https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=198149
--- Comment #16 from commit-h...@freebsd.org --- A commit references this bug: Author: jhb Date: Tue May 19 19:15:20 UTC 2015 New revision: 283123 URL: https://svnweb.freebsd.org/changeset/base/283123 Log: 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 Differential Revision: https://reviews.freebsd.org/D2557 Reviewed by: emaste MFC after: 1 week Sponsored by: Norse Corp, Inc. Changes: head/sys/dev/hwpmc/hwpmc_core.c head/sys/dev/hwpmc/hwpmc_mod.c -- You are receiving this mail because: You are the assignee for the bug. _______________________________________________ freebsd-bugs@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-bugs To unsubscribe, send any mail to "freebsd-bugs-unsubscr...@freebsd.org"
