Re: [PATCH] drm/i915: Queue page flip work with high priority

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 13/09/16 11:31, Imre Deak wrote:
> On ti, 2016-09-13 at 11:24 +0100, Tvrtko Ursulin wrote:
>> On 12/09/16 15:09, Imre Deak wrote:
>>> While user space has control over the scheduling priority of its
>>> page
>>> flipping thread, the corresponding work the driver schedules for
>>> MMIO
>>> flips always runs with normal scheduling priority. This would
>>> hinder an
>>> application that wants more stringent guarantees over flip timing
>>> (to
>>> avoid missing a flip at the next frame count).
>>>
>>> Fix this by scheduling the work with high priority, meaning normal
>>> scheduling policy with -20 nice level.
>>>
>>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97775
>>> Testcase: igt/kms_cursor_legacy
>>> CC: Chris Wilson <chris@chris-wilson.co.uk>
>>> CC: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>>> Signed-off-by: Imre Deak <imre.deak@intel.com>
>>> ---
>>>    drivers/gpu/drm/i915/i915_drv.c      | 7 +++++++
>>>    drivers/gpu/drm/i915/i915_drv.h      | 4 ++++
>>>    drivers/gpu/drm/i915/intel_display.c | 2 +-
>>>    3 files changed, 12 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/i915_drv.c
>>> b/drivers/gpu/drm/i915/i915_drv.c
>>> index 02c34d6..381ef23 100644
>>> --- a/drivers/gpu/drm/i915/i915_drv.c
>>> +++ b/drivers/gpu/drm/i915/i915_drv.c
>>> @@ -756,8 +756,14 @@ static int i915_workqueues_init(struct
>>> drm_i915_private *dev_priv)
>>>    	if (dev_priv->hotplug.dp_wq == NULL)
>>>    		goto out_free_wq;
>>>
>>> +	dev_priv->flip_wq = alloc_workqueue("i915-flip",
>>> WQ_HIGHPRI, 0);
>>> +	if (dev_priv->flip_wq == NULL)
>>> +		goto out_free_dp_wq;
>>> +
>>>    	return 0;
>>>
>>> +out_free_dp_wq:
>>> +	destroy_workqueue(dev_priv->hotplug.dp_wq);
>>>    out_free_wq:
>>>    	destroy_workqueue(dev_priv->wq);
>>>    out_err:
>>> @@ -768,6 +774,7 @@ out_err:
>>>
>>>    static void i915_workqueues_cleanup(struct drm_i915_private
>>> *dev_priv)
>>>    {
>>> +	destroy_workqueue(dev_priv->flip_wq);
>>>    	destroy_workqueue(dev_priv->hotplug.dp_wq);
>>>    	destroy_workqueue(dev_priv->wq);
>>>    }
>>> diff --git a/drivers/gpu/drm/i915/i915_drv.h
>>> b/drivers/gpu/drm/i915/i915_drv.h
>>> index f499fa5..3653ce4 100644
>>> --- a/drivers/gpu/drm/i915/i915_drv.h
>>> +++ b/drivers/gpu/drm/i915/i915_drv.h
>>> @@ -1844,6 +1844,10 @@ struct drm_i915_private {
>>>    	 * result in deadlocks.
>>>    	 */
>>>    	struct workqueue_struct *wq;
>>> +	/**
>>> +	 * flip_wq - High priority flip workqueue.
>>> +	 */
>>> +	struct workqueue_struct *flip_wq;
>>>
>>>    	/* Display functions */
>>>    	struct drm_i915_display_funcs display;
>>> diff --git a/drivers/gpu/drm/i915/intel_display.c
>>> b/drivers/gpu/drm/i915/intel_display.c
>>> index 3c367d0..48433e1 100644
>>> --- a/drivers/gpu/drm/i915/intel_display.c
>>> +++ b/drivers/gpu/drm/i915/intel_display.c
>>> @@ -12278,7 +12278,7 @@ static int intel_crtc_page_flip(struct
>>> drm_crtc *crtc,
>>>
>>>    		work->flip_queued_req = i915_gem_active_get(&obj-
>>>> last_write,
>>>    							    &obj-
>>>> base.dev->struct_mutex);
>>> -		schedule_work(&work->mmio_work);
>>> +		queue_work(dev_priv->flip_wq, &work->mmio_work);
>>>    	} else {
>>>    		request = i915_gem_request_alloc(engine, engine-
>>>> last_context);
>>>    		if (IS_ERR(request)) {
>>>
>>
>> I am curious if just a dedicated wq would be enough, or you have
>> found
>> that it has to be a high-prio one?
>
> I haven't tried a dedicated normal-prio wq. Right, another work in the
> queue could also hold up this one, but the system_wq is unordered, so
> that kind of dependency shouldn't be a problem if that's what you
> meant.

Yes, I've suspicious whether the problem is work start latency and not 
actually the worker priority. Since the flip work item mostly does 
waiting and little CPU activity, I though the former sounded like more 
likely.

Regards,

Tvrtko
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