IMPORT: plock: lower the slope of the exponential back-off

IMPORT: plock: lower the slope of the exponential back-off

Along many tests involving both haproxy's scheduler and forwarded
traffic, various exponents and algorithms were attempted for the EBO
and their effects were measured. It was found that a growth in 1.25^N
limited to 128k cycles consistently gives a better latency than 1.5^N
limited to 256k cycles, without degrading general performance. The
measures of the time to grab a write lock on a 48-thread EPYC show
that the number of occurrences of low times was roughly multiplied by
2-3 while the number of occurrences of times above 64us was reduced
by similar factors, to even reach 300 at 64us and limiting the maximum
time by a factor of 4.

The other variants that were experimented with are:

  m = ((m + (m >> 1)) + 2) & 0x3ffff;            // original
  m = ((m + (m >> 1) + (m >> 3)) + 2) & 0x3ffff;
  m = ((m + (m >> 1) + (m >> 4)) + 2) & 0x3ffff;
  m = ((m + (m >> 1) + (m >> 4)) + 2) & 0x1ffff;
  m = ((m + (m >> 1) + (m >> 4)) + 1) & 0x1ffff;
  m = ((m + (m >> 2) + (m >> 4)) + 1) & 0x1ffff; // lowest CPU on pl_wr test + good perf
  m = ((m + (m >> 2)) + 1) & 0x1ffff;            // even lower cpu usage, lowest max
  m = ((m + (m >> 1) + (m >> 2)) + 1) & 0x1ffff; // correct but slightly higher maxes
  m = ((m + (m >> 1) + (m >> 3)) + 1) & 0x1ffff; // less good than m+m>>2
  m = ((m + (m >> 2) + (m >> 3)) + 1) & 0x1ffff; // better but not as good as m+m>>2
  m = ((m + (m >> 3) + (m >> 4)) + 1) & 0x1ffff; // less good, lower rates on small coounts.
  m = ((m + (m >> 2) + (m >> 3) + (m >> 4)) + 1) & 0x1ffff; // less good as well
  m = ((m & 0x7fff) + (m >> 1) + (m >> 4)) + 2;
  m = ((m & 0xffff) + (m >> 1) + (m >> 4)) + 2;

This is plock commit dddd9ee01c522da33c353e2e4d4fd743d8336ec3.

(cherry picked from commit 253fba01a7adf20303ec65cbe256681df588f065)
Signed-off-by: Willy Tarreau <w@1wt.eu>