[PATCH RFC 0/7] rcutorture: Rewrite RCU priority-boost test around a CPU-hungry reader

[Joel Fernandes <joelagnelf@nvidia.com>]

Hi,

This RFC rewrites rcutorture's RCU priority-boost test so that it no longer
depends on disabling RT throttling and no longer infers boost failure from a
grace period merely taking too long, which is fragile.

The idea: make the to-be-boosted reader need a *lot* of CPU time, more than
throttling could ever give it within the test window.  Each test interval,
one CPU runs a SCHED_NORMAL reader that sits in an RCU read-side critical
section and spins until it has consumed test_boost_reader_ms of CPU time
(default 2000 ms), measured with the task's sum_exec_runtime so time spent
preempted does not count.  A SCHED_FIFO hog on the same CPU tries to starve
it, and a grace period is started so the reader blocks it.  An unboosted
reader only gets what the throttling leaves it, about 5% of the CPU
(~200 ms over the 4 s test_boost_duration), so it cannot finish.  If it does
finish in time, boosting worked.  If not, it is counted as a boost failure.

Related work: Sebastian's recent series on the DL fair_server taking over RT
throttling [1], with its RFC [2], also touches rcutorture's handling of RT
throttling. This patch should simplify or make those patches unnecessary.

Testing, all with kvm.sh on an x86 host, --duration 10 and
--bootargs "rcutorture.test_boost=2" (which forces the test on in scenarios
whose .boot file disables it), 10 minutes per scenario:

  Scenario      Result    Boost failures / intervals
  TREE01        SUCCESS   0 / 70
  TREE02        SUCCESS   0 / 73
  TREE03        SUCCESS   0 / 71
  TREE08        SUCCESS   0 / 74
  TREE09        SUCCESS   0 / 59
  BUSTED-BOOST  FAILURE   82 / 84   (expected, boosting disabled)

BUSTED-BOOST boots with rcutree.boost_disable=1 (patches 5-7), so its failures
show that the test catches broken boosting.  Each failure reports e.g. "Boost
inversion: reader got only 203 of 2000 ms CPU in 4003 jiffies", the CPU time
throttling leaves an unboosted task.

The series applies on top of the rcu/next tree.

[1] https://lore.kernel.org/all/20261110-sched_dl-v2-0-cc9399f7aeb1@linutronix.de/
[2] https://lore.kernel.org/all/20261023161348.t3lpPLQZ@linutronix.de/

Joel Fernandes (7):
  rcutorture: Rewrite RCU priority-boost test around a CPU-hungry reader
  rcutorture: Stop disabling RT throttling for boost testing
  rcutorture: Remove unused check_boost_failed hook
  doc: Document rcutorture.test_boost_reader_ms
  rcu: Add rcutree.boost_disable to disable priority boosting at runtime
  doc: Document rcutree.boost_disable
  rcutorture: Make BUSTED-BOOST disable RCU priority boosting

 .../admin-guide/kernel-parameters.txt         |  14 ++
 kernel/rcu/rcutorture.c                       | 192 +++++++++---------
 kernel/rcu/tree_plugin.h                      |   5 +-
 .../rcutorture/configs/rcu/BUSTED-BOOST.boot  |   1 +
 4 files changed, 116 insertions(+), 96 deletions(-)


base-commit: 44646ae309330530a23e1e4c44fd47d274cfd0ea
-- 
2.54.0