[Bug 198149] [hwpmc] pmcstat -P -t (top mode, process sampling) stops after a while

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Mon Jun 1 18:09:23 UTC 2015


--- Comment #17 from commit-hook at freebsd.org ---
A commit references this bug:

Author: jhb
Date: Mon Jun  1 18:08:58 UTC 2015
New revision: 283886
URL: https://svnweb.freebsd.org/changeset/base/283886

  MFC 283123:
  Fix two bugs that could result in PMC sampling effectively stopping.
  In both cases, the the effect of the bug was that a very small positive
  number was written to the counter. This means that a large number of
  events needed to occur before the next sampling interrupt would trigger.
  Even with very frequently occurring events like clock cycles wrapping all
  the way around could take a long time. Both bugs occurred when updating
  the saved reload count for an outgoing thread on a context switch.

  First, the counter-independent code compares the current reload count
  against the count set when the thread switched in and generates a delta
  to apply to the saved count. If this delta causes the reload counter
  to go negative, it would add a full reload interval to wrap it around to
  a positive value. The fix is to add the full reload interval if the
  resulting counter is zero.

  Second, occasionally the raw counter value read during a context switch
  has actually wrapped, but an interrupt has not yet triggered. In this
  case the existing logic would return a very large reload count (e.g.
  2^48 - 2 if the counter had overflowed by a count of 2). This was seen
  both for fixed-function and programmable counters on an E5-2643.
  Workaround this case by returning a reload count of zero.

  PR:        198149
  Sponsored by:    Norse Corp, Inc.

_U  stable/10/
_U  stable/9/sys/
_U  stable/9/sys/dev/

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