Possible SNB throttling erratum in need of workaround?

[Andy Lutomirski]

I have a box running 3.9.4.  For a few hundred ms, all packages and
cores* exceeded their power limits, and then they all came back to
normal.

Since then, turbo boost went away.

The first thing I tried was writing 0 to msr 0x19C to clear the
throttling "log" bit.  The bit was clear, but still no turbo.  (I only
did this on one package to avoid destroying information.)

I have the performance governor set, so there are never any
software-initiated performance transitions.  As an experiment, I
switched core 0 to powersave (aka very low frequency) and then back to
performance.  The entire package's turbo came back.  I did the same
thing to the cores on package 1 (all of them, sorry), and that
package's turbo boost came back.

Does the kernel need to reprogram CPU frequencies after thermal/power
throttling conditions clear?

My CPUs are:

processor       : 31
vendor_id       : GenuineIntel
cpu family      : 6
model           : 45
model name      : Intel(R) Xeon(R) CPU E5-2690 0 @ 2.90GHz
stepping        : 7
microcode       : 0x710


* Slight lie here.  CPU18 exceeded core limit but not power limit.
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