1 /* SPDX-License-Identifier: GPL-2.0 */ 2 #ifndef _SPARC64_BACKOFF_H 3 #define _SPARC64_BACKOFF_H 4 5 /* The macros in this file implement an exponential backoff facility 6 * for atomic operations. 7 * 8 * When multiple threads compete on an atomic operation, it is 9 * possible for one thread to be continually denied a successful 10 * completion of the compare-and-swap instruction. Heavily 11 * threaded cpu implementations like Niagara can compound this 12 * problem even further. 13 * 14 * When an atomic operation fails and needs to be retried, we spin a 15 * certain number of times. At each subsequent failure of the same 16 * operation we double the spin count, realizing an exponential 17 * backoff. 18 * 19 * When we spin, we try to use an operation that will cause the 20 * current cpu strand to block, and therefore make the core fully 21 * available to any other other runnable strands. There are two 22 * options, based upon cpu capabilities. 23 * 24 * On all cpus prior to SPARC-T4 we do three dummy reads of the 25 * condition code register. Each read blocks the strand for something 26 * between 40 and 50 cpu cycles. 27 * 28 * For SPARC-T4 and later we have a special "pause" instruction 29 * available. This is implemented using writes to register %asr27. 30 * The cpu will block the number of cycles written into the register, 31 * unless a disrupting trap happens first. SPARC-T4 specifically 32 * implements pause with a granularity of 8 cycles. Each strand has 33 * an internal pause counter which decrements every 8 cycles. So the 34 * chip shifts the %asr27 value down by 3 bits, and writes the result 35 * into the pause counter. If a value smaller than 8 is written, the 36 * chip blocks for 1 cycle. 37 * 38 * To achieve the same amount of backoff as the three %ccr reads give 39 * on earlier chips, we shift the backoff value up by 7 bits. (Three 40 * %ccr reads block for about 128 cycles, 1 << 7 == 128) We write the 41 * whole amount we want to block into the pause register, rather than 42 * loop writing 128 each time. 43 */ 44 45 #define BACKOFF_LIMIT (4 * 1024) 46 47 #ifdef CONFIG_SMP 48 49 #define BACKOFF_SETUP(reg) \ 50 mov 1, reg 51 52 #define BACKOFF_LABEL(spin_label, continue_label) \ 53 spin_label 54 55 #define BACKOFF_SPIN(reg, tmp, label) \ 56 mov reg, tmp; \ 57 88: rd %ccr, %g0; \ 58 rd %ccr, %g0; \ 59 rd %ccr, %g0; \ 60 .section .pause_3insn_patch,"ax";\ 61 .word 88b; \ 62 sllx tmp, 7, tmp; \ 63 wr tmp, 0, %asr27; \ 64 clr tmp; \ 65 .previous; \ 66 brnz,pt tmp, 88b; \ 67 sub tmp, 1, tmp; \ 68 set BACKOFF_LIMIT, tmp; \ 69 cmp reg, tmp; \ 70 bg,pn %xcc, label; \ 71 nop; \ 72 ba,pt %xcc, label; \ 73 sllx reg, 1, reg; 74 75 #else 76 77 #define BACKOFF_SETUP(reg) 78 79 #define BACKOFF_LABEL(spin_label, continue_label) \ 80 continue_label 81 82 #define BACKOFF_SPIN(reg, tmp, label) 83 84 #endif 85 86 #endif /* _SPARC64_BACKOFF_H */