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

[Luben Tuikov <ltuikov89@gmail.com>]

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Hi,

On 2023-10-27 12:26, Boris Brezillon wrote:
> On Fri, 27 Oct 2023 16:34:26 +0200
> Danilo Krummrich <dakr@redhat.com> wrote:
> 
>> On 10/27/23 09:17, Boris Brezillon wrote:
>>> Hi Danilo,
>>>
>>> On Thu, 26 Oct 2023 18:13:00 +0200
>>> Danilo Krummrich <dakr@redhat.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);  
>>>
>>> I'm copying my late reply to v2 here so it doesn't get lost:
>>>
>>> I keep thinking it'd be simpler to make this a void function that
>>> updates s_job->submission_credits directly. I also don't see the
>>> problem with doing a sanity check on job->submission_credits. I mean,
>>> if the driver is doing something silly, you can't do much to prevent it
>>> anyway, except warn the user that something wrong has happened. If you
>>> want to
>>>
>>> 	WARN_ON(job->submission_credits == 0 ||
>>> 		job->submission_credits > job_old_submission_credits);
>>>
>>> that's fine. But none of this sanity checking has to do with the
>>> function prototype/semantics, and I'm still not comfortable with this 0  
>>> => no-change. If there's no change, we should just leave  
>>> job->submission_credits unchanged (or return job->submission_credits)
>>> instead of inventing a new special case.  
>>
>> If we can avoid letting drivers change fields of generic structures directly
>> without any drawbacks I think we should avoid it. Currently, drivers shouldn't
>> have the need to mess with job->credits directly. The initial value is set
>> through drm_sched_job_init() and is updated through the return value of
>> update_job_credits().
> 
> Fair enough. I do agree that keeping internal fields out of driver
> hands is a good thing in general, it's just that it's already
> free-for-all in so many places in drm_sched (like the fact drivers

"Free-for-all" doesn't mean we need to follow suit. We should keep
good programming practices, as this patch strives to.

> iterate the pending list in their stop-queue handling) that I didn't
> really see it as an issue. Note that's there's always the option of
> providing drm_sched_job_{update,get}_credits() helpers, with the update
> helper making sure the new credits value is consistent (smaller or
> equal to the old one, and not zero).
> 
>>
>> I'm fine getting rid of the 0 => no-change semantics though. Instead we can just
>> WARN() on 0.
> 
> Yeah, I think that's preferable. It's pretty easy to return the old
> value if the driver has a way to detect when nothing changed (with a
> get helper if you don't want drivers to touch the credits field).
> 
>> However, if we do that I'd also want to change it for
>> drm_sched_job_init() (where 0 currently defaults to 1) such that we accept 0, but
>> WARN() accordingly.
> 
> Sure. You update all drivers anyway, so passing 1 instead of 0 is not a
> big deal, I would say.

At this point in time, we should consider 1 as normal, 0 out of spec and
WARN on it but carry on and (perhaps) reset it to 1. Drivers in the future, may
see a need (i.e. do tricks) to return 0, at which point they'll submit a patch which
does two things, 1) removes the WARN, 2) removes the reset from 0 to 1, and
explain why they need to return 0 to allow (one more) job, but we're nowhere near then yet,
so status quo for now.

I don't see how it makes sense to call drm_sched_job_init(credits:0), and I believe
the code is correct to default to 1 in that case--which defaults to the current
flow control we have, which we want.

> 
>>
>> I think it's consequent to either consistently give 0 a different meaning or just
>> accept it but WARN() on it.
> 
> Using default as a default value makes sense when you're passing

I suppose you meant "using zero as a default value".

> zero-initialized objects that are later extended with new fields, but
> here you update the function prototype and all the call sites, so we're
> better off considering 0 as an invalid value, IMHO.

Yes, absolutely.

You never want to give 0 a meaning, since as you pointed out, it is zero-ed
memory, and as such, can have any meaning you'd like. So yes: WARN on 0;
1 is good and normal.

Regards,
Luben

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