This has been in the ia64 (and hence -mm) trees for a couple of months.

Changelog:
 * Affects only architectures which define CONFIG_TIME_INTERPOLATION
   (currently only IA64)
 * Genericize time interpolation, make time interpolators easily usable
   and provide instructions on how to use the interpolator for other
   architectures.
 * Provide nanosecond resolution for clock_gettime and an accuracy
   up to the time interpolator time base.
 * clock_getres() reports resolution of underlying time basis which
   is typically <50ns and may be 1ns on some systems.
 * Make time interpolator self-tuning to limit time jumps
   and to make the interpolators work correctly on systems with
   broken time base specifications.
 * SMP scalability: Make clock_gettime and gettimeofday scale O(1)
   by removing the cmpxchg for most clocks (tested for up to 512 CPUs)
 * IA64: provide asm fastcall that doubles the performance
   of gettimeofday and clock_gettime on SGI and other IA64 systems
   (asm fastcalls scale O(1) together with the scalability fixes).
 * IA64: provide nojitter kernel option so that IA64 systems with
   correctly synchronized ITC counters may also enjoy the
   scalability enhancements.

Performance measurements for single calls (ITC cycles):

A. 4 way Intel IA64 SMP system (kmart)

ITC offsets:
kmart:/usr/src/noship-tests # dmesg|grep synchr
CPU 1: synchronized ITC with CPU 0 (last diff 1 cycles, maxerr 417 cycles)
CPU 2: synchronized ITC with CPU 0 (last diff 2 cycles, maxerr 417 cycles)
CPU 3: synchronized ITC with CPU 0 (last diff 1 cycles, maxerr 417 cycles)

A.1. Current kernel code

kmart:/usr/src/noship-tests # ./dmt
gettimeofday cycles: 3737 220 215 215 215 215 215 215 215 215
clock_gettime(REAL) cycles: 4058 575 564 576 565 566 558 558 558 558
clock_gettime(MONO) cycles: 1583 621 609 609 609 609 609 609 609 609
clock_gettime(PROCESS) cycles: 71428 298 259 259 259 259 259 259 259 259
clock_gettime(THREAD) cycles: 3982 336 290 298 298 298 298 286 286 286

A.2 New code using cmpxchg

kmart:/usr/src/noship-tests # ./dmt
gettimeofday cycles: 3145 213 216 213 213 213 213 213 213 213
clock_gettime(REAL) cycles: 3185 230 210 210 210 210 210 210 210 210
clock_gettime(MONO) cycles: 284 217 217 216 216 216 216 216 216 216
clock_gettime(PROCESS) cycles: 68857 289 270 259 259 259 259 259 259 259
clock_gettime(THREAD) cycles: 3862 339 298 298 298 298 290 286 286 286

A.3 New code with cmpxchg switched off (nojitter kernel option)

kmart:/usr/src/noship-tests # ./dmt
gettimeofday cycles: 3195 219 219 212 212 212 212 212 212 212
clock_gettime(REAL) cycles: 3003 228 205 205 205 205 205 205 205 205
clock_gettime(MONO) cycles: 279 209 209 209 208 208 208 208 208 208
clock_gettime(PROCESS) cycles: 65849 292 259 259 268 270 270 259 259 259

B. SGI SN2 system running 512 IA64 CPUs.

B.1. Current kernel code

[root@ascender noship-tests]# ./dmt
gettimeofday cycles: 17221 1028 1007 1004 1004 1004 1010 25928 1002 1003
clock_gettime(REAL) cycles: 10388 1099 1055 1044 1064 1063 1051 1056 1061 1056
clock_gettime(MONO) cycles: 2363 96 96 96 96 96 96 96 96 96
clock_gettime(PROCESS) cycles: 46537 804 660 666 666 666 666 666 666 666
clock_gettime(THREAD) cycles: 10945 727 710 684 685 686 685 686 685 686

B.2 New code

ascender:~/noship-tests # ./dmt
gettimeofday cycles: 3874 610 588 588 588 588 588 588 588 588
clock_gettime(REAL) cycles: 3893 612 588 582 588 588 588 588 588 588
clock_gettime(MONO) cycles: 686 595 595 588 588 588 588 588 588 588
clock_gettime(PROCESS) cycles: 290759 322 269 269 259 265 265 265 259 259
clock_gettime(THREAD) cycles: 5153 358 306 298 296 304 290 298 298 298

Scalability of time functions (in time it takes to do a million calls):
=======================================================================

A. 4 way Intel IA SMP system (kmart)
A.1 Current code

kmart:/usr/src/noship-tests # ./todscale -n1000000
 CPUS       WALL  WALL/CPUS
    1      0.192      0.192
    2      1.125      0.563
    4      9.229      2.307

A.2 New code using cmpxchg

kmart:/usr/src/noship-tests # ./todscale
 CPUS       WALL  WALL/CPUS
    1      0.188      0.188
    2      0.457      0.229
    4      0.413      0.103

(the measurement with 4 cpus may fluctuate up to 15.x somehow)

A.3 New code without cmpxchg (nojitter kernel option)

kmart:/usr/src/noship-tests # ./todscale -n10000000
 CPUS       WALL  WALL/CPUS
    1      0.180      0.180
    2      0.180      0.090
    4      0.252      0.063

B. SGI SN2 system running 512 IA64 CPUs.

The system has a global monotonic clock and therefore has
no need for compensation. Current code uses a cmpxchg. New
code has no cmpxchg.

B.1 current code

ascender:~/noship-tests # ./todscale
 CPUS       WALL  WALL/CPUS
    1      0.850      0.850
    2      1.767      0.884
    4      6.124      1.531
    8     20.777      2.597
   16     57.693      3.606
   32    164.688      5.146
   64    456.647      7.135
  128   1093.371      8.542
  256   2778.257     10.853
(System crash at 512 CPUs)

B.2 New code

ascender:~/noship-tests # ./todscale -n1000000
 CPUS       WALL  WALL/CPUS
    1      0.426      0.426
    2      0.429      0.215
    4      0.436      0.109
    8      0.452      0.057
   16      0.454      0.028
   32      0.457      0.014
   64      0.459      0.007
  128      0.466      0.004
  256      0.474      0.002
  512      0.518      0.001

Clock Accuracy
==============
A. 4 CPU SMP system

A.1 Old code

kmart:/usr/src/noship-tests # ./cdisp
          Gettimeofday() = 1092124757.270305000
           CLOCK_REALTIME= 1092124757.270382000 resolution= 0.000976563
          CLOCK_MONOTONIC=         89.696726590 resolution= 0.000976563
 CLOCK_PROCESS_CPUTIME_ID=          0.001242507 resolution= 0.000000001
  CLOCK_THREAD_CPUTIME_ID=          0.001255310 resolution= 0.000000001

A.2 New code

kmart:/usr/src/noship-tests # ./cdisp
          Gettimeofday() = 1092124478.194530000
           CLOCK_REALTIME= 1092124478.194603399 resolution= 0.000000001
          CLOCK_MONOTONIC=         88.198315204 resolution= 0.000000001
 CLOCK_PROCESS_CPUTIME_ID=          0.001241235 resolution= 0.000000001
  CLOCK_THREAD_CPUTIME_ID=          0.001254747 resolution= 0.000000001
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>