backoff.h

/* 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 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 */