1 #ifndef _SPARC64_BACKOFF_H
2 #define _SPARC64_BACKOFF_H
4 /* The macros in this file implement an exponential backoff facility
5 * for atomic operations.
7 * When multiple threads compete on an atomic operation, it is
8 * possible for one thread to be continually denied a successful
9 * completion of the compare-and-swap instruction. Heavily
10 * threaded cpu implementations like Niagara can compound this
11 * problem even further.
13 * When an atomic operation fails and needs to be retried, we spin a
14 * certain number of times. At each subsequent failure of the same
15 * operation we double the spin count, realizing an exponential
18 * When we spin, we try to use an operation that will cause the
19 * current cpu strand to block, and therefore make the core fully
20 * available to any other other runnable strands. There are two
21 * options, based upon cpu capabilities.
23 * On all cpus prior to SPARC-T4 we do three dummy reads of the
24 * condition code register. Each read blocks the strand for something
25 * between 40 and 50 cpu cycles.
27 * For SPARC-T4 and later we have a special "pause" instruction
28 * available. This is implemented using writes to register %asr27.
29 * The cpu will block the number of cycles written into the register,
30 * unless a disrupting trap happens first. SPARC-T4 specifically
31 * implements pause with a granularity of 8 cycles. Each strand has
32 * an internal pause counter which decrements every 8 cycles. So the
33 * chip shifts the %asr27 value down by 3 bits, and writes the result
34 * into the pause counter. If a value smaller than 8 is written, the
35 * chip blocks for 1 cycle.
37 * To achieve the same amount of backoff as the three %ccr reads give
38 * on earlier chips, we shift the backoff value up by 7 bits. (Three
39 * %ccr reads block for about 128 cycles, 1 << 7 == 128) We write the
40 * whole amount we want to block into the pause register, rather than
41 * loop writing 128 each time.
44 #define BACKOFF_LIMIT (4 * 1024)
48 #define BACKOFF_SETUP(reg) \
51 #define BACKOFF_LABEL(spin_label, continue_label) \
54 #define BACKOFF_SPIN(reg, tmp, label) \
59 .section .pause_3insn_patch,"ax";\
67 set BACKOFF_LIMIT, tmp; \
76 #define BACKOFF_SETUP(reg)
78 #define BACKOFF_LABEL(spin_label, continue_label) \
81 #define BACKOFF_SPIN(reg, tmp, label)
85 #endif /* _SPARC64_BACKOFF_H */