[RFC PATCH v2 0/4] Extend rseq with sched_state_ptr field

[Mathieu Desnoyers]

This prototype extends struct rseq with a new sched_state_ptr field,
which points to a structure containing a "on-cpu" flag kept up-to-date
by the scheduler.

It is meant to be used by userspace adaptative mutexes to decide between
busy-wait and futex wait system call (releasing the CPU) behaviors based
on the current state of the mutex owner.

The goal is to improve use-cases where the duration of the critical
sections for a given lock follows a multi-modal distribution, preventing
statistical guesses from doing a good job at choosing between busy-wait
and futex wait behavior.

This is in response to the LWN coverage of 2023 Open Source Summit North
America (https://lwn.net/Articles/931789/) unscheduled slot "Adaptive
spinning in user space" presented by André Almeida.

New in this v2:

- Introduce a "struct rseq_sched_state", which contains the on-cpu
  scheduler flag and a thread ID field. This eliminates false sharing
  on the struct rseq cache lines caused by busy-waiting.

I have favored adding a "thread ID" field to struct rseq_sched_state
rather than adding stores of owner pointer in addition to a
compare-and-swap and store on a uint32_t for lock state to minimize the
number of stores to perform on the fast-path.

Feedback is welcome!

Mathieu

Mathieu Desnoyers (4):
  rseq: Add sched_state field to struct rseq
  selftests/rseq: Add sched_state rseq field and getter
  selftests/rseq: Implement sched state test program
  selftests/rseq: Implement rseq_mutex test program

 include/linux/sched.h                         |  16 +++
 include/uapi/linux/rseq.h                     |  41 ++++++
 kernel/rseq.c                                 |  43 +++++++
 tools/testing/selftests/rseq/.gitignore       |   2 +
 tools/testing/selftests/rseq/Makefile         |   3 +-
 tools/testing/selftests/rseq/rseq-abi.h       |  42 ++++++
 tools/testing/selftests/rseq/rseq.c           |  13 ++
 tools/testing/selftests/rseq/rseq.h           |   5 +
 tools/testing/selftests/rseq/rseq_mutex.c     | 120 ++++++++++++++++++
 .../testing/selftests/rseq/sched_state_test.c |  72 +++++++++++
 10 files changed, 356 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/rseq/rseq_mutex.c
 create mode 100644 tools/testing/selftests/rseq/sched_state_test.c

-- 
2.25.1