctx->hw_id_pin_count)) {
+			list_move_tail(&ctx->hw_id_link, &pinned);
+			continue;
+		}
+
+		GEM_BUG_ON(!ctx->hw_id); /* perma-pinned kernel context */
+		list_del_init(&ctx->hw_id_link);
+		id = ctx->hw_id;
+		break;
+	}
+
+	/*
+	 * Remember how far we got up on the last repossesion scan, so the
+	 * list is kept in a "least recently scanned" order.
+	 */
+	list_splice_tail(&pinned, &i915->contexts.hw_id_list);
+	return id;
+}
+
+static int assign_hw_id(struct drm_i915_private *i915, unsigned int *out)
+{
+	int ret;
+
+	lockdep_assert_held(&i915->contexts.mutex);
+
+	/*
+	 * We prefer to steal/stall ourselves and our users over that of the
+	 * entire system. That may be a little unfair to our users, and
+	 * even hurt high priority clients. The choice is whether to oomkill
+	 * something else, or steal a context id.
+	 */
+	ret = new_hw_id(i915, GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
+	if (unlikely(ret < 0)) {
+		ret = steal_hw_id(i915);
+		if (ret < 0) /* once again for the correct errno code */
+			ret = new_hw_id(i915, GFP_KERNEL);
+		if (ret < 0)
+			return ret;
+	}
+
+	*out = ret;
+	return 0;
+}
+
+static void release_hw_id(struct i915_gem_context *ctx)
+{
+	struct drm_i915_private *i915 = ctx->i915;
+
+	if (list_empty(&ctx->hw_id_link))
+		return;
+
+	mutex_lock(&i915->contexts.mutex);
+	if (!list_empty(&ctx->hw_id_link)) {
+		ida_simple_remove(&i915->contexts.hw_ida, ctx->hw_id);
+		list_del_init(&ctx->hw_id_link);
+	}
+	mutex_unlock(&i915->contexts.mutex);
+}
+
 static void i915_gem_context_free(struct i915_gem_context *ctx)
 {
 	unsigned int n;
 
 	lockdep_assert_held(&ctx->i915->drm.struct_mutex);
 	GEM_BUG_ON(!i915_gem_context_is_closed(ctx));
 
+	release_hw_id(ctx);
 	i915_ppgtt_put(ctx->ppgtt);
 
 	for (n = 0; n < ARRAY_SIZE(ctx->__engine); n++) {
@@ -136,7 +226,6 @@ static void i915_gem_context_free(struct i915_gem_context *ctx)
 
 	list_del(&ctx->link);
 
-	ida_simple_remove(&ctx->i915->contexts.hw_ida, ctx->hw_id);
 	kfree_rcu(ctx, rcu);
 }
 
@@ -190,6 +279,12 @@ static void context_close(struct i915_gem_context *ctx)
 {
 	i915_gem_context_set_closed(ctx);
 
+	/*
+	 * This context will never again be assinged to HW, so we can
+	 * reuse its ID for the next context.
+	 */
+	release_hw_id(ctx);
+
 	/*
 	 * The LUT uses the VMA as a backpointer to unref the object,
 	 * so we need to clear the LUT before we close all the VMA (inside
@@ -203,43 +298,6 @@ static void context_close(struct i915_gem_context *ctx)
 	i915_gem_context_put(ctx);
 }
 
-static int assign_hw_id(struct drm_i915_private *dev_priv, unsigned *out)
-{
-	int ret;
-	unsigned int max;
-
-	if (INTEL_GEN(dev_priv) >= 11) {
-		max = GEN11_MAX_CONTEXT_HW_ID;
-	} else {
-		/*
-		 * When using GuC in proxy submission, GuC consumes the
-		 * highest bit in the context id to indicate proxy submission.
-		 */
-		if (USES_GUC_SUBMISSION(dev_priv))
-			max = MAX_GUC_CONTEXT_HW_ID;
-		else
-			max = MAX_CONTEXT_HW_ID;
-	}
-
-
-	ret = ida_simple_get(&dev_priv->contexts.hw_ida,
-			     0, max, GFP_KERNEL);
-	if (ret < 0) {
-		/* Contexts are only released when no longer active.
-		 * Flush any pending retires to hopefully release some
-		 * stale contexts and try again.
-		 */
-		i915_retire_requests(dev_priv);
-		ret = ida_simple_get(&dev_priv->contexts.hw_ida,
-				     0, max, GFP_KERNEL);
-		if (ret < 0)
-			return ret;
-	}
-
-	*out = ret;
-	return 0;
-}
-
 static u32 default_desc_template(const struct drm_i915_private *i915,
 				 const struct i915_hw_ppgtt *ppgtt)
 {
@@ -276,12 +334,6 @@ __create_hw_context(struct drm_i915_private *dev_priv,
 	if (ctx == NULL)
 		return ERR_PTR(-ENOMEM);
 
