tree a8b8c6d171548ea93b51e9c62236e8b41e3e3ab0
parent e70208e41d62fbb65fb537355c5216f37132ce80
author Willy Tarreau <w@1wt.eu> 1616004623 +0100
committer Christopher Faulet <cfaulet@haproxy.com> 1616075350 +0100

BUG/MINOR: freq_ctr/threads: make use of the last updated global time

The freq counters were using the thread's own time as the start of the
current period. The problem is that in case of contention, it was
occasionally possible to perform non-monotonic updates on the edge of
the next second, because if the upfront thread updates a counter first,
it causes a rotation, then the second thread loses the race from its
older time, and tries again, and detects a different time again, but
in the past so it only updates the counter, then a third thread on the
new date would detect a change again, thus provoking a rotation again.

The effect was triple:
  - rare loss of stored values during certain transitions from one
    period to the next one, causing counters to report 0
  - half of the threads forced to go through the slow path every second
  - difficult convergence when using many threads where the CAS can fail
    a lot and we can observe N(N-1) attempts for N threads to complete

This patch fixes this issue in two ways:
  - first, it now makes use og the monotonic global_now value which also
    happens to be volatile and to carry the latest known time; this way
    time will never jump backwards anymore and only the first thread
    updates it on transition, the other ones do not need to.

  - second, re-read the time in the loop after each failure, because
    if the date changed in the counter, it means that one thread knows
    a more recent one and we need to update. In this case if it matches
    the new current second, the fast path is usable.

This patch relies on previous patch "MINOR: time: export the global_now
variable" and must be backported as far as 1.8.

(cherry picked from commit a1ecbca0a50ccb273bb5cdc9f031408d782a7bcf)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 4713176773060651e38345b922b7efbe700b66e9)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit c25bff6d4bbbd7c2043ad9d278ecb01e9b606a21)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 469176a0c5793e02d5092ec19ecccc16d0e343bb)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>