[PATCH v2 0/2] sched: Fix SCX_ENQ_CPU_SELECTED for queued wakeups

[Kuba Piecuch <jpiecuch@google.com>]

SCX_ENQ_CPU_SELECTED is meant to tell ops.enqueue() whether
ops.select_cpu() ran for a wakeup. However, wakeups delivered through
the ttwu wakelist never have the flag set, because sched_ttwu_pending()
drops WF_RQ_SELECTED. Schedulers that set SCX_OPS_ALLOW_QUEUED_WAKEUP
therefore cannot tell whether ops.select_cpu() was skipped, and
SCX_EV_SELECT_CPU_FALLBACK is undercounted.

Patch 1 carries WF_RQ_SELECTED across the wakelist the same way
WF_MIGRATED already is. Patch 2 adds a selftest. In a 4-vCPU VM, without
patch 1 the test reports that all of the roughly 100k queued wakeups in
a 2s run had ops.select_cpu() called but SCX_ENQ_CPU_SELECTED clear.
With patch 1 there are none.

Patch 1 changes kernel/sched/core.c and include/linux/sched.h, but only
sched_ext sees the effect. Peter, Tejun: I'll leave it to you to decide
whether the series goes through tip or the sched_ext tree.

The bug was found, and the series was drafted, with the help of an AI
coding agent.

Changes in v2:
- Patch 1: Reword the description to distinguish the wakelist path
  before ->select_task_rq() from the one after it, and fix the
  ttwu_do_activate() name (Andrea). Add Acked-by from Tejun and
  Reviewed-by from Andrea.
- Patch 2: Give each ping-pong task its own two CPUs and dispatch to
  SCX_DSQ_LOCAL. The waker and the wakee then always run on different
  CPUs, so the test no longer depends on tasks spreading out on their
  own (Sashiko). Skip on systems with fewer than four CPUs.

v1: https://lore.kernel.org/r/20261023144811.2230393-1-jpiecuch@google.com

Kuba Piecuch (2):
  sched/core: Preserve WF_RQ_SELECTED across queued wakeups
  selftests/sched_ext: Test SCX_ENQ_CPU_SELECTED with queued wakeups

 include/linux/sched.h                         |   6 +-
 kernel/sched/core.c                           |  10 +-
 tools/testing/selftests/sched_ext/Makefile    |   1 +
 .../sched_ext/enq_cpu_selected.bpf.c          | 111 ++++++++++
 .../selftests/sched_ext/enq_cpu_selected.c    | 194 ++++++++++++++++++
 5 files changed, 319 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/sched_ext/enq_cpu_selected.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/enq_cpu_selected.c


base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b
-- 
2.56.0.257.gf7497c7d27-goog