Re: [PATCH v2 3/3] drm/i915: Only allocate preempt context when required

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> If we remove some hardcoded assumptions about the preempt context having
> a fixed id, reserved from use by normal user contexts, we may only
> allocate the i915_gem_context when required. Then the subsequent
> decisions on using preemption reduce to having the preempt context
> available.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Michal Winiarski <michal.winiarski@intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
> Cc: Mika Kuoppala <mika.kuoppala@intel.com>
> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem_context.c          | 29 ++++++++++++------------
>  drivers/gpu/drm/i915/intel_engine_cs.c           |  6 ++---
>  drivers/gpu/drm/i915/intel_guc_submission.c      | 24 +++++++++++---------
>  drivers/gpu/drm/i915/intel_lrc.c                 | 17 +++++++++-----
>  drivers/gpu/drm/i915/intel_ringbuffer.h          |  5 ++++
>  drivers/gpu/drm/i915/selftests/mock_gem_device.c |  6 -----
>  6 files changed, 46 insertions(+), 41 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> index 648e7536ff51..d69c8f1a4e78 100644
> --- a/drivers/gpu/drm/i915/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> @@ -449,10 +449,14 @@ destroy_kernel_context(struct i915_gem_context **ctxp)
>  	i915_gem_context_free(ctx);
>  }
>  
> +static bool needs_preempt_context(struct drm_i915_private *i915)
> +{
> +	return HAS_LOGICAL_RING_PREEMPTION(i915);
> +}
> +
>  int i915_gem_contexts_init(struct drm_i915_private *dev_priv)
>  {
>  	struct i915_gem_context *ctx;
> -	int err;
>  
>  	GEM_BUG_ON(dev_priv->kernel_context);

Please consider adding
GEM_BUG_ON(dev_priv->preempt_context);

here even tho the kernel_context should act as a master guard for init ordering
errors. Even if no other than documenting that we expect a clean slate
in this regard also.

>  
> @@ -468,8 +472,7 @@ int i915_gem_contexts_init(struct drm_i915_private *dev_priv)
>  	ctx = i915_gem_context_create_kernel(dev_priv, I915_PRIORITY_MIN);
>  	if (IS_ERR(ctx)) {
>  		DRM_ERROR("Failed to create default global context\n");
> -		err = PTR_ERR(ctx);
> -		goto err;
> +		return PTR_ERR(ctx);
>  	}
>  	/*
>  	 * For easy recognisablity, we want the kernel context to be 0 and then
> @@ -479,23 +482,18 @@ int i915_gem_contexts_init(struct drm_i915_private *dev_priv)
>  	dev_priv->kernel_context = ctx;
>  
>  	/* highest priority; preempting task */
> -	ctx = i915_gem_context_create_kernel(dev_priv, INT_MAX);
> -	if (IS_ERR(ctx)) {
> -		DRM_ERROR("Failed to create default preempt context\n");
> -		err = PTR_ERR(ctx);
> -		goto err_kernel_context;

It seems this error path has been wrong all along.

