Re: [CI-ping 15/15] drm/i915: Late request cancellations are harmful

[John Harrison <John.C.Harrison@xxxxxxxxx>]

On 13/04/2016 10:57, Daniel Vetter wrote:
> On Tue, Apr 12, 2016 at 09:03:09PM +0100, Chris Wilson wrote:
> > Conceptually, each request is a record of a hardware transaction - we
> > build up a list of pending commands and then either commit them to
> > hardware, or cancel them. However, whilst building up the list of
> > pending commands, we may modify state outside of the request and make
> > references to the pending request. If we do so and then cancel that
> > request, external objects then point to the deleted request leading to
> > both graphical and memory corruption.
> >
> > The easiest example is to consider object/VMA tracking. When we mark an
> > object as active in a request, we store a pointer to this, the most
> > recent request, in the object. Then we want to free that object, we wait
> > for the most recent request to be idle before proceeding (otherwise the
> > hardware will write to pages now owned by the system, or we will attempt
> > to read from those pages before the hardware is finished writing). If
> > the request was cancelled instead, that wait completes immediately. As a
> > result, all requests must be committed and not cancelled if the external
> > state is unknown.
> >
> > All that remains of i915_gem_request_cancel() users are just a couple of
> > extremely unlikely allocation failures, so remove the API entirely.
> >
> > A consequence of committing all incomplete requests is that we generate
> > excess breadcrumbs and fill the ring much more often with dummy work. We
> > have completely undone the outstanding_last_seqno optimisation.
> >
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93907
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> Cc: John Harrison <John.C.Harrison@xxxxxxxxx>
>
> I'd like John's ack on this on too, but patch itself looks sound. Fast r-b
> since we've discussed this a while ago already ...

I think this is going to cause a problem with the scheduler. You are 
effectively saying that the execbuf call cannot fail beyond the point of 
allocating a request. If it gets that far then it must go all the way 
and submit the request to the hardware. With a scheduler, that means 
adding it to the scheduler's queues and tracking it all the way through 
the system to completion. If nothing else, that sounds like a lot of 
extra overhead for no actual work. Or worse if the failure is at a point 
where the request cannot be sent further through the system because it 
was something critical that failed then you are really stuffed.

I'm not sure what the other option would be though, short of being able 
to undo the last read/write object tracking updates.


> Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > ---
> >   drivers/gpu/drm/i915/i915_drv.h            |  2 --
> >   drivers/gpu/drm/i915/i915_gem.c            | 50 ++++++++++++------------------
> >   drivers/gpu/drm/i915/i915_gem_execbuffer.c | 15 +++------
> >   drivers/gpu/drm/i915/intel_display.c       |  2 +-
> >   drivers/gpu/drm/i915/intel_lrc.c           |  4 +--
> >   drivers/gpu/drm/i915/intel_overlay.c       |  8 ++---
> >   6 files changed, 30 insertions(+), 51 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> > index 061ecc43d935..de84dd7be971 100644
> > --- a/drivers/gpu/drm/i915/i915_drv.h
> > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > @@ -2331,7 +2331,6 @@ struct drm_i915_gem_request {
> >   struct drm_i915_gem_request * __must_check
> >   i915_gem_request_alloc(struct intel_engine_cs *engine,
> >   		       struct intel_context *ctx);
> > -void i915_gem_request_cancel(struct drm_i915_gem_request *req);
> >   void i915_gem_request_free(struct kref *req_ref);
> >   int i915_gem_request_add_to_client(struct drm_i915_gem_request *req,
> >   				   struct drm_file *file);
> > @@ -2883,7 +2882,6 @@ int i915_gem_sw_finish_ioctl(struct drm_device *dev, void *data,
> >   			     struct drm_file *file_priv);
> >   void i915_gem_execbuffer_move_to_active(struct list_head *vmas,
> >   					struct drm_i915_gem_request *req);
> > -void i915_gem_execbuffer_retire_commands(struct i915_execbuffer_params *params);
> >   int i915_gem_ringbuffer_submission(struct i915_execbuffer_params *params,
> >   				   struct drm_i915_gem_execbuffer2 *args,
> >   				   struct list_head *vmas);
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index b6879d43dd74..c6f09e7839ea 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -2785,7 +2785,8 @@ __i915_gem_request_alloc(struct intel_engine_cs *engine,
> >   		 * fully prepared. Thus it can be cleaned up using the proper
> >   		 * free code.
> >   		 */
> > -		i915_gem_request_cancel(req);
> > +		intel_ring_reserved_space_cancel(req->ringbuf);
> > +		i915_gem_request_unreference(req);
> >   		return ret;
> >   	}
> >   
> > @@ -2822,13 +2823,6 @@ i915_gem_request_alloc(struct intel_engine_cs *engine,
> >   	return err ? ERR_PTR(err) : req;
> >   }
> >   
> > -void i915_gem_request_cancel(struct drm_i915_gem_request *req)
> > -{
> > -	intel_ring_reserved_space_cancel(req->ringbuf);
> > -
> > -	i915_gem_request_unreference(req);
> > -}
> > -
> >   struct drm_i915_gem_request *
> >   i915_gem_find_active_request(struct intel_engine_cs *engine)
> >   {
> > @@ -3438,12 +3432,9 @@ int i915_gpu_idle(struct drm_device *dev)
> >   				return PTR_ERR(req);
> >   
> >   			ret = i915_switch_context(req);
> > -			if (ret) {
> > -				i915_gem_request_cancel(req);
> > -				return ret;
> > -			}
> > -
> >   			i915_add_request_no_flush(req);
> > +			if (ret)
> > +				return ret;
> >   		}
> >   
> >   		ret = intel_engine_idle(engine);
> > @@ -4943,34 +4934,33 @@ i915_gem_init_hw(struct drm_device *dev)
> >   		req = i915_gem_request_alloc(engine, NULL);
> >   		if (IS_ERR(req)) {
> >   			ret = PTR_ERR(req);
> > -			i915_gem_cleanup_engines(dev);
> > -			goto out;
> > +			break;
> >   		}
> >   
> >   		if (engine->id == RCS) {
> > -			for (j = 0; j < NUM_L3_SLICES(dev); j++)
> > -				i915_gem_l3_remap(req, j);
> > +			for (j = 0; j < NUM_L3_SLICES(dev); j++) {
> > +				ret = i915_gem_l3_remap(req, j);
> > +				if (ret)
> > +					goto err_request;
> > +			}
> >   		}
> >   
> >   		ret = i915_ppgtt_init_ring(req);
> > -		if (ret && ret != -EIO) {
> > -			DRM_ERROR("PPGTT enable %s failed %d\n",
> > -				  engine->name, ret);
> > -			i915_gem_request_cancel(req);
> > -			i915_gem_cleanup_engines(dev);
> > -			goto out;
> > -		}
> > +		if (ret)
> > +			goto err_request;
> >   
> >   		ret = i915_gem_context_enable(req);
> > -		if (ret && ret != -EIO) {
> > -			DRM_ERROR("Context enable %s failed %d\n",
> > +		if (ret)
> > +			goto err_request;
> > +
> > +err_request:
> > +		i915_add_request_no_flush(req);
> > +		if (ret) {
> > +			DRM_ERROR("Failed to enable %s, error=%d\n",
> >   				  engine->name, ret);
> > -			i915_gem_request_cancel(req);
> >   			i915_gem_cleanup_engines(dev);
> > -			goto out;
> > +			break;
> >   		}
> > -
> > -		i915_add_request_no_flush(req);
> >   	}
> >   
> >   out:
> > diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > index 6ee4f00f620c..6f4f2a6cdf93 100644
> > --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> > @@ -1137,7 +1137,7 @@ i915_gem_execbuffer_move_to_active(struct list_head *vmas,
