Date:   Wed, 8 Mar 2023 16:02:37 +0100
From:   Christian König <christian.koenig@....com>
To:     Karol Herbst <kherbst@...hat.com>
Cc:     Asahi Lina <lina@...hilina.net>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Luben Tuikov <luben.tuikov@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Ella Stanforth <ella@...unix.org>,
        Faith Ekstrand <faith.ekstrand@...labora.com>,
        Mary <mary@...y.zone>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, rust-for-linux@...r.kernel.org,
        linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
        linux-sgx@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH RFC 10/18] drm/scheduler: Add can_run_job callback

Am 08.03.23 um 15:43 schrieb Karol Herbst:
> [SNIP]
> "further"? There was no discussion at all,

Yeah, well that is exactly what I wanted to archive.

>   you just started off like
> that. If you think somebody misses that connection, you can point out
> to documentation/videos whatever so the contributor can understand
> what's wrong with an approach. You did that, so that's fine. It's just
> starting off _any_ discussion with a "Well complete NAK" is terrible
> style. I'd feel uncomfortable if that happened to me and I'm sure
> there are enough people like that that we should be more reasonable
> with our replies. Just.. don't.
>
> We are all humans here and people react negatively to such things. And
> if people do it on purpose it just makes it worse.

I completely see your point, I just don't know how to improve it.

I don't stop people like this because I want to make them uncomfortable 
but because I want to prevent further discussions on that topic.

In other words how can I make people notice that this is something 
fundamental while still being polite?

>>>> This is clearly going against the idea of having jobs only depend on
>>>> fences and nothing else which is mandatory for correct memory management.
>>>>
>>> I'm sure it's all documented and there is a design document on how
>>> things have to look like you can point out? Might help to get a better
>>> understanding on how things should be.
>> Yeah, that's the problematic part. We have documented this very
>> extensively:
>> https://www.kernel.org/doc/html/v5.9/driver-api/dma-buf.html#indefinite-dma-fences
>>
>> And both Jason and Daniel gave talks about the underlying problem and
> fyi:
> s/Jason/Faith/g

+1. I wasn't aware of that.

>> try to come up with patches to raise warnings when that happens, but
>> people still keep coming up with the same idea over and over again.
>>
> Yes, and we'll have to tell them over and over again. Nothing wrong
> with that. That's just part of maintaining such a big subsystem. And
> that's definitely not a valid reason to phrase things like above.
>
>> It's just that the technical relationship between preventing jobs from
>> running and with that preventing dma_fences from signaling and the core
>> memory management with page faults and shrinkers waiting for those
>> fences is absolutely not obvious.
>>
>> We had at least 10 different teams from different companies falling into
>> the same trap already and either the patches were rejected of hand or
>> had to painfully reverted or mitigated later on.
>>
> Sure, but that's just part of the job. And pointing out fundamental
> mistakes early on is important, but the situation won't get any better
> by being like that. Yes, we'll have to repeat the same words over and
> over again, and yes that might be annoying, but that's just how it is.

Well I have no problem explaining people why a solution doesn't work.

But what usually happens is that people don't realize that they need to 
back of from a design and completely start over.

Regards,
Christian.

>
>> Regards,
>> Christian.
>>
>>>> If the hw is busy with something you need to return the fence for this
>>>> from the prepare_job callback so that the scheduler can be notified when
>>>> the hw is available again.
>>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>> Signed-off-by: Asahi Lina <lina@...hilina.net>
>>>>> ---
>>>>>     drivers/gpu/drm/scheduler/sched_main.c | 10 ++++++++++
>>>>>     include/drm/gpu_scheduler.h            |  8 ++++++++
>>>>>     2 files changed, 18 insertions(+)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>>>> index 4e6ad6e122bc..5c0add2c7546 100644
>>>>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>>>>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>>>>> @@ -1001,6 +1001,16 @@ static int drm_sched_main(void *param)
>>>>>                 if (!entity)
>>>>>                         continue;
>>>>>
>>>>> +             if (sched->ops->can_run_job) {
>>>>> +                     sched_job = to_drm_sched_job(spsc_queue_peek(&entity->job_queue));
>>>>> +                     if (!sched_job) {
>>>>> +                             complete_all(&entity->entity_idle);
>>>>> +                             continue;
>>>>> +                     }
>>>>> +                     if (!sched->ops->can_run_job(sched_job))
>>>>> +                             continue;
>>>>> +             }
>>>>> +
>>>>>                 sched_job = drm_sched_entity_pop_job(entity);
>>>>>
>>>>>                 if (!sched_job) {
>>>>> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
>>>>> index 9db9e5e504ee..bd89ea9507b9 100644
>>>>> --- a/include/drm/gpu_scheduler.h
>>>>> +++ b/include/drm/gpu_scheduler.h
>>>>> @@ -396,6 +396,14 @@ struct drm_sched_backend_ops {
>>>>>         struct dma_fence *(*prepare_job)(struct drm_sched_job *sched_job,
>>>>>                                          struct drm_sched_entity *s_entity);
>>>>>
>>>>> +     /**
>>>>> +      * @can_run_job: Called before job execution to check whether the
>>>>> +      * hardware is free enough to run the job.  This can be used to
>>>>> +      * implement more complex hardware resource policies than the
>>>>> +      * hw_submission limit.
>>>>> +      */
>>>>> +     bool (*can_run_job)(struct drm_sched_job *sched_job);
>>>>> +
>>>>>         /**
>>>>>              * @run_job: Called to execute the job once all of the dependencies
>>>>>              * have been resolved.  This may be called multiple times, if
>>>>>

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