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