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

Christian Borntraeger
2016-10-21T11:59:12Z

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(-)

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2.5.5