Many thanks to Jann Horn for a detailed review of v0.5:
https://lore.kernel.org/lkml/20210908184905.163787-1-posk@google.com/ (local)

The main change in this patchet vs v0.5 is pinning of
pages containing struct umcg_task of RUNNING workers
and their servers. This allows uaccess operations from
sched_submit_work (preempt-disabled). The pages are
pinned only for RUNNING workers, i.e. at most NR_CPUS
pages are pinned by UMCG at any one time, across the
whole machine.

A smaller change is the move of worker wakeup processing
from sched_update_worker() to exit_to_user_mode_loop(),
to ensure that no kernel locks are held if the worker
is descheduled there.

Commit messages in patches 2 and 3 list more changes.

Peter Oskolkov (5):
  sched/umcg: add WF_CURRENT_CPU and externise ttwu
  sched/umcg: RFC: add userspace atomic helpers
  sched/umcg: RFC: implement UMCG syscalls
  sched/umcg: add Documentation/userspace-api/umcg.rst
  sched/umcg: add Documentation/userspace-api/umcg.txt

 Documentation/userspace-api/umcg.rst   | 550 ++++++++++++++++++
 Documentation/userspace-api/umcg.txt   | 537 ++++++++++++++++++
 arch/x86/entry/syscalls/syscall_64.tbl |   2 +
 fs/exec.c                              |   1 +
 include/linux/sched.h                  |  56 ++
 include/linux/syscalls.h               |   4 +
 include/uapi/asm-generic/unistd.h      |   8 +-
 include/uapi/linux/umcg.h              | 117 ++++
 init/Kconfig                           |  10 +
 kernel/entry/common.c                  |   1 +
 kernel/exit.c                          |   2 +
 kernel/sched/Makefile                  |   1 +
 kernel/sched/core.c                    |  18 +-
 kernel/sched/fair.c                    |   4 +
 kernel/sched/sched.h                   |  15 +-
 kernel/sched/umcg.c                    | 745 +++++++++++++++++++++++++
 kernel/sched/umcg_uaccess.h            | 344 ++++++++++++
 kernel/sys_ni.c                        |   4 +
 18 files changed, 2408 insertions(+), 11 deletions(-)
 create mode 100644 Documentation/userspace-api/umcg.rst
 create mode 100644 Documentation/userspace-api/umcg.txt
 create mode 100644 include/uapi/linux/umcg.h
 create mode 100644 kernel/sched/umcg.c
 create mode 100644 kernel/sched/umcg_uaccess.h

--
2.25.1