Re: [Intel-xe] [PATCH v3 02/13] drm/sched: Convert drm scheduler to use a work queue rather than kthread

[Matthew Brost <matthew.brost@intel.com>]

On Tue, Sep 12, 2023 at 09:29:53AM +0200, Boris Brezillon wrote:
> On Mon, 11 Sep 2023 19:16:04 -0700
> Matthew Brost <matthew.brost@intel.com> wrote:
> 
> > @@ -1071,6 +1063,7 @@ static int drm_sched_main(void *param)
> >   *
> >   * @sched: scheduler instance
> >   * @ops: backend operations for this scheduler
> > + * @submit_wq: workqueue to use for submission. If NULL, the system_wq is used
> >   * @hw_submission: number of hw submissions that can be in flight
> >   * @hang_limit: number of times to allow a job to hang before dropping it
> >   * @timeout: timeout value in jiffies for the scheduler
> > @@ -1084,14 +1077,16 @@ static int drm_sched_main(void *param)
> >   */
> >  int drm_sched_init(struct drm_gpu_scheduler *sched,
> >  		   const struct drm_sched_backend_ops *ops,
> > +		   struct workqueue_struct *submit_wq,
> >  		   unsigned hw_submission, unsigned hang_limit,
> >  		   long timeout, struct workqueue_struct *timeout_wq,
> >  		   atomic_t *score, const char *name, struct device *dev)
> >  {
> > -	int i, ret;
> > +	int i;
> >  	sched->ops = ops;
> >  	sched->hw_submission_limit = hw_submission;
> >  	sched->name = name;
> > +	sched->submit_wq = submit_wq ? : system_wq;
> 
> My understanding is that the new design is based on the idea of
> splitting the drm_sched_main function into work items that can be
> scheduled independently so users/drivers can insert their own
> steps/works without requiring changes to drm_sched. This approach is
> relying on the properties of ordered workqueues (1 work executed at a
> time, FIFO behavior) to guarantee that these steps are still executed
> in order, and one at a time.
> 
> Given what you're trying to achieve I think we should create an ordered
> workqueue instead of using the system_wq when submit_wq is NULL,
> otherwise you lose this ordering/serialization guarantee which both
> the dedicated kthread and ordered wq provide. It will probably work for
> most drivers, but might lead to subtle/hard to spot ordering issues.
> 

I debated chosing between a system_wq or creating an ordered-wq by
default myself. Indeed using the system_wq by default subtlety changes
the behavior as run_job & free_job workers can run in parallel. To be
safe, agree the default use be an ordered-wq. If drivers are fine with
run_job() and free_job() running in parallel, they are free to set
submit_wq == system_wq. Will change in next rev.

Matt

> >  	sched->timeout = timeout;
> >  	sched->timeout_wq = timeout_wq ? : system_wq;
> >  	sched->hang_limit = hang_limit;
> > @@ -1100,23 +1095,15 @@ int drm_sched_init(struct drm_gpu_scheduler *sched,
> >  	for (i = DRM_SCHED_PRIORITY_MIN; i < DRM_SCHED_PRIORITY_COUNT; i++)
> >  		drm_sched_rq_init(sched, &sched->sched_rq[i]);
> >  
> > -	init_waitqueue_head(&sched->wake_up_worker);
> >  	init_waitqueue_head(&sched->job_scheduled);
> >  	INIT_LIST_HEAD(&sched->pending_list);
> >  	spin_lock_init(&sched->job_list_lock);
> >  	atomic_set(&sched->hw_rq_count, 0);
> >  	INIT_DELAYED_WORK(&sched->work_tdr, drm_sched_job_timedout);
> > +	INIT_WORK(&sched->work_submit, drm_sched_main);
> >  	atomic_set(&sched->_score, 0);
> >  	atomic64_set(&sched->job_id_count, 0);
> > -
> > -	/* Each scheduler will run on a seperate kernel thread */
> > -	sched->thread = kthread_run(drm_sched_main, sched, sched->name);
> > -	if (IS_ERR(sched->thread)) {
> > -		ret = PTR_ERR(sched->thread);
> > -		sched->thread = NULL;
> > -		DRM_DEV_ERROR(sched->dev, "Failed to create scheduler for %s.\n", name);
> > -		return ret;
> > -	}
> > +	sched->pause_submit = false;
> >  
> >  	sched->ready = true;
> >  	return 0;