linux-kernel - [PATCH/RFC 0/5] cpu_relax: introduce yield, remove lowlatency

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Date:   Fri, 21 Oct 2016 13:58:53 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Nicholas Piggin <npiggin@...il.com>, linux-kernel@...r.kernel.org,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Noam Camus <noamc@...hip.com>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: [PATCH/RFC 0/5] cpu_relax: introduce yield, remove lowlatency

For spinning loops people did 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 s390
cpu_relax gives up the time slice to the hypervisor. On power cpu_relax
tries to give some of the CPU to the neighbor threads. To reduce the
latency another variant cpu_relax_lowlatency was introduced. Before this
is used in more and more places, lets revert the logic of provide a new
function cpu_relax_yield that can spend some time and for s390 yields
the guest CPU.

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
  threads)
- really give up CPU: use  cpu_relax_yield

The alternative is to keep cpu_relax_lowlatency if there is some need.

Not fully sure about arc/eznps and power, but lets hear first if the
approach is ok.

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
  Remove cpu_relax_lowlatency users
  remove cpu_relax_lowlatency

 arch/alpha/include/asm/processor.h      | 2 +-
 arch/arc/include/asm/processor.h        | 2 ++
 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/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 +-
 41 files changed, 52 insertions(+), 50 deletions(-)

-- 
2.5.5