> +	if (needs_preempt_context(dev_priv)) {
> +		ctx = i915_gem_context_create_kernel(dev_priv, INT_MAX);
> +		if (!IS_ERR(ctx))
> +			dev_priv->preempt_context = ctx;
> +		else
> +			DRM_ERROR("Failed to create preempt context; disabling preemption\n");
>  	}
> -	dev_priv->preempt_context = ctx;
>  
>  	DRM_DEBUG_DRIVER("%s context support initialized\n",
>  			 dev_priv->engine[RCS]->context_size ? "logical" :
>  			 "fake");
>  	return 0;
> -
> -err_kernel_context:
> -	destroy_kernel_context(&dev_priv->kernel_context);
> -err:
> -	return err;
>  }
>  
>  void i915_gem_contexts_lost(struct drm_i915_private *dev_priv)
> @@ -521,7 +519,8 @@ void i915_gem_contexts_fini(struct drm_i915_private *i915)
>  {
>  	lockdep_assert_held(&i915->drm.struct_mutex);
>  
> -	destroy_kernel_context(&i915->preempt_context);
> +	if (i915->preempt_context)
> +		destroy_kernel_context(&i915->preempt_context);
>  	destroy_kernel_context(&i915->kernel_context);
>  
>  	/* Must free all deferred contexts (via flush_workqueue) first */
> diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
> index 7eebfbb95e89..bf634432c9c6 100644
> --- a/drivers/gpu/drm/i915/intel_engine_cs.c
> +++ b/drivers/gpu/drm/i915/intel_engine_cs.c
> @@ -631,7 +631,7 @@ int intel_engine_init_common(struct intel_engine_cs *engine)
>  	 * Similarly the preempt context must always be available so that
>  	 * we can interrupt the engine at any time.
>  	 */
> -	if (HAS_LOGICAL_RING_PREEMPTION(engine->i915)) {
> +	if (engine->i915->preempt_context) {
>  		ring = engine->context_pin(engine,
>  					   engine->i915->preempt_context);
>  		if (IS_ERR(ring)) {
> @@ -656,7 +656,7 @@ int intel_engine_init_common(struct intel_engine_cs *engine)
>  err_breadcrumbs:
>  	intel_engine_fini_breadcrumbs(engine);
>  err_unpin_preempt:
> -	if (HAS_LOGICAL_RING_PREEMPTION(engine->i915))
> +	if (engine->i915->preempt_context)
>  		engine->context_unpin(engine, engine->i915->preempt_context);
>  err_unpin_kernel:
>  	engine->context_unpin(engine, engine->i915->kernel_context);
> @@ -686,7 +686,7 @@ void intel_engine_cleanup_common(struct intel_engine_cs *engine)
>  	if (engine->default_state)
>  		i915_gem_object_put(engine->default_state);
>  
> -	if (HAS_LOGICAL_RING_PREEMPTION(engine->i915))
> +	if (engine->i915->preempt_context)
>  		engine->context_unpin(engine, engine->i915->preempt_context);
>  	engine->context_unpin(engine, engine->i915->kernel_context);
>  }
> diff --git a/drivers/gpu/drm/i915/intel_guc_submission.c b/drivers/gpu/drm/i915/intel_guc_submission.c
> index 4ea65df05e02..b43b58cc599b 100644
> --- a/drivers/gpu/drm/i915/intel_guc_submission.c
> +++ b/drivers/gpu/drm/i915/intel_guc_submission.c
> @@ -688,7 +688,7 @@ static void guc_dequeue(struct intel_engine_cs *engine)
>  		goto unlock;
>  
>  	if (port_isset(port)) {
> -		if (HAS_LOGICAL_RING_PREEMPTION(engine->i915)) {
> +		if (engine->i915->preempt_context) {
>  			struct guc_preempt_work *preempt_work =
>  				&engine->i915->guc.preempt_work[engine->id];
>  
> @@ -984,17 +984,19 @@ static int guc_clients_create(struct intel_guc *guc)
>  	}
>  	guc->execbuf_client = client;
>  
> -	client = guc_client_alloc(dev_priv,
> -				  INTEL_INFO(dev_priv)->ring_mask,
> -				  GUC_CLIENT_PRIORITY_KMD_HIGH,
> -				  dev_priv->preempt_context);
> -	if (IS_ERR(client)) {
> -		DRM_ERROR("Failed to create GuC client for preemption!\n");
> -		guc_client_free(guc->execbuf_client);
> -		guc->execbuf_client = NULL;
> -		return PTR_ERR(client);
> +	if (dev_priv->preempt_context) {
> +		client = guc_client_alloc(dev_priv,
> +					  INTEL_INFO(dev_priv)->ring_mask,
> +					  GUC_CLIENT_PRIORITY_KMD_HIGH,
> +					  dev_priv->preempt_context);
> +		if (IS_ERR(client)) {
> +			DRM_ERROR("Failed to create GuC client for preemption!\n");
> +			guc_client_free(guc->execbuf_client);
> +			guc->execbuf_client = NULL;
> +			return PTR_ERR(client);
> +		}
> +		guc->preempt_client = client;
>  	}
> -	guc->preempt_client = client;
>  
>  	return 0;
>  }
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 5390894001f0..221b62d72c72 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -161,7 +161,6 @@
>  #define EXECLISTS_REQUEST_SIZE 64 /* bytes */
>  #define WA_TAIL_DWORDS 2
>  #define WA_TAIL_BYTES (sizeof(u32) * WA_TAIL_DWORDS)
> -#define PREEMPT_ID 0x1
>  
>  static int execlists_context_deferred_alloc(struct i915_gem_context *ctx,
>  					    struct intel_engine_cs *engine);
> @@ -448,7 +447,8 @@ static void inject_preempt_context(struct intel_engine_cs *engine)
>  		&engine->i915->preempt_context->engine[engine->id];
>  	unsigned int n;
>  
> -	GEM_BUG_ON(engine->i915->preempt_context->hw_id != PREEMPT_ID);
> +	GEM_BUG_ON(engine->execlists.preempt_complete_status !=
> +		   upper_32_bits(ce->lrc_desc));
>  	GEM_BUG_ON(!IS_ALIGNED(ce->ring->size, WA_TAIL_BYTES));
>  
>  	memset(ce->ring->vaddr + ce->ring->tail, 0, WA_TAIL_BYTES);
> @@ -528,7 +528,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>  		if (!execlists_is_active(execlists, EXECLISTS_ACTIVE_HWACK))
>  			goto unlock;
>  
> -		if (HAS_LOGICAL_RING_PREEMPTION(engine->i915) &&
> +		if (engine->i915->preempt_context &&
>  		    rb_entry(rb, struct i915_priolist, node)->priority >
>  		    max(last->priotree.priority, 0)) {
>  			/*
> @@ -844,7 +844,7 @@ static void execlists_submission_tasklet(unsigned long data)
>  			GEM_BUG_ON(status & GEN8_CTX_STATUS_IDLE_ACTIVE);
>  
>  			if (status & GEN8_CTX_STATUS_COMPLETE &&
> -			    buf[2*head + 1] == PREEMPT_ID) {
> +			    buf[2*head + 1] == execlists->preempt_complete_status) {
>  				GEM_TRACE("%s preempt-idle\n", engine->name);
>  
>  				execlists_cancel_port_requests(execlists);
> @@ -1910,7 +1910,7 @@ static void execlists_set_default_submission(struct intel_engine_cs *engine)
>  	engine->i915->caps.scheduler =
>  		I915_SCHEDULER_CAP_ENABLED |
>  		I915_SCHEDULER_CAP_PRIORITY;
> -	if (HAS_LOGICAL_RING_PREEMPTION(engine->i915))
> +	if (engine->i915->preempt_context)
>  		engine->i915->caps.scheduler |= I915_SCHEDULER_CAP_PREEMPTION;
>  }
>  
> @@ -1988,6 +1988,11 @@ static int logical_ring_init(struct intel_engine_cs *engine)
>  	engine->execlists.elsp =
>  		engine->i915->regs + i915_mmio_reg_offset(RING_ELSP(engine));
>  
> +	engine->execlists.preempt_complete_status = ~0u;

