Optimize backtrace code for perf PMI handler.

From: Andi Kleen
Date: Fri Sep 26 2014 - 19:31:28 EST

While doing perf record -g I regularly exceed the time quote for the NMI:

This leads to perf shortening the period, which causes various problems.

This patchkit optimizes two sources of longer latencies in the NMI backtrace
code. There's probably more work needed to fix other sources of latencies


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