From fd851a3cdc196bfc1d229b5f22369069af532bf8 Mon Sep 17 00:00:00 2001 From: Nicholas Piggin Date: Mon, 29 May 2017 12:22:23 +1000 Subject: [PATCH] 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 Signed-off-by: Nicholas Piggin Signed-off-by: Michael Ellerman --- include/linux/processor.h | 70 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 70 insertions(+) create mode 100644 include/linux/processor.h diff --git a/include/linux/processor.h b/include/linux/processor.h new file mode 100644 index 000000000000..da0c5e56ca02 --- /dev/null +++ b/include/linux/processor.h @@ -0,0 +1,70 @@ +/* Misc low level processor primitives */ +#ifndef _LINUX_PROCESSOR_H +#define _LINUX_PROCESSOR_H + +#include + +/* + * spin_begin is used before beginning a busy-wait loop, and must be paired + * with spin_end when the loop is exited. spin_cpu_relax must be called + * within the loop. + * + * The loop body should be as small and fast as possible, on the order of + * tens of instructions/cycles as a guide. It should and avoid calling + * cpu_relax, or any "spin" or sleep type of primitive including nested uses + * of these primitives. It should not lock or take any other resource. + * Violations of these guidelies will not cause a bug, but may cause sub + * optimal performance. + * + * These loops are optimized to be used where wait times are expected to be + * less than the cost of a context switch (and associated overhead). + * + * Detection of resource owner and decision to spin or sleep or guest-yield + * (e.g., spin lock holder vcpu preempted, or mutex owner not on CPU) can be + * tested within the loop body. + */ +#ifndef spin_begin +#define spin_begin() +#endif + +#ifndef spin_cpu_relax +#define spin_cpu_relax() cpu_relax() +#endif + +/* + * spin_cpu_yield may be called to yield (undirected) to the hypervisor if + * necessary. This should be used if the wait is expected to take longer + * than context switch overhead, but we can't sleep or do a directed yield. + */ +#ifndef spin_cpu_yield +#define spin_cpu_yield() cpu_relax_yield() +#endif + +#ifndef spin_end +#define spin_end() +#endif + +/* + * spin_until_cond can be used to wait for a condition to become true. It + * may be expected that the first iteration will true in the common case + * (no spinning), so that callers should not require a first "likely" test + * for the uncontended case before using this primitive. + * + * Usage and implementation guidelines are the same as for the spin_begin + * primitives, above. + */ +#ifndef spin_until_cond +#define spin_until_cond(cond) \ +do { \ + if (unlikely(!(cond))) { \ + spin_begin(); \ + do { \ + spin_cpu_relax(); \ + } while (!(cond)); \ + spin_end(); \ + } \ +} while (0) + +#endif + +#endif /* _LINUX_PROCESSOR_H */ -- 2.20.1