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fd851a3cdc
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>
71 lines
2.1 KiB
C
71 lines
2.1 KiB
C
/* Misc low level processor primitives */
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#ifndef _LINUX_PROCESSOR_H
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#define _LINUX_PROCESSOR_H
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#include <asm/processor.h>
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/*
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* spin_begin is used before beginning a busy-wait loop, and must be paired
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* with spin_end when the loop is exited. spin_cpu_relax must be called
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* within the loop.
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*
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* The loop body should be as small and fast as possible, on the order of
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* tens of instructions/cycles as a guide. It should and avoid calling
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* cpu_relax, or any "spin" or sleep type of primitive including nested uses
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* of these primitives. It should not lock or take any other resource.
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* Violations of these guidelies will not cause a bug, but may cause sub
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* optimal performance.
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*
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* These loops are optimized to be used where wait times are expected to be
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* less than the cost of a context switch (and associated overhead).
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*
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* Detection of resource owner and decision to spin or sleep or guest-yield
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* (e.g., spin lock holder vcpu preempted, or mutex owner not on CPU) can be
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* tested within the loop body.
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*/
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#ifndef spin_begin
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#define spin_begin()
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#endif
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#ifndef spin_cpu_relax
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#define spin_cpu_relax() cpu_relax()
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#endif
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/*
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* spin_cpu_yield may be called to yield (undirected) to the hypervisor if
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* necessary. This should be used if the wait is expected to take longer
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* than context switch overhead, but we can't sleep or do a directed yield.
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*/
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#ifndef spin_cpu_yield
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#define spin_cpu_yield() cpu_relax_yield()
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#endif
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#ifndef spin_end
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#define spin_end()
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#endif
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/*
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* spin_until_cond can be used to wait for a condition to become true. It
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* may be expected that the first iteration will true in the common case
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* (no spinning), so that callers should not require a first "likely" test
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* for the uncontended case before using this primitive.
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*
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* Usage and implementation guidelines are the same as for the spin_begin
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* primitives, above.
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*/
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#ifndef spin_until_cond
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#define spin_until_cond(cond) \
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do { \
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if (unlikely(!(cond))) { \
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spin_begin(); \
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do { \
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spin_cpu_relax(); \
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} while (!(cond)); \
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spin_end(); \
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} \
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} while (0)
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#endif
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#endif /* _LINUX_PROCESSOR_H */
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