-	ret = assign_hw_id(dev_priv, &ctx->hw_id);
-	if (ret) {
-		kfree(ctx);
-		return ERR_PTR(ret);
-	}
-
 	kref_init(&ctx->ref);
 	list_add_tail(&ctx->link, &dev_priv->contexts.list);
 	ctx->i915 = dev_priv;
@@ -295,6 +347,7 @@ __create_hw_context(struct drm_i915_private *dev_priv,
 
 	INIT_RADIX_TREE(&ctx->handles_vma, GFP_KERNEL);
 	INIT_LIST_HEAD(&ctx->handles_list);
+	INIT_LIST_HEAD(&ctx->hw_id_link);
 
 	/* Default context will never have a file_priv */
 	ret = DEFAULT_CONTEXT_HANDLE;
@@ -421,15 +474,35 @@ i915_gem_context_create_gvt(struct drm_device *dev)
 	return ctx;
 }
 
+static void
+destroy_kernel_context(struct i915_gem_context **ctxp)
+{
+	struct i915_gem_context *ctx;
+
+	/* Keep the context ref so that we can free it immediately ourselves */
+	ctx = i915_gem_context_get(fetch_and_zero(ctxp));
+	GEM_BUG_ON(!i915_gem_context_is_kernel(ctx));
+
+	context_close(ctx);
+	i915_gem_context_free(ctx);
+}
+
 struct i915_gem_context *
 i915_gem_context_create_kernel(struct drm_i915_private *i915, int prio)
 {
 	struct i915_gem_context *ctx;
+	int err;
 
 	ctx = i915_gem_create_context(i915, NULL);
 	if (IS_ERR(ctx))
 		return ctx;
 
+	err = i915_gem_context_pin_hw_id(ctx);
+	if (err) {
+		destroy_kernel_context(&ctx);
+		return ERR_PTR(err);
+	}
+
 	i915_gem_context_clear_bannable(ctx);
 	ctx->sched.priority = prio;
 	ctx->ring_size = PAGE_SIZE;
@@ -439,17 +512,19 @@ i915_gem_context_create_kernel(struct drm_i915_private *i915, int prio)
 	return ctx;
 }
 
-static void
-destroy_kernel_context(struct i915_gem_context **ctxp)
+static void init_contexts(struct drm_i915_private *i915)
 {
-	struct i915_gem_context *ctx;
+	mutex_init(&i915->contexts.mutex);
+	INIT_LIST_HEAD(&i915->contexts.list);
 
-	/* Keep the context ref so that we can free it immediately ourselves */
-	ctx = i915_gem_context_get(fetch_and_zero(ctxp));
-	GEM_BUG_ON(!i915_gem_context_is_kernel(ctx));
+	/* Using the simple ida interface, the max is limited by sizeof(int) */
+	BUILD_BUG_ON(MAX_CONTEXT_HW_ID > INT_MAX);
+	BUILD_BUG_ON(GEN11_MAX_CONTEXT_HW_ID > INT_MAX);
+	ida_init(&i915->contexts.hw_ida);
+	INIT_LIST_HEAD(&i915->contexts.hw_id_list);
 
-	context_close(ctx);
-	i915_gem_context_free(ctx);
+	INIT_WORK(&i915->contexts.free_work, contexts_free_worker);
+	init_llist_head(&i915->contexts.free_list);
 }
 
 static bool needs_preempt_context(struct drm_i915_private *i915)
@@ -470,14 +545,7 @@ int i915_gem_contexts_init(struct drm_i915_private *dev_priv)
 	if (ret)
 		return ret;
 
-	INIT_LIST_HEAD(&dev_priv->contexts.list);
-	INIT_WORK(&dev_priv->contexts.free_work, contexts_free_worker);
-	init_llist_head(&dev_priv->contexts.free_list);
-
-	/* Using the simple ida interface, the max is limited by sizeof(int) */
-	BUILD_BUG_ON(MAX_CONTEXT_HW_ID > INT_MAX);
-	BUILD_BUG_ON(GEN11_MAX_CONTEXT_HW_ID > INT_MAX);
-	ida_init(&dev_priv->contexts.hw_ida);
+	init_contexts(dev_priv);
 
