arch/sparc/include/asm/backoff.h

   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 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 */