On Tue, 4 Feb 2025 16:30:53 +0100
Peter Zijlstra [off-list ref] wrote:

If you go back and reread that initial thread, you'll find the 50us is
below the scheduling latency that random test box already had.

I'm sure more modern systems will have a lower number, and slower
systems will have a larger number, but we got to pick a number :/

I'm fine with making it 20us. Or whatever. Its just a stupid number.

But yes. If we're going to be doing this, there is absolutely no reason
not to allow DEADLINE/FIFO threads the same. Misbehaving FIFO is already
a problem, and we can make DL-CBS enforcement punch through it if we
have to.

And less retries on the RSEQ for FIFO can equally improve performance.

There is no difference between a 'malicious/broken' userspace consuming
the entire window in userspace (50us, 20us whatever it will be) and
doing a system call which we know will cause similar delays because it
does in-kernel locking.

This is where we will disagree for the reasons I explained in my second
email. This feature affects other tasks. And no, making it 20us doesn't
make it better. Because from what I get from you, if we implement this, it
will be available for all preemption methods (including PREEMPT_RT), where
we do have less than 50us latency, and and even a 20us will break those
applications.

This was supposed to be only a hint to the kernel, not a complete feature
that is hard coded and will override how other tasks behave. As system
calls themselves can make how things are scheduled depending on the
preemption method, I didn't want to add something that will change how
things are scheduled that ignores the preemption method that was chosen.

-- Steve