diff options
Diffstat (limited to 'arch/sparc/include/asm/backoff.h')
| -rw-r--r-- | arch/sparc/include/asm/backoff.h | 69 |
1 files changed, 59 insertions, 10 deletions
diff --git a/arch/sparc/include/asm/backoff.h b/arch/sparc/include/asm/backoff.h index db3af0d30fb1..4e02086b839c 100644 --- a/arch/sparc/include/asm/backoff.h +++ b/arch/sparc/include/asm/backoff.h @@ -1,6 +1,46 @@ #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 @@ -11,16 +51,25 @@ #define BACKOFF_LABEL(spin_label, continue_label) \ spin_label -#define BACKOFF_SPIN(reg, tmp, label) \ - mov reg, tmp; \ -88: 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; +#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 |