[RFC PATCH 0/4 v0.3] sched/UMCG

[Peter Oskolkov <posk@posk.io>]

This is another attempt at implementing UMCG, based on
discussion in https://lore.kernel.org/patchwork/cover/1433967/ and
https://lore.kernel.org/lkml/20210708194638.128950-1-posk@google.com/

Most of the "why" is covered here (some details are obsolete):
https://lore.kernel.org/patchwork/cover/1433967/#1632328

At a high level, UMCG servers/workers provide the foundation
for an M:N threading library, as described in the link above.

Joel Fernandes has also once mentioned that he had a use case
for a wake+bring-the-wakee-to-the-current-CPU operation,
so this is now also supported via UMCG_WF_CURRENT_CPU flag
(patch 3).

Patch 1: add WF_CURRENT_CPU and tweak ttwu - same as last time
Patch 2: add X86_64 helpers to work atomically with userspace values
Patch 3: add helpers to work with single-linked lists in userspace
Patch 4: implement UMCG kernel-side

In this version of the patchset I used only userspace/TLS
data, as suggested by Peter Zijlstra. With the exception
of one issue (see patch 3 commit message) everything seems
to be working great.

This TLS-only approach makes the userspace code a bit more
involved, so I'm not posting libumcg/selftests with this
patchset to focus on the kernel side only.

v0.2->v0.3 chages:
- split patch 2 into two paches: atomic ops and llist ops
- rework atomic ops in patch 2 to avoid "stac/clac nonsense"
- make llist kernel-side operations constant time (no indefinite
  spinning)
- make task wakeup work without spinning/retries

I'm not aware of any issues with this patchset other than
what's mentioned below. In short, it seems that "SMP basics":
block/wake detection, worker "scheduling" by servers, etc.
all work.

TODO:
  - combine cmpxchg_user_32/64 functions into a macro in patch 2
  - implement timeout handling
  - imlement worker preemption
  - more testing
  - manpages, docs, and similar
  - attach libumbc and selftest patches

Peter Oskolkov (4):
  sched: add WF_CURRENT_CPU and externise ttwu
  sched/umcg: RFC: add userspace atomic helpers
  sched/umcg: RFC: add userspace sll helpers
  sched/umcg: RFC: implement UMCG syscalls

 arch/x86/entry/syscalls/syscall_64.tbl |   2 +
 include/linux/sched.h                  |   6 +
 include/linux/syscalls.h               |   4 +
 include/uapi/asm-generic/unistd.h      |   8 +-
 include/uapi/linux/umcg.h              | 259 +++++++++++++
 init/Kconfig                           |  10 +
 kernel/exit.c                          |   7 +
 kernel/sched/Makefile                  |   1 +
 kernel/sched/core.c                    |  20 +-
 kernel/sched/fair.c                    |   4 +
 kernel/sched/sched.h                   |  15 +-
 kernel/sched/umcg.c                    | 485 +++++++++++++++++++++++++
 kernel/sched/umcg.h                    | 298 +++++++++++++++
 kernel/sys_ni.c                        |   4 +
 14 files changed, 1112 insertions(+), 11 deletions(-)
 create mode 100644 include/uapi/linux/umcg.h
 create mode 100644 kernel/sched/umcg.c
 create mode 100644 kernel/sched/umcg.h

--
2.25.1