> >   	}
> >   }
> >   
> > -void
> > +static void
> >   i915_gem_execbuffer_retire_commands(struct i915_execbuffer_params *params)
> >   {
> >   	/* Unconditionally force add_request to emit a full flush. */
> > @@ -1322,7 +1322,6 @@ i915_gem_ringbuffer_submission(struct i915_execbuffer_params *params,
> >   	trace_i915_gem_ring_dispatch(params->request, params->dispatch_flags);
> >   
> >   	i915_gem_execbuffer_move_to_active(vmas, params->request);
> > -	i915_gem_execbuffer_retire_commands(params);
> >   
> >   	return 0;
> >   }
> > @@ -1624,7 +1623,7 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
> >   
> >   	ret = i915_gem_request_add_to_client(req, file);
> >   	if (ret)
> > -		goto err_batch_unpin;
> > +		goto err_request;
> >   
> >   	/*
> >   	 * Save assorted stuff away to pass through to *_submission().
> > @@ -1641,6 +1640,8 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
> >   	params->request                 = req;
> >   
> >   	ret = dev_priv->gt.execbuf_submit(params, args, &eb->vmas);
> > +err_request:
> > +	i915_gem_execbuffer_retire_commands(params);
> >   
> >   err_batch_unpin:
> >   	/*
> > @@ -1657,14 +1658,6 @@ err:
> >   	i915_gem_context_unreference(ctx);
> >   	eb_destroy(eb);
> >   
> > -	/*
> > -	 * If the request was created but not successfully submitted then it
> > -	 * must be freed again. If it was submitted then it is being tracked
> > -	 * on the active request list and no clean up is required here.
> > -	 */
> > -	if (ret && !IS_ERR_OR_NULL(req))
> > -		i915_gem_request_cancel(req);
> > -
> >   	mutex_unlock(&dev->struct_mutex);
> >   
> >   pre_mutex_err:
> > diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> > index b1b457864e17..3cae596d10a3 100644
> > --- a/drivers/gpu/drm/i915/intel_display.c
> > +++ b/drivers/gpu/drm/i915/intel_display.c
> > @@ -11664,7 +11664,7 @@ cleanup_unpin:
> >   	intel_unpin_fb_obj(fb, crtc->primary->state->rotation);
> >   cleanup_pending:
> >   	if (!IS_ERR_OR_NULL(request))
> > -		i915_gem_request_cancel(request);
> > +		i915_add_request_no_flush(request);
> >   	atomic_dec(&intel_crtc->unpin_work_count);
> >   	mutex_unlock(&dev->struct_mutex);
> >   cleanup:
> > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > index b8f6b96472a6..6fc24deaa16a 100644
> > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > @@ -1010,7 +1010,6 @@ int intel_execlists_submission(struct i915_execbuffer_params *params,
> >   	trace_i915_gem_ring_dispatch(params->request, params->dispatch_flags);
> >   
> >   	i915_gem_execbuffer_move_to_active(vmas, params->request);
> > -	i915_gem_execbuffer_retire_commands(params);
> >   
> >   	return 0;
> >   }
> > @@ -2679,13 +2678,12 @@ int intel_lr_context_deferred_alloc(struct intel_context *ctx,
> >   		}
> >   
> >   		ret = engine->init_context(req);
> > +		i915_add_request_no_flush(req);
> >   		if (ret) {
> >   			DRM_ERROR("ring init context: %d\n",
> >   				ret);
> > -			i915_gem_request_cancel(req);
> >   			goto error_ringbuf;
> >   		}
> > -		i915_add_request_no_flush(req);
> >   	}
> >   	return 0;
> >   
> > diff --git a/drivers/gpu/drm/i915/intel_overlay.c b/drivers/gpu/drm/i915/intel_overlay.c
> > index 6694e9230cd5..bcc3b6a016d8 100644
> > --- a/drivers/gpu/drm/i915/intel_overlay.c
> > +++ b/drivers/gpu/drm/i915/intel_overlay.c
> > @@ -247,7 +247,7 @@ static int intel_overlay_on(struct intel_overlay *overlay)
> >   
> >   	ret = intel_ring_begin(req, 4);
> >   	if (ret) {
> > -		i915_gem_request_cancel(req);
> > +		i915_add_request_no_flush(req);
> >   		return ret;
> >   	}
> >   
> > @@ -290,7 +290,7 @@ static int intel_overlay_continue(struct intel_overlay *overlay,
> >   
> >   	ret = intel_ring_begin(req, 2);
> >   	if (ret) {
> > -		i915_gem_request_cancel(req);
> > +		i915_add_request_no_flush(req);
> >   		return ret;
> >   	}
> >   
> > @@ -356,7 +356,7 @@ static int intel_overlay_off(struct intel_overlay *overlay)
> >   
> >   	ret = intel_ring_begin(req, 6);
> >   	if (ret) {
> > -		i915_gem_request_cancel(req);
> > +		i915_add_request_no_flush(req);
> >   		return ret;
> >   	}
> >   
> > @@ -431,7 +431,7 @@ static int intel_overlay_release_old_vid(struct intel_overlay *overlay)
> >   
> >   		ret = intel_ring_begin(req, 2);
> >   		if (ret) {
> > -			i915_gem_request_cancel(req);
> > +			i915_add_request_no_flush(req);
> >   			return ret;
> >   		}
> >   
> > --
> > 2.8.0.rc3
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