This is to ensure that we catch a rogue status? Atleast we will
bug on of preempting with this status as the EXECLIST_ACTIVE_PREEMPT
will be false.

> +	if (engine->i915->preempt_context)
> +		engine->execlists.preempt_complete_status =
> +			upper_32_bits(engine->i915->preempt_context->engine[engine->id].lrc_desc);


We have not upgraded the descriptor yet, so just use preempt_context->hw_id;

I would also like duplicate, from i915_gem_context.c, the assertion that
hw_id needs to be <= INT_MAX but didn't find a good spot.

-Mika

> +
>  	return 0;
>  
>  error:
> @@ -2250,7 +2255,7 @@ populate_lr_context(struct i915_gem_context *ctx,
>  	if (!engine->default_state)
>  		regs[CTX_CONTEXT_CONTROL + 1] |=
>  			_MASKED_BIT_ENABLE(CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT);
> -	if (ctx->hw_id == PREEMPT_ID)
> +	if (ctx == ctx->i915->preempt_context)
>  		regs[CTX_CONTEXT_CONTROL + 1] |=
>  			_MASKED_BIT_ENABLE(CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT |
>  					   CTX_CTRL_ENGINE_CTX_SAVE_INHIBIT);
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> index c5ff203e42d6..03be8995f415 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> @@ -279,6 +279,11 @@ struct intel_engine_execlists {
>  	 * @csb_use_mmio: access csb through mmio, instead of hwsp
>  	 */
>  	bool csb_use_mmio;
> +
> +	/**
> +	 * @preempt_complete_status: expected CSB upon completing preemption
> +	 */
> +	u32 preempt_complete_status;
>  };
>  
>  #define INTEL_ENGINE_CS_MAX_NAME 8
> diff --git a/drivers/gpu/drm/i915/selftests/mock_gem_device.c b/drivers/gpu/drm/i915/selftests/mock_gem_device.c
> index 1bc61f3f76fc..3175db70cc6e 100644
> --- a/drivers/gpu/drm/i915/selftests/mock_gem_device.c
> +++ b/drivers/gpu/drm/i915/selftests/mock_gem_device.c
> @@ -243,16 +243,10 @@ struct drm_i915_private *mock_gem_device(void)
>  	if (!i915->kernel_context)
>  		goto err_engine;
>  
> -	i915->preempt_context = mock_context(i915, NULL);
> -	if (!i915->preempt_context)
> -		goto err_kernel_context;
> -
>  	WARN_ON(i915_gemfs_init(i915));
>  
>  	return i915;
>  
> -err_kernel_context:
> -	i915_gem_context_put(i915->kernel_context);
>  err_engine:
>  	for_each_engine(engine, i915, id)
>  		mock_engine_free(engine);
> -- 
> 2.15.1
>
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