From 2c3c1048746a4622d8c89a29670120dc8fab93c4 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 7 Apr 2024 20:49:45 +0200 Subject: Adding upstream version 6.1.76. Signed-off-by: Daniel Baumann --- arch/sparc/include/asm/backoff.h | 86 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 86 insertions(+) create mode 100644 arch/sparc/include/asm/backoff.h (limited to 'arch/sparc/include/asm/backoff.h') diff --git a/arch/sparc/include/asm/backoff.h b/arch/sparc/include/asm/backoff.h new file mode 100644 index 000000000..597a22953 --- /dev/null +++ b/arch/sparc/include/asm/backoff.h @@ -0,0 +1,86 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _SPARC64_BACKOFF_H +#define _SPARC64_BACKOFF_H + +/* The macros in this file implement an exponential backoff facility + * for atomic operations. + * + * When multiple threads compete on an atomic operation, it is + * possible for one thread to be continually denied a successful + * completion of the compare-and-swap instruction. Heavily + * threaded cpu implementations like Niagara can compound this + * problem even further. + * + * When an atomic operation fails and needs to be retried, we spin a + * certain number of times. At each subsequent failure of the same + * operation we double the spin count, realizing an exponential + * backoff. + * + * When we spin, we try to use an operation that will cause the + * current cpu strand to block, and therefore make the core fully + * available to any other runnable strands. There are two + * options, based upon cpu capabilities. + * + * On all cpus prior to SPARC-T4 we do three dummy reads of the + * condition code register. Each read blocks the strand for something + * between 40 and 50 cpu cycles. + * + * For SPARC-T4 and later we have a special "pause" instruction + * available. This is implemented using writes to register %asr27. + * The cpu will block the number of cycles written into the register, + * unless a disrupting trap happens first. SPARC-T4 specifically + * implements pause with a granularity of 8 cycles. Each strand has + * an internal pause counter which decrements every 8 cycles. So the + * chip shifts the %asr27 value down by 3 bits, and writes the result + * into the pause counter. If a value smaller than 8 is written, the + * chip blocks for 1 cycle. + * + * To achieve the same amount of backoff as the three %ccr reads give + * on earlier chips, we shift the backoff value up by 7 bits. (Three + * %ccr reads block for about 128 cycles, 1 << 7 == 128) We write the + * whole amount we want to block into the pause register, rather than + * loop writing 128 each time. + */ + +#define BACKOFF_LIMIT (4 * 1024) + +#ifdef CONFIG_SMP + +#define BACKOFF_SETUP(reg) \ + mov 1, reg + +#define BACKOFF_LABEL(spin_label, continue_label) \ + spin_label + +#define BACKOFF_SPIN(reg, tmp, label) \ + mov reg, tmp; \ +88: rd %ccr, %g0; \ + rd %ccr, %g0; \ + rd %ccr, %g0; \ + .section .pause_3insn_patch,"ax";\ + .word 88b; \ + sllx tmp, 7, tmp; \ + wr tmp, 0, %asr27; \ + clr tmp; \ + .previous; \ + brnz,pt tmp, 88b; \ + sub tmp, 1, tmp; \ + set BACKOFF_LIMIT, tmp; \ + cmp reg, tmp; \ + bg,pn %xcc, label; \ + nop; \ + ba,pt %xcc, label; \ + sllx reg, 1, reg; + +#else + +#define BACKOFF_SETUP(reg) + +#define BACKOFF_LABEL(spin_label, continue_label) \ + continue_label + +#define BACKOFF_SPIN(reg, tmp, label) + +#endif + +#endif /* _SPARC64_BACKOFF_H */ -- cgit v1.2.3