[PATCH v3 0/5] sched_ext: Support high-performance monotonically non-decreasing clock

[Changwoo Min <multics69@gmail.com>]

Many BPF schedulers (such as scx_central, scx_lavd, scx_rusty, scx_bpfland,
and scx_flash) frequently call bpf_ktime_get_ns() for tracking tasks' runtime
properties. If supported, bpf_ktime_get_ns() eventually reads a hardware
timestamp counter (TSC). However, reading a hardware TSC is not
performant in some hardware platforms, degrading IPC.

This patchset addresses the performance problem of reading hardware TSC
by leveraging the rq clock in the scheduler core, introducing a
scx_bpf_clock_get_ns() function for BPF schedulers. Whenever the rq clock
is fresh and valid, scx_bpf_clock_get_ns() provides the rq clock, which is
already updated by the scheduler core (update_rq_clock), so it can reduce
reading the hardware TSC.

When the rq lock is released (rq_unpin_lock) or a long-running
operations are done by the BPF scheduler (ops.running, ops.update_idle),
the rq clock is invalidated, so a subsequent scx_bpf_clock_get_ns() call
gets the fresh sched_clock for the caller.

In addition, scx_bpf_clock_get_ns() guarantees the clock is
monotonically non-decreasing for the same CPU, so the clock cannot go
backward in the same CPU.

Using scx_bpf_clock_get_ns() reduces the number of reading hardware TSC
by 40-70% (65% for scx_lavd, 58% for scx_bpfland, and 43% for scx_rusty)
for the following benchmark.

    perf bench -f simple sched messaging -t -g 20 -l 6000

The patchset begins by managing the status of rq clock in the scheduler
core, then implementing scx_bpf_clock_get_ns(), and finally applying it
to the BPF schedulers.

ChangeLog v2 -> v3:
  - To avoid unnecessarily modifying cache lines, scx_rq_clock_update()
    and scx_rq_clock_stale() update the clock and flags only when a
    sched_ext scheduler is enabled.

ChangeLog v1 -> v2:
  - Rename SCX_RQ_CLK_UPDATED to SCX_RQ_CLK_VALID to denote the validity
    of an rq clock clearly.
  - Rearrange the clock and flags fields in struct scx_rq to make sure
    they are in the same cacheline to minimize the cache misses 
  - Add an additional explanation to the commit message in the 2/5 patch
    describing when the rq clock will be reused with an example.
  - Fix typos
  - Rebase the code to the tip of Tejun's sched_ext repo

Changwoo Min (5):
  sched_ext: Implement scx_rq_clock_update/stale()
  sched_ext: Manage the validity of scx_rq_clock
  sched_ext: Implement scx_bpf_clock_get_ns()
  sched_ext: Add scx_bpf_clock_get_ns() for BPF scheduler
  sched_ext: Replace bpf_ktime_get_ns() to scx_bpf_clock_get_ns()

 kernel/sched/core.c                      |  6 +-
 kernel/sched/ext.c                       | 74 ++++++++++++++++++++++++
 kernel/sched/sched.h                     | 50 +++++++++++-----
 tools/sched_ext/include/scx/common.bpf.h |  1 +
 tools/sched_ext/include/scx/compat.bpf.h |  5 ++
 tools/sched_ext/scx_central.bpf.c        |  4 +-
 tools/sched_ext/scx_flatcg.bpf.c         |  2 +-
 7 files changed, 123 insertions(+), 19 deletions(-)

-- 
2.47.1