[PATCH v7 00/11] support "task_isolated" mode for nohz_full

[Chris Metcalf]

The cover email for the patch series is getting a little unwieldy
so I will provide a terser summary here, and just update the
list of changes from version to version.  Please see the previous
versions linked by the In-Reply-To for more detailed comments
about changes in earlier versions of the patch series.

v7:
  The main change in this version is a change in where we call
  task_isolation_enter().  The arm64 code only invokes the
  context_tracking code right at kernel entry, and right at kernel
  exit, and the exit point is too late for task isolation; one of my
  test cases, when run on arm64, showed that a signal delivered while
  task isolation is waiting for the timer interrupt to quiesce was not
  properly handled before returning to userspace.  The tilegx code
  properly handled that case because it ran user_exit() in the
  work-pending loop.  But since arm64 calls user_exit() later, it was
  too late to go back and handle the signal.  I decided to make the
  task isolation work explicit in the "work" loop done on return to
  userspace, and although I could have done this by hacking up the
  arm64 assembly code for this purpose, I decided to follow the x86
  approach and use the prepare_exit_to_usermode() model where
  architectures handles work looping in C code.  I added that support
  to arm64 and tile as a pre-requisite change, then modified the loop
  in C to call task isolation appropriately.  This both makes the
  slowpath return-to-user code more maintainable for arm64 and tile
  going forward, and also avoids some of the subtlety where the
  context tracking code was being asked to invoke task isolation at
  user_enter() time.

  As a result of this change, I have moved all the
  architecture-specific changes to individual patches: two patches to
  switch arm64 and tile to the prepare_exit_to_usermode() loop, and
  three patches (one each for x86, arm64, and tile) to add the
  necessary call to task_isolation(), plus changes to check at syscall
  entry for strict mode.

  In addition, since arm64 doesn't use exception_enter(), I added an
  explicit call to task_isolation_exception() in do_mem_abort() so
  that page faults would be properly flagged in strict mode.

  I also added an RCU_LOCKDEP_WARN() at Andy Lutomirski's suggestion.

  And, the patch series is rebased to v4.3-rc1.

v6:
  restructured to be a "task_isolation" mode not a "cpu_isolated"
  mode (Frederic)

v5:
  rebased on kernel v4.2-rc3
  converted to use CONFIG_CPU_ISOLATED and separate .c and .h files
  incorporates Christoph Lameter's quiet_vmstat() call

v4:
  rebased on kernel v4.2-rc1
  added support for detecting CPU_ISOLATED_STRICT syscalls on arm64

v3:
  remove dependency on cpu_idle subsystem (Thomas Gleixner)
  use READ_ONCE instead of ACCESS_ONCE in tick_nohz_cpu_isolated_enter
  use seconds for console messages instead of jiffies (Thomas Gleixner)
  updated commit description for patch 5/5

v2:
  rename "dataplane" to "cpu_isolated"
  drop ksoftirqd suppression changes (believed no longer needed)
  merge previous "QUIESCE" functionality into baseline functionality
  explicitly track syscalls and exceptions for "STRICT" functionality
  allow configuring a signal to be delivered for STRICT mode failures
  move debug tracking to irq_enter(), not irq_exit()

General summary:

The existing nohz_full mode does a nice job of suppressing extraneous
kernel interrupts for cores that desire it.  However, there is a need
for a more deterministic mode that rigorously disallows kernel
interrupts, even at a higher cost in user/kernel transition time:
for example, high-speed networking applications running userspace
drivers that will drop packets if they are ever interrupted.

These changes attempt to provide an initial draft of such a framework;
the changes do not add any overhead to the usual non-nohz_full mode,
and only very small overhead to the typical nohz_full mode.  The
kernel must be built with CONFIG_TASK_ISOLATION to take advantage of
this new mode.  A prctl() option (PR_SET_TASK_ISOLATION) is added to
control whether processes have requested this stricter semantics, and
within that prctl() option we provide a number of different bits for
more precise control.  Additionally, we add a new command-line boot
argument to facilitate debugging where unexpected interrupts are being
delivered from.

