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). _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
