[PATCH drm-misc-next v3] drm/sched: implement dynamic job-flow control

[Christian König]

Am 27.10.23 um 09:39 schrieb Boris Brezillon:
> On Fri, 27 Oct 2023 09:35:01 +0200
> Christian König<christian.koenig at amd.com>  wrote:
>
>> Am 27.10.23 um 09:32 schrieb Boris Brezillon:
>>> On Fri, 27 Oct 2023 09:22:12 +0200
>>> Christian König<christian.koenig at amd.com>  wrote:
>>>   
>>>>> +
>>>>> +	/**
>>>>> +	 * @update_job_credits: Called once the scheduler is considering this
>>>>> +	 * job for execution.
>>>>> +	 *
>>>>> +	 * Drivers may use this to update the job's submission credits, which is
>>>>> +	 * useful to e.g. deduct the number of native fences which have been
>>>>> +	 * signaled meanwhile.
>>>>> +	 *
>>>>> +	 * The callback must either return the new number of submission credits
>>>>> +	 * for the given job, or zero if no update is required.
>>>>> +	 *
>>>>> +	 * This callback is optional.
>>>>> +	 */
>>>>> +	u32 (*update_job_credits)(struct drm_sched_job *sched_job);
>>>> Why do we need an extra callback for this?
>>>>
>>>> Just document that prepare_job() is allowed to reduce the number of
>>>> credits the job might need.
>>> ->prepare_job() is called only once if the returned fence is NULL, but
>>> we need this credit-update to happen every time a job is considered for
>>> execution by the scheduler.
>> But the job is only considered for execution once. How do you see that
>> this is called multiple times?
> Nope, it's not. If drm_sched_can_queue() returns false, the scheduler
> will go look for another entity that has a job ready for execution, and
> get back to this entity later, and test drm_sched_can_queue() again.
> Basically, any time drm_sched_can_queue() is called, the job credits
> update should happen, so we have an accurate view of how many credits
> this job needs.

Well, that is the handling which I already rejected because it creates 
unfairness between processes. When you consider the credits needed 
*before* scheduling jobs with a lower credit count are always preferred 
over jobs with a higher credit count.
What you can do is to look at the credits of a job *after* it was picked 
up for scheduling so that you can scheduler more jobs.

Regards,
Christian.
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