Code that is conceptually similar has been in use in Tilera's
Multicore Development Environment since 2008, known as Zero-Overhead
Linux, and has seen wide adoption by a range of customers.  This patch
series represents the first serious attempt to upstream that
functionality.  Although the current state of the kernel isn't quite
ready to run with absolutely no kernel interrupts (for example,
workqueues on task_isolation cores still remain to be dealt with), this
patch series provides a way to make dynamic tradeoffs between avoiding
kernel interrupts on the one hand, and making voluntary calls in and
out of the kernel more expensive, for tasks that want it.

The series (based currently on v4.3-rc1) is available at:

  git://git.kernel.org/pub/scm/linux/kernel/git/cmetcalf/linux-tile.git dataplane

Note: I have not removed the commit to disable the 1Hz timer tick
fallback that was nack'ed by PeterZ, pending a decision on that thread
as to what to do (https://lkml.org/lkml/2015/5/8/555); also since if
we remove the 1Hz tick, task_isolation threads will never re-enter
userspace since a tick will always be pending.

Chris Metcalf (10):
  task_isolation: add initial support
  task_isolation: support PR_TASK_ISOLATION_STRICT mode
  task_isolation: provide strict mode configurable signal
  task_isolation: add debug boot flag
  nohz: task_isolation: allow tick to be fully disabled
  arch/x86: enable task isolation functionality
  arch/arm64: adopt prepare_exit_to_usermode() model from x86
  arch/arm64: enable task isolation functionality
  arch/tile: adopt prepare_exit_to_usermode() model from x86
  arch/tile: enable task isolation functionality

Christoph Lameter (1):
  vmstat: provide a function to quiet down the diff processing

 Documentation/kernel-parameters.txt  |   7 ++
 arch/arm64/include/asm/thread_info.h |  18 +++--
 arch/arm64/kernel/entry.S            |   6 +-
 arch/arm64/kernel/ptrace.c           |  10 ++-
 arch/arm64/kernel/signal.c           |  36 +++++++---
 arch/arm64/mm/fault.c                |   8 +++
 arch/tile/include/asm/processor.h    |   2 +-
 arch/tile/include/asm/thread_info.h  |   8 ++-
 arch/tile/kernel/intvec_32.S         |  46 ++++---------
 arch/tile/kernel/intvec_64.S         |  49 +++++---------
 arch/tile/kernel/process.c           |  92 ++++++++++++++-----------
 arch/tile/kernel/ptrace.c            |   3 +
 arch/tile/mm/homecache.c             |   5 +-
 arch/x86/entry/common.c              |  45 ++++++++++---
 include/linux/context_tracking.h     |  11 ++-
 include/linux/isolation.h            |  42 ++++++++++++
 include/linux/sched.h                |   3 +
 include/linux/vmstat.h               |   2 +
 include/uapi/linux/prctl.h           |   8 +++
 init/Kconfig                         |  20 ++++++
 kernel/Makefile                      |   1 +
 kernel/context_tracking.c            |   9 ++-
 kernel/irq_work.c                    |   5 +-
 kernel/isolation.c                   | 127 +++++++++++++++++++++++++++++++++++
 kernel/sched/core.c                  |  21 ++++++
 kernel/signal.c                      |   5 ++
 kernel/smp.c                         |   4 ++
 kernel/softirq.c                     |   7 ++
 kernel/sys.c                         |   8 +++
 kernel/time/tick-sched.c             |   3 +-
 mm/vmstat.c                          |  14 ++++
 31 files changed, 477 insertions(+), 148 deletions(-)
 create mode 100644 include/linux/isolation.h
 create mode 100644 kernel/isolation.c

-- 
2.1.2

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