[Bug 194796] New: pmcstat(8) paniced a box

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=194796

            Bug ID: 194796
           Summary: pmcstat(8) paniced a box
           Product: Base System
           Version: 10.0-STABLE
          Hardware: Any
                OS: Any
            Status: Needs Triage
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: hiren at FreeBSD.org

Did "pmcstat -S instructions -T" on a 10-stable (ish) sandybridge xeon box and
it paniced.

Nov  3 13:51:11 <kern.crit> vxs162401 kernel: panic: [pmc,4186] cpu 3 didn't
find a sample to collect
Nov  3 13:51:11 <kern.crit> vxs162401 kernel: cpuid = 3
Nov  3 13:51:11 <kern.crit> vxs162401 kernel: KDB: stack backtrace:
Nov  3 13:51:11 <kern.crit> vxs162401 kernel: db_trace_self_wrapper() at
db_trace_self_wrapper+0x2b/frame 0xfffffe1
06dd0d840
Nov  3 13:51:11 <kern.crit> vxs162401 kernel: kdb_backtrace() at
kdb_backtrace+0x39/frame 0xfffffe106dd0d8f0
Nov  3 13:51:11 <kern.crit> vxs162401 kernel: vpanic() at vpanic+0x126/frame
0xfffffe106dd0d930
Nov  3 13:51:11 <kern.crit> vxs162401 kernel: kassert_panic() at
kassert_panic+0x136/frame 0xfffffe106dd0d9a0
Nov  3 13:51:11 <kern.crit> vxs162401 kernel: pmc_capture_user_callchain() at
pmc_capture_user_callchain+0x124/fram
e 0xfffffe106dd0d9f0
Nov  3 13:51:11 <kern.crit> vxs162401 kernel: pmc_hook_handler() at
pmc_hook_handler+0x6bf/frame 0xfffffe106dd0dab0
Nov  3 13:51:11 <kern.crit> vxs162401 kernel: outofnmi() at outofnmi+0x23/frame
0xffff5be0
Nov  3 13:51:11 <kern.crit> vxs162401 kernel: Uptime: 4d14h51m21s

>From chat:

14:38 <@davide> hiren: I see nothing in the code preventing the thread to be
rescheduled into another cpu from a first look
14:38 <@adrian> *burp*
14:38 <@davide> at the end of pmc_capture_user_callchain() you call
sched_unpin() but there's no matching sched_pin() call


Need to investigate more.

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