Re: [PATCH v4] drm: Don't free jobs in wait_event_interruptible()

[Grodzovsky, Andrey]

On 9/26/19 11:23 AM, Steven Price wrote:
> On 26/09/2019 16:14, Grodzovsky, Andrey wrote:
>> On 9/26/19 10:16 AM, Steven Price wrote:
>>> drm_sched_cleanup_jobs() attempts to free finished jobs, however because
>>> it is called as the condition of wait_event_interruptible() it must not
>>> sleep. Unfortuantly some free callbacks (notibly for Panfrost) do sleep.
>>>
>>> Instead let's rename drm_sched_cleanup_jobs() to
>>> drm_sched_get_cleanup_job() and simply return a job for processing if
>>> there is one. The caller can then call the free_job() callback outside
>>> the wait_event_interruptible() where sleeping is possible before
>>> re-checking and returning to sleep if necessary.
>>>
>>> Signed-off-by: Steven Price <steven.price@xxxxxxx>
>>> ---
>>> Changes from v3:
>>>    * drm_sched_main() re-arms the timeout for the next job after calling
>>>      free_job()
>>>
>>>    drivers/gpu/drm/scheduler/sched_main.c | 45 +++++++++++++++-----------
>>>    1 file changed, 26 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>> index 9a0ee74d82dc..148468447ba9 100644
>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>> @@ -622,43 +622,41 @@ static void drm_sched_process_job(struct dma_fence *f, struct dma_fence_cb *cb)
>>>    }
>>>    
>>>    /**
>>> - * drm_sched_cleanup_jobs - destroy finished jobs
>>> + * drm_sched_get_cleanup_job - fetch the next finished job to be destroyed
>>>     *
>>>     * @sched: scheduler instance
>>>     *
>>> - * Remove all finished jobs from the mirror list and destroy them.
>>> + * Returns the next finished job from the mirror list (if there is one)
>>> + * ready for it to be destroyed.
>>>     */
>>> -static void drm_sched_cleanup_jobs(struct drm_gpu_scheduler *sched)
>>> +static struct drm_sched_job *
>>> +drm_sched_get_cleanup_job(struct drm_gpu_scheduler *sched)
>>>    {
>>> +	struct drm_sched_job *job = NULL;
>>>    	unsigned long flags;
>>>    
>>>    	/* Don't destroy jobs while the timeout worker is running */
>>>    	if (sched->timeout != MAX_SCHEDULE_TIMEOUT &&
>>>    	    !cancel_delayed_work(&sched->work_tdr))
>>> -		return;
>>> -
>>> +		return NULL;
>>>    
>>> -	while (!list_empty(&sched->ring_mirror_list)) {
>>> -		struct drm_sched_job *job;
>>> +	spin_lock_irqsave(&sched->job_list_lock, flags);
>>>    
>>> -		job = list_first_entry(&sched->ring_mirror_list,
>>> +	job = list_first_entry_or_null(&sched->ring_mirror_list,
>>>    				       struct drm_sched_job, node);
>>> -		if (!dma_fence_is_signaled(&job->s_fence->finished))
>>> -			break;
>>>    
>>> -		spin_lock_irqsave(&sched->job_list_lock, flags);
>>> +	if (job && dma_fence_is_signaled(&job->s_fence->finished)) {
>>>    		/* remove job from ring_mirror_list */
>>>    		list_del_init(&job->node);
>>> -		spin_unlock_irqrestore(&sched->job_list_lock, flags);
>>> -
>>> -		sched->ops->free_job(job);
>>> +	} else {
>>> +		job = NULL;
>>> +		/* queue timeout for next job */
>>> +		drm_sched_start_timeout(sched);
>>>    	}
>>>    
>>> -	/* queue timeout for next job */
>>> -	spin_lock_irqsave(&sched->job_list_lock, flags);
>>> -	drm_sched_start_timeout(sched);
>>>    	spin_unlock_irqrestore(&sched->job_list_lock, flags);
>>>    
>>> +	return job;
>>>    }
>>>    
>>>    /**
>>> @@ -698,12 +696,21 @@ static int drm_sched_main(void *param)
>>>    		struct drm_sched_fence *s_fence;
>>>    		struct drm_sched_job *sched_job;
>>>    		struct dma_fence *fence;
>>> +		struct drm_sched_job *cleanup_job = NULL;
>>>    
>>>    		wait_event_interruptible(sched->wake_up_worker,
>>> -					 (drm_sched_cleanup_jobs(sched),
>>> +					 (cleanup_job = drm_sched_get_cleanup_job(sched)) ||
>>>    					 (!drm_sched_blocked(sched) &&
>>>    					  (entity = drm_sched_select_entity(sched))) ||
>>> -					 kthread_should_stop()));
>>> +					 kthread_should_stop());
>>> +
>>> +		while (cleanup_job) {
>>> +			sched->ops->free_job(cleanup_job);
>>> +			/* queue timeout for next job */
>>> +			drm_sched_start_timeout(sched);
>>> +
>>> +			cleanup_job = drm_sched_get_cleanup_job(sched);
>>> +		}
>>
>> Why drm_sched_start_timeout is called both here and inside
>> drm_sched_get_cleanup_job ? And also why call it multiple times in the
>> loop instead of only once after the loop is done ?
> Christian pointed out to be that the first thing
> drm_sched_get_cleanup_job does is call cancel_delayed_work(), and if
> that returns false then it bails out with a NULL return. So to actually
> get another job (if one exists) the timeout has to be restarted.


For this case where timeout work already in progress note that 
drm_sched_job_timedout restarts the timeout in it's end so it should be 
ok to restart the timeout unconditionally inside 
drm_sched_get_cleanup_job as it was done before.


>
> It's also necessary to restart the timeout in the case where the return
> is NULL which is handled in the function itself.
>
> TBH I'm not sure whether this while loop is worth it - it may be better
> to replace it with simply:
>
> 	if (cleanup_job) {
> 		sched->ops->free_job(cleanup_job);
> 		/* queue timeout for next job */
> 		drm_sched_start_timeout(sched);
> 	}
>
> The outer loop would then handle the next call to
> drm_sched_get_cleanup_job() as necessary.


What outer loop ?

Andrey

>
> Steve