[RFC PATCH 0/3 v0.2] RFC: sched/UMCG

From: Peter Oskolkov
Date: Thu Jul 08 2021 - 15:46:50 EST


This is another attempt at implementing UMCG, based on
discussion in https://lore.kernel.org/patchwork/cover/1433967/

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.

In addition, servers without workers can be used as "basic"
UMCG tasks if wait/wake/context-switch are the only desired
operations.

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: 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.


TODO:
- put atomic helpers from patch 2 into their proper place (unless
keeping them in kernel/sched/umcg.h is OK)
- fix the wake server issue in preempt disable block (see patch 3)
- implement timeout handling
- imlement worker preemption
- more testing
- manpages, docs, and similar
- attach libumbc and selftest patches


Peter Oskolkov (3):
sched: add WF_CURRENT_CPU and externise ttwu
sched/umcg: RFC: add userspace atomic 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 | 246 +++++++++++++
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 | 481 +++++++++++++++++++++++++
kernel/sched/umcg.h | 271 ++++++++++++++
kernel/sys_ni.c | 4 +
14 files changed, 1068 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

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2.25.1