You seem to be missing the fact that latency guarantees only work if you
can make progress. If a low priority process is pre-empted owning a
resource (quite likely) then you won't get your good latency. To
handle those cases you get into priority boosting, and all sorts of lock
complexity - so that the task that owns the resource temporarily can borrow
your priority in order that you can make progress at your needed speed.
That gets horrendously complex, and you get huge chains of priority
dependancies including hardware level ones.
The low latency patches don't make that problem go away, but they achieve
equivalent real world latencies up to at least the point you have to do
priority handling of that kind.
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