linux_dsm_epyc7002/include
Nicholas Piggin fd851a3cdc spin loop primitives for busy waiting
Current busy-wait loops are implemented by repeatedly calling cpu_relax()
to give an arch option for a low-latency option to improve power and/or
SMT resource contention.

This poses some difficulties for powerpc, which has SMT priority setting
instructions (priorities determine how ifetch cycles are apportioned).
powerpc's cpu_relax() is implemented by setting a low priority then
setting normal priority. This has several problems:

 - Changing thread priority can have some execution cost and potential
   impact to other threads in the core. It's inefficient to execute them
   every time around a busy-wait loop.

 - Depending on implementation details, a `low ; medium` sequence may
   not have much if any affect. Some software with similar pattern
   actually inserts a lot of nops between, in order to cause a few fetch
   cycles with the low priority.

 - The busy-wait loop runs with regular priority. This might only be a few
   fetch cycles, but if there are several threads running such loops, they
   could cause a noticable impact on a non-idle thread.

Implement spin_begin, spin_end primitives that can be used around busy
wait loops, which default to no-ops. And spin_cpu_relax which defaults to
cpu_relax.

This will allow architectures to hook the entry and exit of busy-wait
loops, and will allow powerpc to set low SMT priority at entry, and
normal priority at exit.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
2017-06-28 22:49:11 +10:00
..
acpi More ACPI updates for v4.12-rc1 2017-05-10 09:35:42 -07:00
asm-generic powerpc: Link warning for orphan sections 2017-05-30 14:59:51 +10:00
clocksource
crypto Merge branch 'next' of git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security 2017-05-03 08:50:52 -07:00
drm mm, vmalloc: use __GFP_HIGHMEM implicitly 2017-05-08 17:15:13 -07:00
dt-bindings Sort of on the quieter side this time, which is probably due more 2017-05-10 13:38:18 -07:00
keys
kvm KVM: arm/arm64: Fix bug when registering redist iodevs 2017-05-18 11:18:12 +02:00
linux spin loop primitives for busy waiting 2017-06-28 22:49:11 +10:00
math-emu
media
memory
misc
net ipv4: add reference counting to metrics 2017-05-26 14:57:07 -04:00
pcmcia
ras
rdma smc_diag.h: fix include from userland 2017-05-11 00:18:39 +09:00
rxrpc
scsi Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/nab/target-pending 2017-05-12 11:44:13 -07:00
soc powerpc updates for 4.12 part 2 2017-05-12 10:04:09 -07:00
sound ASoC: Updates for v4.12 2017-05-02 08:25:25 +02:00
target target/user: PGR Support 2017-05-01 22:21:45 -07:00
trace Merge branch 'next' of git://git.kernel.org/pub/scm/linux/kernel/git/rzhang/linux 2017-05-12 11:58:45 -07:00
uapi USB fixes for 4.12-rc2 2017-05-20 08:52:34 -07:00
video uapi: export all headers under uapi directories 2017-05-11 00:21:54 +09:00
xen xen: Implement EFI reset_system callback 2017-05-02 12:06:50 +02:00