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Date:   Tue, 6 Oct 2020 12:01:13 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Rob Clark <robdclark@gmail.com>, Qais Yousef <qais.yousef@arm.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
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        Rob Clark <robdclark@chromium.org>,
        open list <linux-kernel@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH v2 0/3] drm: commit_work scheduling
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On Tue, Oct 06, 2020 at 11:08:59AM +0200, Daniel Vetter wrote:
> vblank work needs to preempt commit work.
> 
> Right now we don't have any driver requiring this, but if we e.g. roll out
> the gamma table update for i915, then this _has_ to happen in the vblank
> period.
> 
> Whereas the commit work can happen in there, but it can also be delayed a
> bit (until the vblank worker has finished) we will not miss any additional
> deadline due to that.
> 
> So that's why we have 2 levels. I'm not even sure you can model that with
> SCHED_DEADLINE, since essentially we need a few usec of cpu time very
> vblank (16ms normally), but thos few usec _must_ be scheduled within a
> very specific time slot or we're toast. And that vblank period is only
> 1-2ms usually.

Depends a bit on what the hardware gets us. If for example we're
provided an interrupt on vblank start, then that could wake a DEADLINE
job with (given your numbers above):

 .sched_period = 16ms,
 .sched_deadline = 1-2ms,
 .sched_runtime = 1-2ms,

The effective utilization of that task would be: 1-2/16.

> deadline has the upshot that it compose much better than SCHED_FIFO:
> Everyone just drops their deadline requirements onto the scheduler,
> scheduler makes sure it's all obeyed (or rejects your request).
> 
> The trouble is we'd need to know how long a commit takes, worst case, on a
> given platform. And for that you need to measure stuff, and we kinda can't
> spend a few minutes at boot-up going through the combinatorial maze of
> atomic commits to make sure we have it all.
> 
> So I think in practice letting userspace set the right rt priority/mode is
> the only way to go here :-/

Or you can have it adjust it's expected runtime as the system runs
(always keeping a little extra room over what you measure to make sure).

Given you have period > deadline, you can simply start with runtime =
deadline and adjust downwards during use (carefully).