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

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Message-ID: <a9215c37-61cd-4fbc-9f80-217daacd96bd@gmail.com>
Date:   Fri, 27 Oct 2023 10:32:52 -0400
From:   Luben Tuikov <ltuikov89@...il.com>
To:     Boris Brezillon <boris.brezillon@...labora.com>,
        Danilo Krummrich <dakr@...hat.com>
Cc:     matthew.brost@...el.com, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, faith@...strand.net,
        luben.tuikov@....com, christian.koenig@....com
Subject: Re: [PATCH drm-misc-next v3] drm/sched: implement dynamic job-flow
 control

On 2023-10-27 04:25, Boris Brezillon wrote:
> Hi Danilo,
> 
> On Thu, 26 Oct 2023 18:13:00 +0200
> Danilo Krummrich <dakr@...hat.com> wrote:
> 
>> Currently, job flow control is implemented simply by limiting the number
>> of jobs in flight. Therefore, a scheduler is initialized with a credit
>> limit that corresponds to the number of jobs which can be sent to the
>> hardware.
>>
>> This implies that for each job, drivers need to account for the maximum
>> job size possible in order to not overflow the ring buffer.
>>
>> However, there are drivers, such as Nouveau, where the job size has a
>> rather large range. For such drivers it can easily happen that job
>> submissions not even filling the ring by 1% can block subsequent
>> submissions, which, in the worst case, can lead to the ring run dry.
>>
>> In order to overcome this issue, allow for tracking the actual job size
>> instead of the number of jobs. Therefore, add a field to track a job's
>> credit count, which represents the number of credits a job contributes
>> to the scheduler's credit limit.
>>
>> Signed-off-by: Danilo Krummrich <dakr@...hat.com>
>> ---
>> Changes in V2:
>> ==============
>>   - fixed up influence on scheduling fairness due to consideration of a job's
>>     size
>>     - If we reach a ready entity in drm_sched_select_entity() but can't actually
>>       queue a job from it due to size limitations, just give up and go to sleep
>>       until woken up due to a pending job finishing, rather than continue to try
>>       other entities.
>>   - added a callback to dynamically update a job's credits (Boris)
> 
> This callback seems controversial. I'd suggest dropping it, so the
> patch can be merged.

Sorry, why is it controversial? (I did read back-and-forth above, but it wasn't clear
why it is /controversial/.)

I believe only drivers are privy to changes in the credit availability as their
firmware and hardware executes new jobs and finishes others, and so this "update"
here is essential--leaving it only to prepare_job() wouldn't quite fulfill the vision
of why the credit mechanism introduced by this patch in the first place.
-- 
Regards,
Luben

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