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@xxxxxxxxxx/ 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
