[PATCH 0/2] sched/fair: Stop core-scheduled siblings trading rescheduling IPIs

[Fernand Sieber <sieberf@amazon.com>]

Since v6.17, two SMT siblings under core scheduling can keep sending each
other rescheduling IPIs, thousands per CPU per second, while each keeps
running the same task.

Commit 0b9ca2dcabc3 ("sched/fair: Always trigger resched at the end of a
protected period") makes update_curr() ask for a reschedule once the
running entity's protected period has expired. set_protect_slice() only
rearms that period on a fresh pick (SNT_PICK), so an entity that is picked
again keeps an expired vprot and every later update_curr() on it asks
again. Under core scheduling the ask is remote: a core wide pick calls
pick_task() - hence update_curr() - on every sibling's runqueue, so each
sibling IPIs the other, which schedules, re-picks the same entity, renews
nothing, and IPIs back.

Patch 1 renews the protection on a repick. Patch 2 adds a selftest that
cookies one SMT pair, runs two spinners on each sibling and fails above
1000 rescheduling IPIs per CPU per second. It reproduces in a 4 vCPU QEMU
guest (-smp 4,sockets=1,cores=2,threads=2) with CONFIG_SCHED_CORE=y and
CONFIG_PREEMPT_LAZY=n: without patch 1 it fails 6 runs out of 7, at
808-2123 IPIs/CPU/s; with it all 7 pass, at 224-271.

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

Fernand Sieber (2):
  sched/fair: Renew slice protection when current is picked again
  selftests/sched: Add a core scheduling rescheduling IPI test

 kernel/sched/fair.c                           |   7 +
 tools/testing/selftests/sched/.gitignore      |   1 +
 tools/testing/selftests/sched/Makefile        |   4 +-
 tools/testing/selftests/sched/config          |   2 +-
 .../selftests/sched/cs_resched_ipi_test.c     | 377 ++++++++++++++++++
 5 files changed, 388 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/sched/cs_resched_ipi_test.c

-- 
2.47.3




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