On Mon, Dec 12, 2022 at 11:33:12AM -1000, Tejun Heo wrote: > > But this.. afaict that means that: > > > > - the whole EXT thing is incompatible with SCHED_CORE > > Can you expand on why this would be? I didn't test against SCHED_CORE, so am > sure things might be broken but can't think of a reason why it'd be > fundamentally incompatible. For starters, SCHED_CORE doesn't use __pick_next_task() (much). But I think you're going to have more trouble with prio_less() (which is the 3rd implementation of the scheduling function :/) > > - the whole EXT thing can be trivially starved by the presence of a > > single CFS/BATCH/IDLE task. > > It's a simliar situation w/ RT vs. CFS, which is resolved via RT having > starvation avoidance. That is a horrible situation as is, FIFO/RR are very crap scheduling policies for a number of reasons but we're stuck with them due to history and POSIX :-(, that is not something you should justify anything with. In fact, it should be an example of what to avoid. Specifically, FIFO/RR fail at the fundamentals of OS abstractions -- they provide neither resource distribution nor isolation. > Here, the way it's handled is a bit different, SCX has > a watchdog mechanism implemented in "[PATCH 18/31] sched_ext: Implement > runnable task stall watchdog", so if SCX tasks hang for whatever reason > including being starved by CFS, it will get aborted and all tasks will be > handed back to CFS. IOW, it's treated like any other BPF scheduler errors > that can lead to stalls and recovered the same way. That all sounds quite terrible.. :/ When the scheduler isn't available it should be an error to switch a task to the policy, when there are tasks in the policy, it must not go away. The policy itself should never cause policy changes.