[PATCH 0/5] sched: Fix DL server starvation under core scheduling

[Fernand Sieber <sieberf@amazon.com>]

Under core scheduling the class a DL server reserves CPU time for gets
almost nothing. With the default fair server (50 ms / 1 s), a CFS task
spinning on one SMT sibling beside a cookied SCHED_FIFO task on the
other measures 0.000% of its CPU instead of the reserved 5%. The ext
server starves sched_ext tasks the same way.

Two independent mechanisms are at fault. The core-wide selection
mishandles the rq->dl_server hand-off: a stale link is passed to idle
(which then forces the siblings idle and trips the forced-idle WARN), a
sibling's link is lost on the fast path, and picks are ranked before
p->dl_server is set (patches 1-3, Fixes: bd9bbc96e835). Independently,
a deferred server is charged its rq's forced-idle time, so its runtime
runs out while it is still throttled, replenish_dl_new_period() pushes
the activation a full period out, and the next period of forced idle
pushes it again (patch 4, Fixes: a110a81c52a9). Either fix alone leaves
the share at ~0%.

Patch 5 adds cs_dl_server_test to selftests/sched: a cookied FIFO
spinner on one sibling, an uncookied CFS spinner on the other, and a
check of the CFS share of that CPU against half the fair server's
reservation, 2.5% by default. Run under QEMU with -smp 4,cores=2,threads=2.

Over a 5 s window, plain tip fails 10/10 at 0.000%, and with the whole
series it passes 31/31 at 3.99-5.19%.

This series was prepared with the help of an AI coding assistant. I
have reviewed and tested all of it.

Fernand Sieber (5):
  sched/core: Clear stale rq->dl_server when core-sched replaces a pick
  sched/core: Keep a sibling's DL server link in the core-wide pick
  sched/core: Rank DL server picks as deadline in the core-wide pick
  sched/deadline: Don't charge core-sched forced idle to the DL servers
  selftests/sched: Add a core-sched forced-idle DL server test

 kernel/sched/core.c                           |  59 ++-
 kernel/sched/idle.c                           |   7 +
 kernel/sched/sched.h                          |  15 +
 tools/testing/selftests/sched/.gitignore      |   1 +
 tools/testing/selftests/sched/Makefile        |   4 +-
 tools/testing/selftests/sched/config          |   4 +-
 .../selftests/sched/cs_dl_server_test.c       | 402 ++++++++++++++++++
 7 files changed, 471 insertions(+), 21 deletions(-)
 create mode 100644 tools/testing/selftests/sched/cs_dl_server_test.c


base-commit: d7e670601af6c564ccf6d3b2ddf70ee4f41a6d1c
-- 
2.47.3




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