 	/* lowest priority; idle task */
 	ctx = i915_gem_context_create_kernel(dev_priv, I915_PRIORITY_MIN);
@@ -487,9 +555,13 @@ int i915_gem_contexts_init(struct drm_i915_private *dev_priv)
 	}
 	/*
 	 * For easy recognisablity, we want the kernel context to be 0 and then
-	 * all user contexts will have non-zero hw_id.
+	 * all user contexts will have non-zero hw_id. Kernel contexts are
+	 * permanently pinned, so that we never suffer a stall and can
+	 * use them from any allocation context (e.g. for evicting other
+	 * contexts and from inside the shrinker).
 	 */
 	GEM_BUG_ON(ctx->hw_id);
+	GEM_BUG_ON(!atomic_read(&ctx->hw_id_pin_count));
 	dev_priv->kernel_context = ctx;
 
 	/* highest priority; preempting task */
@@ -527,6 +599,7 @@ void i915_gem_contexts_fini(struct drm_i915_private *i915)
 	destroy_kernel_context(&i915->kernel_context);
 
 	/* Must free all deferred contexts (via flush_workqueue) first */
+	GEM_BUG_ON(!list_empty(&i915->contexts.hw_id_list));
 	ida_destroy(&i915->contexts.hw_ida);
 }
 
@@ -932,6 +1005,33 @@ int i915_gem_context_reset_stats_ioctl(struct drm_device *dev,
 	return ret;
 }
 
+int __i915_gem_context_pin_hw_id(struct i915_gem_context *ctx)
+{
+	struct drm_i915_private *i915 = ctx->i915;
+	int err = 0;
+
+	mutex_lock(&i915->contexts.mutex);
+
+	GEM_BUG_ON(i915_gem_context_is_closed(ctx));
+
+	if (list_empty(&ctx->hw_id_link)) {
+		GEM_BUG_ON(atomic_read(&ctx->hw_id_pin_count));
+
+		err = assign_hw_id(i915, &ctx->hw_id);
+		if (err)
+			goto out_unlock;
+
+		list_add_tail(&ctx->hw_id_link, &i915->contexts.hw_id_list);
+	}
+
+	GEM_BUG_ON(atomic_read(&ctx->hw_id_pin_count) == ~0u);
+	atomic_inc(&ctx->hw_id_pin_count);
+
+out_unlock:
+	mutex_unlock(&i915->contexts.mutex);
+	return err;
+}
+
 #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
 #include "selftests/mock_context.c"
 #include "selftests/i915_gem_context.c"

drivers/gpu/drm/i915/i915_gem_context.h

@@ -134,8 +134,16 @@ struct i915_gem_context {
 	 * functions like fault reporting, PASID, scheduling. The
 	 * &drm_i915_private.context_hw_ida is used to assign a unqiue
 	 * id for the lifetime of the context.
+	 *
+	 * @hw_id_pin_count: - number of times this context had been pinned
+	 * for use (should be, at most, once per engine).
+	 *
+	 * @hw_id_link: - all contexts with an assigned id are tracked
+	 * for possible repossession.
 	 */
 	unsigned int hw_id;
+	atomic_t hw_id_pin_count;
+	struct list_head hw_id_link;
 
 	/**
 	 * @user_handle: userspace identifier
@@ -254,6 +262,21 @@ static inline void i915_gem_context_set_force_single_submission(struct i915_gem_
 	__set_bit(CONTEXT_FORCE_SINGLE_SUBMISSION, &ctx->flags);
 }
 
+int __i915_gem_context_pin_hw_id(struct i915_gem_context *ctx);
+static inline int i915_gem_context_pin_hw_id(struct i915_gem_context *ctx)
+{
+	if (atomic_inc_not_zero(&ctx->hw_id_pin_count))
+		return 0;
+
+	return __i915_gem_context_pin_hw_id(ctx);
+}
+
+static inline void i915_gem_context_unpin_hw_id(struct i915_gem_context *ctx)
+{
+	GEM_BUG_ON(atomic_read(&ctx->hw_id_pin_count) == 0u);
+	atomic_dec(&ctx->hw_id_pin_count);
+}
+
 static inline bool i915_gem_context_is_default(const struct i915_gem_context *c)
 {
 	return c->user_handle == DEFAULT_CONTEXT_HANDLE;