[Xen-devel] [GIT PULL v2 0/5] cpu_relax: drop lowlatency, introduce yield

[Christian Borntraeger <borntraeger@xxxxxxxxxx>]

Peter,

here is v2 with some improved patch descriptions and some fixes. The
previous version has survived one day of linux-next and I only changed
small parts.
So unless there is some other issue, feel free to pull (or to apply
the patches) to tip/locking.

The following changes since commit 07d9a380680d1c0eb51ef87ff2eab5c994949e69:

  Linux 4.9-rc2 (2016-10-23 17:10:14 -0700)

are available in the git repository at:

  git://git.kernel.org/pub/scm/linux/kernel/git/borntraeger/linux.git  
tags/cpurelax

for you to fetch changes up to dcc37f9044436438360402714b7544a8e8779b07:

  processor.h: remove cpu_relax_lowlatency (2016-10-25 09:49:57 +0200)

----------------------------------------------------------------
cpu_relax: drop lowlatency, introduce yield

For spinning loops people do often use barrier() or cpu_relax().
For most architectures cpu_relax and barrier are the same, but on
some architectures cpu_relax can add some latency.
For example on power,sparc64 and arc, cpu_relax can shift the CPU
towards other hardware threads in an SMT environment.
On s390 cpu_relax does even more, it uses an hypercall to the
hypervisor to give up the timeslice.
In contrast to the SMT yielding this can result in larger latencies.
In some places this latency is unwanted, so another variant
"cpu_relax_lowlatency" was introduced. Before this is used in more
and more places, lets revert the logic and provide a cpu_relax_yield
that can be called in places where yielding is more important than
latency. By default this is the same as cpu_relax on all architectures.

So my proposal boils down to:
- lowest latency: use barrier() or mb() if necessary
- low latency: use cpu_relax (e.g. might give up some cpu for the other
  _hardware_ threads)
- really give up CPU: use  cpu_relax_yield

PS: In the long run I would also try to provide for s390 something
like cpu_relax_yield_to with a cpu number (or just add that to
cpu_relax_yield), since a yield_to is always better than a yield as
long as we know the waiter.

----------------------------------------------------------------
Christian Borntraeger (5):
      processor.h: introduce cpu_relax_yield
      stop_machine: yield CPU during stop machine
      s390: make cpu_relax a barrier again
      processor.h: Remove cpu_relax_lowlatency users
      processor.h: remove cpu_relax_lowlatency

 arch/alpha/include/asm/processor.h      | 2 +-
 arch/arc/include/asm/processor.h        | 4 ++--
 arch/arm/include/asm/processor.h        | 2 +-
 arch/arm64/include/asm/processor.h      | 2 +-
 arch/avr32/include/asm/processor.h      | 2 +-
 arch/blackfin/include/asm/processor.h   | 2 +-
 arch/c6x/include/asm/processor.h        | 2 +-
 arch/cris/include/asm/processor.h       | 2 +-
 arch/frv/include/asm/processor.h        | 2 +-
 arch/h8300/include/asm/processor.h      | 2 +-
 arch/hexagon/include/asm/processor.h    | 2 +-
 arch/ia64/include/asm/processor.h       | 2 +-
 arch/m32r/include/asm/processor.h       | 2 +-
 arch/m68k/include/asm/processor.h       | 2 +-
 arch/metag/include/asm/processor.h      | 2 +-
 arch/microblaze/include/asm/processor.h | 2 +-
 arch/mips/include/asm/processor.h       | 2 +-
 arch/mn10300/include/asm/processor.h    | 2 +-
 arch/nios2/include/asm/processor.h      | 2 +-
 arch/openrisc/include/asm/processor.h   | 2 +-
 arch/parisc/include/asm/processor.h     | 2 +-
 arch/powerpc/include/asm/processor.h    | 2 +-
 arch/s390/include/asm/processor.h       | 4 ++--
 arch/s390/kernel/processor.c            | 4 ++--
 arch/score/include/asm/processor.h      | 2 +-
 arch/sh/include/asm/processor.h         | 2 +-
 arch/sparc/include/asm/processor_32.h   | 2 +-
 arch/sparc/include/asm/processor_64.h   | 2 +-
 arch/tile/include/asm/processor.h       | 2 +-
 arch/unicore32/include/asm/processor.h  | 2 +-
 arch/x86/include/asm/processor.h        | 2 +-
 arch/x86/um/asm/processor.h             | 2 +-
 arch/xtensa/include/asm/processor.h     | 2 +-
 drivers/gpu/drm/i915/i915_gem_request.c | 2 +-
 drivers/vhost/net.c                     | 4 ++--
 kernel/locking/mcs_spinlock.h           | 4 ++--
 kernel/locking/mutex.c                  | 4 ++--
 kernel/locking/osq_lock.c               | 6 +++---
 kernel/locking/qrwlock.c                | 6 +++---
 kernel/locking/rwsem-xadd.c             | 4 ++--
 kernel/stop_machine.c                   | 2 +-
 lib/lockref.c                           | 2 +-
 42 files changed, 53 insertions(+), 53 deletions(-)


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