scx_bpfland: introduce --lowlatency option

[arighi]

Introduce the new --lowlatency option, which enables switching between the default pure vruntime-based scheduling (more optimized for server workloads) and a deadline-based scheduling (better suited for low-latency workloads).

When the low-latency mode is activated, a task's deadline is calculated as its vruntime, adjusted by a bonus proportional to the task's average number of voluntary context switches (the more voluntary context switches, the shorter the deadline).

This feature enhances the prioritization of interactive tasks even more, proportionally to their average voluntary context switches, also within the two main global queues (priority / shared) and it helps to maintain interactive workloads always responsive, even in presence of heavy non-interactive background work.

Low-latency mode allows to prevent audio cracking even in presence of a large amount of short-lived tasks with pseudo-interactive behavior (i.e, hackbench) and it enables achieving approximately a +33% average frames-per-second (FPS) in the typical "gaming while building the kernel" benchmark.

However, it can also amplify the de-prioritization of CPU-intensive tasks, making this option more suitable for specific low-latency scenarios. Therefore the low-latency mode is disabled by default and it can only be enabled via the --lowlatency option.

[sirlucjan]

❯ systemctl status scx                   
● scx.service - Start scx_scheduler
     Loaded: loaded (/usr/lib/systemd/system/scx.service; enabled; preset: disabled)
     Active: active (running) since Thu 2024-08-22 11:37:22 CEST; 1h 21min ago
 Invocation: 49cc28cb6fd34bc7b2ee344058f1331e
   Main PID: 54323 (scx_bpfland)
      Tasks: 2 (limit: 37769)
     Memory: 11.7M (peak: 18.9M)
        CPU: 369ms
     CGroup: /system.slice/scx.service
             └─54323 scx_bpfland --lowlatency

sie 22 12:58:39 cachyos bash[54323]: 10:58:39 [INFO] [scx_bpfland] tasks -> run:  4/4  int: 1  wait: 8    | nvcsw: 3    | dispatch -> dir: 11076920 prio: 8574830 shr: 63675
sie 22 12:58:40 cachyos bash[54323]: 10:58:40 [INFO] [scx_bpfland] tasks -> run:  2/4  int: 1  wait: 7    | nvcsw: 3    | dispatch -> dir: 11076920 prio: 8576487 shr: 67132
sie 22 12:58:41 cachyos bash[54323]: 10:58:41 [INFO] [scx_bpfland] tasks -> run:  4/4  int: 1  wait: 7    | nvcsw: 8    | dispatch -> dir: 11076920 prio: 8577757 shr: 70390
sie 22 12:58:42 cachyos bash[54323]: 10:58:42 [INFO] [scx_bpfland] tasks -> run:  4/4  int: 1  wait: 9    | nvcsw: 6    | dispatch -> dir: 11076920 prio: 8580116 shr: 73552
sie 22 12:58:43 cachyos bash[54323]: 10:58:43 [INFO] [scx_bpfland] tasks -> run:  3/4  int: 2  wait: 8    | nvcsw: 6    | dispatch -> dir: 11076920 prio: 8582535 shr: 76798
sie 22 12:58:44 cachyos bash[54323]: 10:58:44 [INFO] [scx_bpfland] tasks -> run:  4/4  int: 1  wait: 8    | nvcsw: 8    | dispatch -> dir: 11076920 prio: 8584137 shr: 79877
sie 22 12:58:45 cachyos bash[54323]: 10:58:45 [INFO] [scx_bpfland] tasks -> run:  4/4  int: 1  wait: 8    | nvcsw: 8    | dispatch -> dir: 11076920 prio: 8586224 shr: 83010
sie 22 12:58:46 cachyos bash[54323]: 10:58:46 [INFO] [scx_bpfland] tasks -> run:  3/4  int: 2  wait: 10   | nvcsw: 8    | dispatch -> dir: 11076920 prio: 8587724 shr: 86406
sie 22 12:58:47 cachyos bash[54323]: 10:58:47 [INFO] [scx_bpfland] tasks -> run:  4/4  int: 1  wait: 8    | nvcsw: 3    | dispatch -> dir: 11076920 prio: 8589042 shr: 89807
sie 22 12:58:48 cachyos bash[54323]: 10:58:48 [INFO] [scx_bpfland] tasks -> run:  3/4  int: 1  wait: 7    | nvcsw: 4    | dispatch -> dir: 11076920 prio: 8590517 shr: 92993

I have been using for a few days, everything works without regression.

Tested-by: Piotr Gorski ([email protected])

[arighi]

@sirlucjan thanks, added your tested-by line.