MuQSS by itself for 4.8:
MuQSS by itself for 4.7:
A week has passed since the last major update to BFS and -ck was posted, allowing me to concentrate on receiving and responding to any bug reports. As it turns out, there were very few apart from the recurring local_softirq_pending warning/stalls. This is nice because it means MuQSS is mostly ~stable now. Mainline has even had more "stable" releases in the same time as MuQSS for 4.8, moving to 4.8.6 in the interim.
In this version I've added aggressive handling of pending softirqs in the hope the warnings and stalls all go away. The true reason the handling of softirqs are being dropped still escapes me but is likely related to the fact that MuQSS does a lot of lockless rescheduling across CPUs to decrease overhead but this does not give guarantees that locking would.
Additionally, I've added a number of APIs to the kernel to do specified millisecond schedule timeouts which use the highres timers which are mandatory now for MuQSS. The reason for doing this is there are many timeouts in the kernel that specify values below 10ms and the timer resolution at 100Hz only guarantees timeouts under 20ms.
I've also added a code sweep across the entire kernel looking for timeout calls under 50ms and use the new interface in its place. Additionally there are numerous places where schedule_timeout(1) are used in the kernel where a "minimum timeout" is expected, yet this is entirely Hz dependent, again being up to 20ms in duration. I've replaced all these with a 1ms timeout, emulating what would happen on a 1000Hz kernel, but without the overhead of running the higher Hz kernel. I'm not entirely sure this will equate to any real world improvements but the fact it's used in things like audio drivers worries me that it might.
Finally I've replaced the standard msleep call from userspace to use highres timers, in case there are userspace applications that expects msleep to actually give some kind of sleep that resembles what's asked of it, instead of something Hz limited, in case this is leading to slowdowns in userspace due to assumptions on the userspace coders' part. Calls to msleep() from userspace now give 100us accuracy at 100Hz instead of 20ms.
All these timing changes add overhead since they're trying to emulate the timing accuracy of running at 1000Hz but in a latency-focused scheduler I believe they're appropriate, and they do not incur the overhead that actually changing Hz would incur. Additionally they add accuracy to timers and timeouts that 1000Hz does not afford.
In the -ck tarball of broken-out patches, I've kept these timer changes separate to allow the muqss scheduler to be applied by itself should they prove problematic, and they will make merging with future kernels easier.