rseq: Implement time slice extension mechanism

This is a follow up on the V1 version:

     https://lore.kernel.org/20250908225709.144709889@linutron...

Time slice extensions are an attempt to provide opportunistic priority
ceiling without the overhead of an actual priority ceiling protocol, but
also without the guarantees such a protocol provides.

The intent is to avoid situations where a user space thread is interrupted
in a critical section and scheduled out, while holding a resource on which
the preempting thread or other threads in the system might block on. That
obviously prevents those threads from making progress in the worst case for
at least a full time slice. Especially in the context of user space
spinlocks, which are a patently bad idea to begin with, but that's also
true for other mechanisms.

This series uses the existing RSEQ user memory to implement it.

Changes vs. V1:

   - Rebase on the newest RSEQ and uaccess changes

   - Use seperate bytes for request and grant and lift the atomic operation
     requirement for user space - Mathieu

   - Kconfig indentation, fix typos and expressions - Randy

   - Provide an extra stub for the !RSEQ case - Prateek

   - Use the proper name in sys_ni.c and add comment - Prateek

   - Return 1 from __setup() - Prateek


The uaccess and RSEQ modifications on which this series is based can be
found here:

    https://lore.kernel.org/20251022104005.907410538@linutron...

and in git:

    git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git rseq/cid

For your convenience all of it is also available as a conglomerate from
git:

    git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git rseq/slice

Thanks,

	tglx
---
Peter Zilstra (1):
      sched: Provide and use set_need_resched_current()

Thomas Gleixner (11):
      rseq: Add fields and constants for time slice extension
      rseq: Provide static branch for time slice extensions
      rseq: Add statistics for time slice extensions
      rseq: Add prctl() to enable time slice extensions
      rseq: Implement sys_rseq_slice_yield()
      rseq: Implement syscall entry work for time slice extensions
      rseq: Implement time slice extension enforcement timer
      rseq: Reset slice extension when scheduled
      rseq: Implement rseq_grant_slice_extension()
      entry: Hook up rseq time slice extension
      selftests/rseq: Implement time slice extension test

 Documentation/userspace-api/index.rst       |    1 
 Documentation/userspace-api/rseq.rst        |  118 ++++++++++
 arch/alpha/kernel/syscalls/syscall.tbl      |    1 
 arch/arm/tools/syscall.tbl                  |    1 
 arch/arm64/tools/syscall_32.tbl             |    1 
 arch/m68k/kernel/syscalls/syscall.tbl       |    1 
 arch/microblaze/kernel/syscalls/syscall.tbl |    1 
 arch/mips/kernel/syscalls/syscall_n32.tbl   |    1 
 arch/mips/kernel/syscalls/syscall_n64.tbl   |    1 
 arch/mips/kernel/syscalls/syscall_o32.tbl   |    1 
 arch/parisc/kernel/syscalls/syscall.tbl     |    1 
 arch/powerpc/kernel/syscalls/syscall.tbl    |    1 
 arch/s390/kernel/syscalls/syscall.tbl       |    1 
 arch/s390/mm/pfault.c                       |    3 
 arch/sh/kernel/syscalls/syscall.tbl         |    1 
 arch/sparc/kernel/syscalls/syscall.tbl      |    1 
 arch/x86/entry/syscalls/syscall_32.tbl      |    1 
 arch/x86/entry/syscalls/syscall_64.tbl      |    1 
 arch/xtensa/kernel/syscalls/syscall.tbl     |    1 
 include/linux/entry-common.h                |    2 
 include/linux/rseq.h                        |   11 +
 include/linux/rseq_entry.h                  |  190 ++++++++++++++++-
 include/linux/rseq_types.h                  |   28 ++
 include/linux/sched.h                       |    7 
 include/linux/syscalls.h                    |    1 
 include/linux/thread_info.h                 |   16 -
 include/uapi/asm-generic/unistd.h           |    5 
 include/uapi/linux/prctl.h                  |   10 
 include/uapi/linux/rseq.h                   |   38 +++
 init/Kconfig                                |   12 +
 kernel/entry/common.c                       |   14 +
 kernel/entry/syscall-common.c               |   11 -
 kernel/rcu/tiny.c                           |    8 
 kernel/rcu/tree.c                           |   14 -
 kernel/rcu/tree_exp.h                       |    3 
 kernel/rcu/tree_plugin.h                    |    9 
 kernel/rcu/tree_stall.h                     |    3 
 kernel/rseq.c                               |  304 ++++++++++++++++++++++++++++
 kernel/sys.c                                |    6 
 kernel/sys_ni.c                             |    1 
 scripts/syscall.tbl                         |    1 
 tools/testing/selftests/rseq/.gitignore     |    1 
 tools/testing/selftests/rseq/Makefile       |    5 
 tools/testing/selftests/rseq/rseq-abi.h     |   27 ++
 tools/testing/selftests/rseq/slice_test.c   |  198 ++++++++++++++++++
 45 files changed, 1011 insertions(+), 52 deletions(-)